[PATCH 0/7] TTY Keyboard Status Request

[PATCH 0/7] TTY Keyboard Status Request

[Arseny Maslennikov]

This patch series introduces TTY keyboard status request, a feature of
the n_tty line discipline that reserves a character in struct termios
(^T by default) and reacts to it by printing a short informational line
to the terminal and sending a Unix signal to the tty's foreground
process group. The processes may, in response to the signal, output a
textual description of what they're doing.

The feature has been present in a similar form at least in
Free/Open/NetBSD; it would be nice to have something like this in Linux
as well. There is an LKML thread[1] where users have previously
expressed the rationale for this.

The current implementation does not break existing kernel API in any
way, since, fortunately, all the architectures supported by the kernel
happen to have at least 1 free byte in the termios control character
array.

The series should cleanly apply to tty-next.

To thoroughly test these, one might need at least a patched stty among
other tools, so I've brought up a simple initrd generator[2] which can
be used to create a lightweight environment to boot up in a VM and to
fiddle with.

[1] https://lore.kernel.org/lkml/1415200663.3247743.187387481.75CE9317@webmail.messagingengine.com/
[2] https://github.com/porrided/tty-kb-status-userspace

Arseny Maslennikov (7):
  signal.h: Define SIGINFO on all architectures
  tty: termios: Reserve space for VSTATUS in .c_cc
  n_tty: Send SIGINFO to fg pgrp on status request character
  linux/signal.h: Ignore SIGINFO by default in new tasks
  tty: Add NOKERNINFO lflag to termios
  n_tty: ->ops->write: Cut core logic out to a separate function
  n_tty: Provide an informational line on VSTATUS receipt

 arch/alpha/include/asm/termios.h         |   4 +-
 arch/alpha/include/uapi/asm/termbits.h   |   2 +
 arch/arm/include/uapi/asm/signal.h       |   1 +
 arch/h8300/include/uapi/asm/signal.h     |   1 +
 arch/ia64/include/asm/termios.h          |   4 +-
 arch/ia64/include/uapi/asm/signal.h      |   1 +
 arch/ia64/include/uapi/asm/termbits.h    |   2 +
 arch/m68k/include/uapi/asm/signal.h      |   1 +
 arch/mips/include/asm/termios.h          |   4 +-
 arch/mips/include/uapi/asm/signal.h      |   1 +
 arch/mips/include/uapi/asm/termbits.h    |   2 +
 arch/parisc/include/asm/termios.h        |   4 +-
 arch/parisc/include/uapi/asm/signal.h    |   1 +
 arch/parisc/include/uapi/asm/termbits.h  |   2 +
 arch/powerpc/include/asm/termios.h       |   4 +-
 arch/powerpc/include/uapi/asm/signal.h   |   1 +
 arch/powerpc/include/uapi/asm/termbits.h |   2 +
 arch/s390/include/asm/termios.h          |   4 +-
 arch/s390/include/uapi/asm/signal.h      |   1 +
 arch/sparc/include/asm/termios.h         |   4 +-
 arch/sparc/include/uapi/asm/signal.h     |   2 +
 arch/sparc/include/uapi/asm/termbits.h   |   2 +
 arch/x86/include/uapi/asm/signal.h       |   1 +
 arch/xtensa/include/uapi/asm/signal.h    |   1 +
 arch/xtensa/include/uapi/asm/termbits.h  |   2 +
 drivers/tty/Makefile                     |   3 +-
 drivers/tty/n_tty.c                      |  69 ++++-
 drivers/tty/n_tty_status.c               | 363 +++++++++++++++++++++++
 include/asm-generic/termios.h            |   4 +-
 include/linux/signal.h                   |   5 +-
 include/linux/tty.h                      |   7 +-
 include/uapi/asm-generic/signal.h        |   1 +
 include/uapi/asm-generic/termbits.h      |   2 +
 33 files changed, 475 insertions(+), 33 deletions(-)
 create mode 100644 drivers/tty/n_tty_status.c

-- 
2.20.1

[PATCH 1/7] signal.h: Define SIGINFO on all architectures

[Arseny Maslennikov]

This complementary patch defines SIGINFO as a synonym for SIGPWR
on every architecture supported by the kernel.
The particular signal number chosen does not really matter and is only
required for the related tty functionality to work properly,
so if it does not suite expectations, any suggestions are warmly
welcome.

SIGPWR looks like a nice candidate for this role, because it is
defined on every supported arch; it is currently only used to inform
PID 1 of power failures, and daemons that care about low-level
events do not tend to have a controlling terminal.

However, on sparcs SIGPWR is a synonym for SIGLOST, a signal unique
to that architecture, with a narrow set of intended uses that do not
combine well with interactively requesting status.
SIGLOST is not used by any kernel code at the moment.
I'm not sure there is a more reasonable alternative right now.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 arch/arm/include/uapi/asm/signal.h     | 1 +
 arch/h8300/include/uapi/asm/signal.h   | 1 +
 arch/ia64/include/uapi/asm/signal.h    | 1 +
 arch/m68k/include/uapi/asm/signal.h    | 1 +
 arch/mips/include/uapi/asm/signal.h    | 1 +
 arch/parisc/include/uapi/asm/signal.h  | 1 +
 arch/powerpc/include/uapi/asm/signal.h | 1 +
 arch/s390/include/uapi/asm/signal.h    | 1 +
 arch/sparc/include/uapi/asm/signal.h   | 2 ++
 arch/x86/include/uapi/asm/signal.h     | 1 +
 arch/xtensa/include/uapi/asm/signal.h  | 1 +
 include/uapi/asm-generic/signal.h      | 1 +
 12 files changed, 13 insertions(+)

diff --git a/arch/arm/include/uapi/asm/signal.h b/arch/arm/include/uapi/asm/signal.h
index 9b4185ba4f8a..b80b53a17267 100644
--- a/arch/arm/include/uapi/asm/signal.h
+++ b/arch/arm/include/uapi/asm/signal.h
@@ -50,6 +50,7 @@ typedef unsigned long sigset_t;
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
diff --git a/arch/h8300/include/uapi/asm/signal.h b/arch/h8300/include/uapi/asm/signal.h
index e15521037348..7a2b783af22b 100644
--- a/arch/h8300/include/uapi/asm/signal.h
+++ b/arch/h8300/include/uapi/asm/signal.h
@@ -50,6 +50,7 @@ typedef unsigned long sigset_t;
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
diff --git a/arch/ia64/include/uapi/asm/signal.h b/arch/ia64/include/uapi/asm/signal.h
index aa98ff1b9e22..b4c98cb17165 100644
--- a/arch/ia64/include/uapi/asm/signal.h
+++ b/arch/ia64/include/uapi/asm/signal.h
@@ -45,6 +45,7 @@
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 /* signal 31 is no longer "unused", but the SIGUNUSED macro remains for backwards compatibility */
 #define	SIGUNUSED	31
diff --git a/arch/m68k/include/uapi/asm/signal.h b/arch/m68k/include/uapi/asm/signal.h
index 915cc755a184..a0b4e4108cb8 100644
--- a/arch/m68k/include/uapi/asm/signal.h
+++ b/arch/m68k/include/uapi/asm/signal.h
@@ -50,6 +50,7 @@ typedef unsigned long sigset_t;
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
diff --git a/arch/mips/include/uapi/asm/signal.h b/arch/mips/include/uapi/asm/signal.h
index 53104b10aae2..975a6f0d3b0b 100644
--- a/arch/mips/include/uapi/asm/signal.h
+++ b/arch/mips/include/uapi/asm/signal.h
@@ -43,6 +43,7 @@ typedef unsigned long old_sigset_t;		/* at least 32 bits */
 #define SIGCHLD		18	/* Child status has changed (POSIX).  */
 #define SIGCLD		SIGCHLD /* Same as SIGCHLD (System V).	*/
 #define SIGPWR		19	/* Power failure restart (System V).  */
+#define SIGINFO		SIGPWR	/* Keyboard status request (4.2 BSD). */
 #define SIGWINCH	20	/* Window size change (4.3 BSD, Sun).  */
 #define SIGURG		21	/* Urgent condition on socket (4.2 BSD).  */
 #define SIGIO		22	/* I/O now possible (4.2 BSD).	*/
diff --git a/arch/parisc/include/uapi/asm/signal.h b/arch/parisc/include/uapi/asm/signal.h
index d38563a394f2..fe2e00d590ac 100644
--- a/arch/parisc/include/uapi/asm/signal.h
+++ b/arch/parisc/include/uapi/asm/signal.h
@@ -22,6 +22,7 @@
 #define SIGUSR2		17
 #define SIGCHLD		18
 #define SIGPWR		19
+#define SIGINFO		SIGPWR
 #define SIGVTALRM	20
 #define SIGPROF		21
 #define SIGIO		22
diff --git a/arch/powerpc/include/uapi/asm/signal.h b/arch/powerpc/include/uapi/asm/signal.h
index 85b0a7aa43e7..e7f3885905b4 100644
--- a/arch/powerpc/include/uapi/asm/signal.h
+++ b/arch/powerpc/include/uapi/asm/signal.h
@@ -53,6 +53,7 @@ typedef struct {
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
diff --git a/arch/s390/include/uapi/asm/signal.h b/arch/s390/include/uapi/asm/signal.h
index 9a14a611ed82..12ee62987971 100644
--- a/arch/s390/include/uapi/asm/signal.h
+++ b/arch/s390/include/uapi/asm/signal.h
@@ -58,6 +58,7 @@ typedef unsigned long sigset_t;
 #define SIGLOST         29
 */
 #define SIGPWR          30
+#define SIGINFO         SIGPWR
 #define SIGSYS		31
 #define SIGUNUSED       31
 
diff --git a/arch/sparc/include/uapi/asm/signal.h b/arch/sparc/include/uapi/asm/signal.h
index ff9505923b9a..b655163198bb 100644
--- a/arch/sparc/include/uapi/asm/signal.h
+++ b/arch/sparc/include/uapi/asm/signal.h
@@ -71,6 +71,8 @@
 #define SIGWINCH	28
 #define SIGLOST		29
 #define SIGPWR		SIGLOST
+/* XXX: is it OK for SIGINFO to collide with LOST? */
+#define SIGINFO		SIGPWR
 #define SIGUSR1		30
 #define SIGUSR2		31
 
diff --git a/arch/x86/include/uapi/asm/signal.h b/arch/x86/include/uapi/asm/signal.h
index e5745d593dc7..1539bb28826c 100644
--- a/arch/x86/include/uapi/asm/signal.h
+++ b/arch/x86/include/uapi/asm/signal.h
@@ -55,6 +55,7 @@ typedef unsigned long sigset_t;
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
diff --git a/arch/xtensa/include/uapi/asm/signal.h b/arch/xtensa/include/uapi/asm/signal.h
index 005dec5bfde4..d644234305de 100644
--- a/arch/xtensa/include/uapi/asm/signal.h
+++ b/arch/xtensa/include/uapi/asm/signal.h
@@ -65,6 +65,7 @@ typedef struct {
 #define SIGPOLL		SIGIO
 /* #define SIGLOST		29 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
diff --git a/include/uapi/asm-generic/signal.h b/include/uapi/asm-generic/signal.h
index 5c716a952cbe..9f9a1db0d43c 100644
--- a/include/uapi/asm-generic/signal.h
+++ b/include/uapi/asm-generic/signal.h
@@ -43,6 +43,7 @@
 #define SIGLOST		29
 */
 #define SIGPWR		30
+#define SIGINFO		SIGPWR
 #define SIGSYS		31
 #define	SIGUNUSED	31
 
-- 
2.20.1

[PATCH 2/7] tty: termios: Reserve space for VSTATUS in .c_cc

[Arseny Maslennikov]

There was no architecture we had to increase NCCS on,
so the size of struct termios did not change.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 arch/alpha/include/asm/termios.h         | 4 ++--
 arch/alpha/include/uapi/asm/termbits.h   | 1 +
 arch/ia64/include/asm/termios.h          | 4 ++--
 arch/ia64/include/uapi/asm/termbits.h    | 1 +
 arch/mips/include/asm/termios.h          | 4 ++--
 arch/mips/include/uapi/asm/termbits.h    | 1 +
 arch/parisc/include/asm/termios.h        | 4 ++--
 arch/parisc/include/uapi/asm/termbits.h  | 1 +
 arch/powerpc/include/asm/termios.h       | 4 ++--
 arch/powerpc/include/uapi/asm/termbits.h | 1 +
 arch/s390/include/asm/termios.h          | 4 ++--
 arch/sparc/include/asm/termios.h         | 4 ++--
 arch/sparc/include/uapi/asm/termbits.h   | 1 +
 arch/xtensa/include/uapi/asm/termbits.h  | 1 +
 include/asm-generic/termios.h            | 4 ++--
 include/uapi/asm-generic/termbits.h      | 1 +
 16 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/arch/alpha/include/asm/termios.h b/arch/alpha/include/asm/termios.h
index b7c77bb1bfd2..3f6e9995f415 100644
--- a/arch/alpha/include/asm/termios.h
+++ b/arch/alpha/include/asm/termios.h
@@ -8,9 +8,9 @@
 	werase=^W	kill=^U		reprint=^R	sxtc=\0
 	intr=^C		quit=^\		susp=^Z		<OSF/1 VDSUSP>
 	start=^Q	stop=^S		lnext=^V	discard=^U
-	vmin=\1		vtime=\0
+	vmin=\1		vtime=\0	vstatus=^T
 */
-#define INIT_C_CC "\004\000\000\177\027\025\022\000\003\034\032\000\021\023\026\025\001\000"
+#define INIT_C_CC "\004\000\000\177\027\025\022\000\003\034\032\000\021\023\026\025\001\000\024"
 
 /*
  * Translate a "termio" structure into a "termios". Ugh.
diff --git a/arch/alpha/include/uapi/asm/termbits.h b/arch/alpha/include/uapi/asm/termbits.h
index 4575ba34a0ea..bb895d467033 100644
--- a/arch/alpha/include/uapi/asm/termbits.h
+++ b/arch/alpha/include/uapi/asm/termbits.h
@@ -70,6 +70,7 @@ struct ktermios {
 #define VDISCARD 15
 #define VMIN 16
 #define VTIME 17
+#define VSTATUS 18
 
 /* c_iflag bits */
 #define IGNBRK	0000001
diff --git a/arch/ia64/include/asm/termios.h b/arch/ia64/include/asm/termios.h
index 589c026444cc..b262e010baf2 100644
--- a/arch/ia64/include/asm/termios.h
+++ b/arch/ia64/include/asm/termios.h
@@ -15,9 +15,9 @@
 	eof=^D		vtime=\0	vmin=\1		sxtc=\0
 	start=^Q	stop=^S		susp=^Z		eol=\0
 	reprint=^R	discard=^U	werase=^W	lnext=^V
-	eol2=\0
+	eol2=\0		status=^T
 */
-#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0"
+#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0\024"
 
 /*
  * Translate a "termio" structure into a "termios". Ugh.
diff --git a/arch/ia64/include/uapi/asm/termbits.h b/arch/ia64/include/uapi/asm/termbits.h
index 000a1a297c75..86898e4c5905 100644
--- a/arch/ia64/include/uapi/asm/termbits.h
+++ b/arch/ia64/include/uapi/asm/termbits.h
@@ -67,6 +67,7 @@ struct ktermios {
 #define VWERASE 14
 #define VLNEXT 15
 #define VEOL2 16
+#define VSTATUS 17
 
 /* c_iflag bits */
 #define IGNBRK	0000001
diff --git a/arch/mips/include/asm/termios.h b/arch/mips/include/asm/termios.h
index bc29eeacc55a..9086e4f641f4 100644
--- a/arch/mips/include/asm/termios.h
+++ b/arch/mips/include/asm/termios.h
@@ -17,9 +17,9 @@
  *	vmin=\1		vtime=\0	eol2=\0		swtc=\0
  *	start=^Q	stop=^S		susp=^Z		vdsusp=
  *	reprint=^R	discard=^U	werase=^W	lnext=^V
- *	eof=^D		eol=\0
+ *	eof=^D		eol=\0		status=^T
  */
-#define INIT_C_CC "\003\034\177\025\1\0\0\0\021\023\032\0\022\017\027\026\004\0"
+#define INIT_C_CC "\003\034\177\025\1\0\0\0\021\023\032\0\022\017\027\026\004\0\024"
 
 #include <linux/string.h>
 
diff --git a/arch/mips/include/uapi/asm/termbits.h b/arch/mips/include/uapi/asm/termbits.h
index dfeffba729b7..3dd60924f0ef 100644
--- a/arch/mips/include/uapi/asm/termbits.h
+++ b/arch/mips/include/uapi/asm/termbits.h
@@ -78,6 +78,7 @@ struct ktermios {
 #define VLNEXT		15		/* Literal-next character [IEXTEN].  */
 #define VEOF		16		/* End-of-file character [ICANON].  */
 #define VEOL		17		/* End-of-line character [ICANON].  */
+#define VSTATUS		18		/* Status request character [ISIG]. */
 
 /* c_iflag bits */
 #define IGNBRK	0000001		/* Ignore break condition.  */
diff --git a/arch/parisc/include/asm/termios.h b/arch/parisc/include/asm/termios.h
index cded9dc90c1b..37828b15a428 100644
--- a/arch/parisc/include/asm/termios.h
+++ b/arch/parisc/include/asm/termios.h
@@ -9,9 +9,9 @@
 	eof=^D		vtime=\0	vmin=\1		sxtc=\0
 	start=^Q	stop=^S		susp=^Z		eol=\0
 	reprint=^R	discard=^U	werase=^W	lnext=^V
-	eol2=\0
+	eol2=\0		status=^T
 */
-#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0"
+#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0\024"
 
 /*
  * Translate a "termio" structure into a "termios". Ugh.
diff --git a/arch/parisc/include/uapi/asm/termbits.h b/arch/parisc/include/uapi/asm/termbits.h
index 40e920f8d683..ecca3b7d0c66 100644
--- a/arch/parisc/include/uapi/asm/termbits.h
+++ b/arch/parisc/include/uapi/asm/termbits.h
@@ -58,6 +58,7 @@ struct ktermios {
 #define VWERASE 14
 #define VLNEXT 15
 #define VEOL2 16
+#define VSTATUS 17
 
 
 /* c_iflag bits */
diff --git a/arch/powerpc/include/asm/termios.h b/arch/powerpc/include/asm/termios.h
index 205de8f8a9d3..263707fc3ec9 100644
--- a/arch/powerpc/include/asm/termios.h
+++ b/arch/powerpc/include/asm/termios.h
@@ -10,8 +10,8 @@
 
 #include <uapi/asm/termios.h>
 
-/*                   ^C  ^\ del  ^U  ^D   1   0   0   0   0  ^W  ^R  ^Z  ^Q  ^S  ^V  ^U  */
-#define INIT_C_CC "\003\034\177\025\004\001\000\000\000\000\027\022\032\021\023\026\025" 
+/*                   ^C  ^\ del  ^U  ^D   1   0   0   0   0  ^W  ^R  ^Z  ^Q  ^S  ^V  ^U  ^T  */
+#define INIT_C_CC "\003\034\177\025\004\001\000\000\000\000\027\022\032\021\023\026\025\024"
 
 #include <asm-generic/termios-base.h>
 
diff --git a/arch/powerpc/include/uapi/asm/termbits.h b/arch/powerpc/include/uapi/asm/termbits.h
index ed18bc61f63d..c85e98d75e76 100644
--- a/arch/powerpc/include/uapi/asm/termbits.h
+++ b/arch/powerpc/include/uapi/asm/termbits.h
@@ -62,6 +62,7 @@ struct ktermios {
 #define VSTOP		14
 #define VLNEXT		15
 #define VDISCARD	16
+#define VSTATUS		17
 
 /* c_iflag bits */
 #define IGNBRK	0000001
diff --git a/arch/s390/include/asm/termios.h b/arch/s390/include/asm/termios.h
index 46fa3020b41e..c84e49ed76b4 100644
--- a/arch/s390/include/asm/termios.h
+++ b/arch/s390/include/asm/termios.h
@@ -14,9 +14,9 @@
 	eof=^D		vtime=\0	vmin=\1		sxtc=\0
 	start=^Q	stop=^S		susp=^Z		eol=\0
 	reprint=^R	discard=^U	werase=^W	lnext=^V
-	eol2=\0
+	eol2=\0		status=^T
 */
-#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0"
+#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0\024"
 
 #define user_termios_to_kernel_termios(k, u) copy_from_user(k, u, sizeof(struct termios2))
 #define kernel_termios_to_user_termios(u, k) copy_to_user(u, k, sizeof(struct termios2))
diff --git a/arch/sparc/include/asm/termios.h b/arch/sparc/include/asm/termios.h
index 4a558efdfa93..249c95cf285b 100644
--- a/arch/sparc/include/asm/termios.h
+++ b/arch/sparc/include/asm/termios.h
@@ -20,9 +20,9 @@
 	eof=^D		eol=\0		eol2=\0		sxtc=\0
 	start=^Q	stop=^S		susp=^Z		dsusp=^Y
 	reprint=^R	discard=^U	werase=^W	lnext=^V
-	vmin=\1         vtime=\0
+	vmin=\1		vtime=\0	status=^T
 */
-#define INIT_C_CC "\003\034\177\025\004\000\000\000\021\023\032\031\022\025\027\026\001"
+#define INIT_C_CC "\003\034\177\025\004\000\000\000\021\023\032\031\022\025\027\026\001\000\024"
 
 /*
  * Translate a "termio" structure into a "termios". Ugh.
diff --git a/arch/sparc/include/uapi/asm/termbits.h b/arch/sparc/include/uapi/asm/termbits.h
index ce5ad5d0f105..a1638c9cde8b 100644
--- a/arch/sparc/include/uapi/asm/termbits.h
+++ b/arch/sparc/include/uapi/asm/termbits.h
@@ -80,6 +80,7 @@ struct ktermios {
 #define VDISCARD 13
 #define VWERASE  14
 #define VLNEXT   15
+#define VSTATUS  16
 
 /* Kernel keeps vmin/vtime separated, user apps assume vmin/vtime is
  * shared with eof/eol
diff --git a/arch/xtensa/include/uapi/asm/termbits.h b/arch/xtensa/include/uapi/asm/termbits.h
index d4206a7c5138..77969cb275b8 100644
--- a/arch/xtensa/include/uapi/asm/termbits.h
+++ b/arch/xtensa/include/uapi/asm/termbits.h
@@ -72,6 +72,7 @@ struct ktermios {
 #define VWERASE 14
 #define VLNEXT 15
 #define VEOL2 16
+#define VSTATUS 17
 
 /* c_iflag bits */
 
diff --git a/include/asm-generic/termios.h b/include/asm-generic/termios.h
index b1398d0d4a1d..06fbfb87c991 100644
--- a/include/asm-generic/termios.h
+++ b/include/asm-generic/termios.h
@@ -10,9 +10,9 @@
 	eof=^D		vtime=\0	vmin=\1		sxtc=\0
 	start=^Q	stop=^S		susp=^Z		eol=\0
 	reprint=^R	discard=^U	werase=^W	lnext=^V
-	eol2=\0
+	eol2=\0		status=^T
 */
-#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0"
+#define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0\024"
 
 /*
  * Translate a "termio" structure into a "termios". Ugh.
diff --git a/include/uapi/asm-generic/termbits.h b/include/uapi/asm-generic/termbits.h
index 2fbaf9ae89dd..11bb142ba844 100644
--- a/include/uapi/asm-generic/termbits.h
+++ b/include/uapi/asm-generic/termbits.h
@@ -58,6 +58,7 @@ struct ktermios {
 #define VWERASE 14
 #define VLNEXT 15
 #define VEOL2 16
+#define VSTATUS 17
 
 /* c_iflag bits */
 #define IGNBRK	0000001
-- 
2.20.1

[PATCH 3/7] n_tty: Send SIGINFO to fg pgrp on status request character

[Arseny Maslennikov]

No kerninfo line is printed yet.

No existing implementation of this on any Unix-like system echoes
the status character; no existing implementation discards or flushes
pending input on VSTATUS receipt.

There are existing popular TUI applications (e. g. mutt) that only
turn off icanon and not iexten, but still do not expect any special
treatment of the status request character — thus we require all three:
isig, icanon and iexten to trigger this.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 drivers/tty/n_tty.c | 15 +++++++++++++--
 include/linux/tty.h |  1 +
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index f9c584244f72..29f33798a6cd 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -79,6 +79,10 @@
 #define ECHO_BLOCK		256
 #define ECHO_DISCARD_WATERMARK	N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)
 
+#define SIG_FLUSHING_MASK ( \
+	rt_sigmask(SIGINT) | rt_sigmask(SIGQUIT) | \
+	rt_sigmask(SIGTSTP)			 )
+#define SIG_FLUSHING(sig) ((1 << sig) & SIG_FLUSHING_MASK)
 
 #undef N_TTY_TRACE
 #ifdef N_TTY_TRACE
@@ -1122,7 +1126,7 @@ static void isig(int sig, struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	if (L_NOFLSH(tty)) {
+	if (L_NOFLSH(tty) || (!SIG_FLUSHING(sig))) {
 		/* signal only */
 		__isig(sig, tty);
 
@@ -1244,7 +1248,8 @@ n_tty_receive_signal_char(struct tty_struct *tty, int signal, unsigned char c)
 	if (I_IXON(tty))
 		start_tty(tty);
 	if (L_ECHO(tty)) {
-		echo_char(c, tty);
+		if (c != STATUS_CHAR(tty))
+			echo_char(c, tty);
 		commit_echoes(tty);
 	} else
 		process_echoes(tty);
@@ -1294,6 +1299,9 @@ n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
 		} else if (c == SUSP_CHAR(tty)) {
 			n_tty_receive_signal_char(tty, SIGTSTP, c);
 			return 0;
+		} else if (c == STATUS_CHAR(tty)) {
+			n_tty_receive_signal_char(tty, SIGINFO, c);
+			return 0;
 		}
 	}
 
@@ -1848,6 +1856,9 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 			set_bit(INTR_CHAR(tty), ldata->char_map);
 			set_bit(QUIT_CHAR(tty), ldata->char_map);
 			set_bit(SUSP_CHAR(tty), ldata->char_map);
+			if (L_ICANON(tty) && L_IEXTEN(tty)) {
+				set_bit(STATUS_CHAR(tty), ldata->char_map);
+			}
 		}
 		clear_bit(__DISABLED_CHAR, ldata->char_map);
 		ldata->raw = 0;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index bfa4e2ee94a9..38d8ffe7f0e3 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -120,6 +120,7 @@ struct tty_bufhead {
 #define WERASE_CHAR(tty) ((tty)->termios.c_cc[VWERASE])
 #define LNEXT_CHAR(tty)	((tty)->termios.c_cc[VLNEXT])
 #define EOL2_CHAR(tty) ((tty)->termios.c_cc[VEOL2])
+#define STATUS_CHAR(tty) ((tty)->termios.c_cc[VSTATUS])
 
 #define _I_FLAG(tty, f)	((tty)->termios.c_iflag & (f))
 #define _O_FLAG(tty, f)	((tty)->termios.c_oflag & (f))
-- 
2.20.1

[PATCH 4/7] linux/signal.h: Ignore SIGINFO by default in new tasks

[Arseny Maslennikov]

This matches the behaviour of other Unix-like systems that have SIGINFO
and causes less harm to processes that do not install handlers for this
signal, making the keyboard status character non-fatal for them.

This is implemented with the assumption that SIGINFO is defined
to be equivalent to SIGPWR; still, there is no reason for PWR to
result in termination of the signal recipient anyway — it does not
indicate there is a fatal problem with the recipient's execution
context (like e.g. FPE/ILL do), and we have TERM/KILL for explicit
termination requests.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 include/linux/signal.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/signal.h b/include/linux/signal.h
index 9702016734b1..c365754ea647 100644
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -360,7 +360,7 @@ extern bool unhandled_signal(struct task_struct *tsk, int sig);
  *	|  SIGSYS/SIGUNUSED  |	coredump 	|
  *	|  SIGSTKFLT         |	terminate	|
  *	|  SIGWINCH          |	ignore   	|
- *	|  SIGPWR            |	terminate	|
+ *	|  SIGPWR            |	ignore   	|
  *	|  SIGRTMIN-SIGRTMAX |	terminate       |
  *	+--------------------+------------------+
  *	|  non-POSIX signal  |  default action  |
@@ -411,7 +411,8 @@ extern bool unhandled_signal(struct task_struct *tsk, int sig);
 
 #define SIG_KERNEL_IGNORE_MASK (\
         rt_sigmask(SIGCONT)   |  rt_sigmask(SIGCHLD)   | \
-	rt_sigmask(SIGWINCH)  |  rt_sigmask(SIGURG)    )
+	rt_sigmask(SIGWINCH)  |  rt_sigmask(SIGURG)    | \
+	rt_sigmask(SIGINFO)			       )
 
 #define SIG_SPECIFIC_SICODES_MASK (\
 	rt_sigmask(SIGILL)    |  rt_sigmask(SIGFPE)    | \
-- 
2.20.1

[PATCH 5/7] tty: Add NOKERNINFO lflag to termios

[Arseny Maslennikov]

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 arch/alpha/include/uapi/asm/termbits.h   | 1 +
 arch/ia64/include/uapi/asm/termbits.h    | 1 +
 arch/mips/include/uapi/asm/termbits.h    | 1 +
 arch/parisc/include/uapi/asm/termbits.h  | 1 +
 arch/powerpc/include/uapi/asm/termbits.h | 1 +
 arch/sparc/include/uapi/asm/termbits.h   | 1 +
 arch/xtensa/include/uapi/asm/termbits.h  | 1 +
 include/linux/tty.h                      | 1 +
 include/uapi/asm-generic/termbits.h      | 1 +
 9 files changed, 9 insertions(+)

diff --git a/arch/alpha/include/uapi/asm/termbits.h b/arch/alpha/include/uapi/asm/termbits.h
index bb895d467033..850dd8dc4c51 100644
--- a/arch/alpha/include/uapi/asm/termbits.h
+++ b/arch/alpha/include/uapi/asm/termbits.h
@@ -204,6 +204,7 @@ struct ktermios {
 #define PENDIN	0x20000000
 #define IEXTEN	0x00000400
 #define EXTPROC	0x10000000
+#define NOKERNINFO	0x40000000
 
 /* Values for the ACTION argument to `tcflow'.  */
 #define	TCOOFF		0
diff --git a/arch/ia64/include/uapi/asm/termbits.h b/arch/ia64/include/uapi/asm/termbits.h
index 86898e4c5905..f777b99bc5ab 100644
--- a/arch/ia64/include/uapi/asm/termbits.h
+++ b/arch/ia64/include/uapi/asm/termbits.h
@@ -190,6 +190,7 @@ struct ktermios {
 #define PENDIN	0040000
 #define IEXTEN	0100000
 #define EXTPROC	0200000
+#define NOKERNINFO	0400000
 
 /* tcflow() and TCXONC use these */
 #define	TCOOFF		0
diff --git a/arch/mips/include/uapi/asm/termbits.h b/arch/mips/include/uapi/asm/termbits.h
index 3dd60924f0ef..2755cfd1497e 100644
--- a/arch/mips/include/uapi/asm/termbits.h
+++ b/arch/mips/include/uapi/asm/termbits.h
@@ -206,6 +206,7 @@ struct ktermios {
 #define TOSTOP	0100000		/* Send SIGTTOU for background output.	*/
 #define ITOSTOP TOSTOP
 #define EXTPROC 0200000		/* External processing on pty */
+#define NOKERNINFO 0400000		/* Disable kernel output from VSTATUS. */
 
 /* ioctl (fd, TIOCSERGETLSR, &result) where result may be as below */
 #define TIOCSER_TEMT	0x01	/* Transmitter physically empty */
diff --git a/arch/parisc/include/uapi/asm/termbits.h b/arch/parisc/include/uapi/asm/termbits.h
index ecca3b7d0c66..c7ba145adf02 100644
--- a/arch/parisc/include/uapi/asm/termbits.h
+++ b/arch/parisc/include/uapi/asm/termbits.h
@@ -183,6 +183,7 @@ struct ktermios {
 #define PENDIN  0040000
 #define IEXTEN  0100000
 #define EXTPROC	0200000
+#define NOKERNINFO	0400000
 
 /* tcflow() and TCXONC use these */
 #define	TCOOFF		0
diff --git a/arch/powerpc/include/uapi/asm/termbits.h b/arch/powerpc/include/uapi/asm/termbits.h
index c85e98d75e76..79dbcc546217 100644
--- a/arch/powerpc/include/uapi/asm/termbits.h
+++ b/arch/powerpc/include/uapi/asm/termbits.h
@@ -192,6 +192,7 @@ struct ktermios {
 #define PENDIN	0x20000000
 #define IEXTEN	0x00000400
 #define EXTPROC	0x10000000
+#define NOKERNINFO	0x40000000
 
 /* Values for the ACTION argument to `tcflow'.  */
 #define	TCOOFF		0
diff --git a/arch/sparc/include/uapi/asm/termbits.h b/arch/sparc/include/uapi/asm/termbits.h
index a1638c9cde8b..108c039ddefb 100644
--- a/arch/sparc/include/uapi/asm/termbits.h
+++ b/arch/sparc/include/uapi/asm/termbits.h
@@ -225,6 +225,7 @@ struct ktermios {
 #define PENDIN	0x00004000
 #define IEXTEN	0x00008000
 #define EXTPROC	0x00010000
+#define NOKERNINFO	0x00020000
 
 /* modem lines */
 #define TIOCM_LE	0x001
diff --git a/arch/xtensa/include/uapi/asm/termbits.h b/arch/xtensa/include/uapi/asm/termbits.h
index 77969cb275b8..6155e1c81164 100644
--- a/arch/xtensa/include/uapi/asm/termbits.h
+++ b/arch/xtensa/include/uapi/asm/termbits.h
@@ -199,6 +199,7 @@ struct ktermios {
 #define PENDIN	0040000
 #define IEXTEN	0100000
 #define EXTPROC	0200000
+#define NOKERNINFO	0400000
 
 /* tcflow() and TCXONC use these */
 
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 38d8ffe7f0e3..07189f18f93d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -186,6 +186,7 @@ struct tty_bufhead {
 #define L_PENDIN(tty)	_L_FLAG((tty), PENDIN)
 #define L_IEXTEN(tty)	_L_FLAG((tty), IEXTEN)
 #define L_EXTPROC(tty)	_L_FLAG((tty), EXTPROC)
+#define L_NOKERNINFO(tty)	_L_FLAG((tty), NOKERNINFO)
 
 struct device;
 struct signal_struct;
diff --git a/include/uapi/asm-generic/termbits.h b/include/uapi/asm-generic/termbits.h
index 11bb142ba844..6219803d6f4d 100644
--- a/include/uapi/asm-generic/termbits.h
+++ b/include/uapi/asm-generic/termbits.h
@@ -181,6 +181,7 @@ struct ktermios {
 #define PENDIN	0040000
 #define IEXTEN	0100000
 #define EXTPROC	0200000
+#define NOKERNINFO 0400000
 
 /* tcflow() and TCXONC use these */
 #define	TCOOFF		0
-- 
2.20.1

[PATCH 6/7] n_tty: ->ops->write: Cut core logic out to a separate function

[Arseny Maslennikov]

To simplify internal re-use of the line discipline's write method,
we isolate the work it does to its own function.

Since in-kernel callers might not refer to the tty through a file,
the struct file* argument might make no sense, so we also stop
tty_io_nonblock() from dereferencing file too early, allowing
to pass NULL as the referring file here.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 drivers/tty/n_tty.c | 33 ++++++++++++++++++++++-----------
 include/linux/tty.h |  2 +-
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 29f33798a6cd..7d7d84adcffe 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2309,22 +2309,15 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
  *		  lock themselves)
  */
 
-static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
-			   const unsigned char *buf, size_t nr)
+static ssize_t do_n_tty_write(struct tty_struct *tty, struct file *file,
+			      const unsigned char *buf, size_t nr)
 {
 	const unsigned char *b = buf;
 	DEFINE_WAIT_FUNC(wait, woken_wake_function);
 	int c;
 	ssize_t retval = 0;
 
-	/* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
-	if (L_TOSTOP(tty) && file->f_op->write != redirected_tty_write) {
-		retval = tty_check_change(tty);
-		if (retval)
-			return retval;
-	}
-
-	down_read(&tty->termios_rwsem);
+	lockdep_assert_held_read(&tty->termios_rwsem);
 
 	/* Write out any echoed characters that are still pending */
 	process_echoes(tty);
@@ -2392,10 +2385,28 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
 	remove_wait_queue(&tty->write_wait, &wait);
 	if (nr && tty->fasync)
 		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	up_read(&tty->termios_rwsem);
 	return (b - buf) ? b - buf : retval;
 }
 
+static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
+			   const unsigned char *buf, size_t nr)
+{
+	ssize_t retval = 0;
+
+	/* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
+	if (L_TOSTOP(tty) && file->f_op->write != redirected_tty_write) {
+		retval = tty_check_change(tty);
+		if (retval)
+			return retval;
+	}
+
+	down_read(&tty->termios_rwsem);
+	retval = do_n_tty_write(tty, file, buf, nr);
+	up_read(&tty->termios_rwsem);
+
+	return retval;
+}
+
 /**
  *	n_tty_poll		-	poll method for N_TTY
  *	@tty: terminal device
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 07189f18f93d..ddb97704956d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -388,7 +388,7 @@ static inline void tty_set_flow_change(struct tty_struct *tty, int val)
 
 static inline bool tty_io_nonblock(struct tty_struct *tty, struct file *file)
 {
-	return file->f_flags & O_NONBLOCK ||
+	return (file && file->f_flags & O_NONBLOCK) ||
 		test_bit(TTY_LDISC_CHANGING, &tty->flags);
 }
 
-- 
2.20.1

[PATCH 7/7] n_tty: Provide an informational line on VSTATUS receipt

[Arseny Maslennikov]

If the three termios local flags isig, icanon, iexten are enabled
and the local flag nokerninfo is disabled for a tty governed
by the n_tty line discipline, then on receiving the keyboard status
character n_tty will generate a status message and write it out to
the tty before sending SIGINFO to the tty's foreground process group.

This kerninfo line contains information about the current system load
as well as some properties of "the most interesting" process in the
tty's current foreground process group, namely:
 - its PID as seen inside its deepest PID namespace;
   * the whole process group ought to be in a single PID namespace,
     so this is actually deterministic
 - its saved command name truncated to 16 bytes (task_struct::comm);
   * at the time of writing TASK_COMM_LEN == 16
 - its state and some related bits, procps-style;
 - for S and D: its symbolic wait channel, if available; or a short
   description for other process states instead;
 - its user, system and real rusage time values;
 - its resident set size (as well as the high watermark) in kilobytes.

The "most interesting" process is chosen as follows:
 - runnables over everything
 - uninterruptibles over everything else
 - among 2 runnables pick the biggest utime + stime
 - any unresolved ties are decided in favour of greatest PID.

While the kerninfo line is not very useful for debugging the kernel
itself, since we have much more powerful debugging tools, it still gives
the user behind the terminal some meaningful feedback to a VSTATUS that
works even if no processes respond.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
---
 drivers/tty/Makefile       |   3 +-
 drivers/tty/n_tty.c        |  22 +++
 drivers/tty/n_tty_status.c | 354 +++++++++++++++++++++++++++++++++++++
 include/linux/sched.h      |   7 +
 include/linux/tty.h        |   3 +
 5 files changed, 388 insertions(+), 1 deletion(-)
 create mode 100644 drivers/tty/n_tty_status.c

diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index 020b1cd9294f..8bb5efb40b81 100644
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -1,7 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
-obj-$(CONFIG_TTY)		+= tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \
+obj-$(CONFIG_TTY)		+= tty_io.o tty_ioctl.o tty_ldisc.o \
 				   tty_buffer.o tty_port.o tty_mutex.o \
 				   tty_ldsem.o tty_baudrate.o tty_jobctrl.o \
+				   n_tty.o n_tty_status.o \
 				   n_null.o
 obj-$(CONFIG_LEGACY_PTYS)	+= pty.o
 obj-$(CONFIG_UNIX98_PTYS)	+= pty.o
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 7d7d84adcffe..74e6b4bf86bd 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -34,6 +34,7 @@
 #include <linux/signal.h>
 #include <linux/fcntl.h>
 #include <linux/sched.h>
+#include <linux/kallsyms.h>
 #include <linux/interrupt.h>
 #include <linux/tty.h>
 #include <linux/timer.h>
@@ -79,6 +80,8 @@
 #define ECHO_BLOCK		256
 #define ECHO_DISCARD_WATERMARK	N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)
 
+#define STATUS_LINE_LEN (2 * KSYM_NAME_LEN)
+
 #define SIG_FLUSHING_MASK ( \
 	rt_sigmask(SIGINT) | rt_sigmask(SIGQUIT) | \
 	rt_sigmask(SIGTSTP)			 )
@@ -158,6 +161,8 @@ static inline unsigned char *echo_buf_addr(struct n_tty_data *ldata, size_t i)
 	return &ldata->echo_buf[i & (N_TTY_BUF_SIZE - 1)];
 }
 
+static void n_tty_status_line(struct tty_struct *tty);
+
 /* If we are not echoing the data, perhaps this is a secret so erase it */
 static void zero_buffer(struct tty_struct *tty, u8 *buffer, int size)
 {
@@ -1300,6 +1305,8 @@ n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
 			n_tty_receive_signal_char(tty, SIGTSTP, c);
 			return 0;
 		} else if (c == STATUS_CHAR(tty)) {
+			if (!L_NOKERNINFO(tty))
+				n_tty_status_line(tty);
 			n_tty_receive_signal_char(tty, SIGINFO, c);
 			return 0;
 		}
@@ -2489,6 +2496,21 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
 	}
 }
 
+static void n_tty_status_line(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	char *msg, *buf;
+	msg = buf = kzalloc(STATUS_LINE_LEN, GFP_KERNEL);
+	tty_sprint_status_line(tty, buf + 1, STATUS_LINE_LEN - 1);
+	/* The only current caller of this takes output_lock for us. */
+	if (ldata->column != 0)
+		*msg = '\n';
+	else
+		msg++;
+	do_n_tty_write(tty, NULL, msg, strlen(msg));
+	kfree(buf);
+}
+
 static struct tty_ldisc_ops n_tty_ops = {
 	.magic           = TTY_LDISC_MAGIC,
 	.name            = "n_tty",
diff --git a/drivers/tty/n_tty_status.c b/drivers/tty/n_tty_status.c
new file mode 100644
index 000000000000..818caf04ad82
--- /dev/null
+++ b/drivers/tty/n_tty_status.c
@@ -0,0 +1,354 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* TODO:
+ *
+ * - Should we care about TIDs in the same thread group?
+ */
+
+#include <linux/kallsyms.h>
+#include <linux/pid.h>
+#include <linux/sched.h>
+#include <linux/sched/signal.h>
+#include <linux/sched/loadavg.h>
+#include <linux/sched/cputime.h>
+#include <linux/sched/mm.h>
+#include <linux/slab.h>
+#include <linux/tty.h>
+
+#define BCOMPARE(lbool, rbool) ((lbool) << 1 | (rbool))
+#define BCOMPARE_NONE 0
+#define BCOMPARE_RIGHT 1
+#define BCOMPARE_LEFT 2
+#define BCOMPARE_BOTH (BCOMPARE_LEFT | BCOMPARE_RIGHT)
+
+/*
+ * Select the most interesting task of two.
+ *
+ * The implemented approach is simple for now:
+ * - pick runnable
+ * - if no runnables, pick uninterruptible
+ * - if tie between runnables, pick highest utime + stime
+ * - if a tie is not broken by the above, pick highest pid nr.
+ *
+ * For reference, here's the one used in FreeBSD:
+ * - pick runnables over anything
+ * - if both runnables, pick highest cpu utilization
+ * - if no runnables, pick shortest sleep time (the scheduler
+ *   actually takes care to maintain this statistic)
+ * - other ties are decided in favour of youngest process.
+ */
+static struct task_struct *__better_proc_R(struct task_struct *a,
+		struct task_struct *b)
+{
+	unsigned long flags;
+	u64 atime, btime, tgutime, tgstime;
+	struct task_struct *ret;
+
+	if (!lock_task_sighand(a, &flags))
+		goto out_a_unlocked;
+	thread_group_cputime_adjusted(a, &tgutime, &tgstime);
+	atime = tgutime + tgstime;
+	unlock_task_sighand(a, &flags);
+
+	if (!lock_task_sighand(b, &flags))
+		goto out_b_unlocked;
+	thread_group_cputime_adjusted(b, &tgutime, &tgstime);
+	btime = tgutime + tgstime;
+	unlock_task_sighand(b, &flags);
+
+	ret = atime > btime ? a : b;
+
+	return ret;
+
+out_b_unlocked:
+out_a_unlocked:
+	return task_pid_vinr(a) > task_pid_vinr(b) ? a : b;
+}
+
+static struct task_struct *__better_proc(struct task_struct *a,
+		struct task_struct *b)
+{
+	if (!pid_alive(a))
+		return b;
+	if (!pid_alive(b))
+		return a;
+
+	switch (BCOMPARE(a->state == TASK_RUNNING,
+			b->state == TASK_RUNNING)) {
+	case BCOMPARE_LEFT:
+		return a;
+	case BCOMPARE_RIGHT:
+		return b;
+	case BCOMPARE_BOTH:
+		return __better_proc_R(a, b);
+	}
+
+	switch (BCOMPARE(a->state == TASK_UNINTERRUPTIBLE,
+			b->state == TASK_UNINTERRUPTIBLE)) {
+	case BCOMPARE_LEFT:
+		return a;
+	case BCOMPARE_RIGHT:
+		return b;
+	case BCOMPARE_BOTH:
+		break;
+	}
+
+	/* TODO: Perhaps we should check something else... */
+	return task_pid_vinr(a) > task_pid_vinr(b) ? a : b;
+}
+
+/*
+ * Weed out NULLs.
+ * a and b are pointers to (struct task_struct); therefore no problem
+ * with triple evaluation.
+ */
+#define better_proc(a, b) ((a) ? ((b) ? __better_proc((a), (b)) : (a)) : (b))
+
+static int scnprint_load(char *msgp, size_t size)
+{
+	unsigned long la[3];
+
+	get_avenrun(la, FIXED_1/200, 0);
+	return scnprintf(msgp, size, "load: %lu.%02lu; ",
+			LOAD_INT(la[0]), LOAD_FRAC(la[0]));
+}
+
+static int scnprint_task(char *msgp, size_t size, struct task_struct *task)
+{
+	char commname[TASK_COMM_LEN];
+
+	get_task_comm(commname, task);
+	return scnprintf(msgp, size, "%d/%s:", task_pid_vinr(task), commname);
+}
+
+static int scnprint_rusage(char *msgp, ssize_t size,
+		struct task_struct *task, struct mm_struct *mm)
+{
+	struct rusage ru;
+	struct timeval utime, stime;
+	struct timespec rtime;
+	u64 now;
+	int ret = 0;
+	int psz = 0;
+
+	getrusage(task, RUSAGE_BOTH, &ru);
+	now = ktime_get_ns();
+
+	utime = ru.ru_utime;
+	stime = ru.ru_stime;
+	rtime.tv_nsec = now - task->start_time;
+	rtime.tv_sec = rtime.tv_nsec / 1000000000;
+	rtime.tv_nsec %= 1000000000;
+
+	psz = scnprintf(msgp, size,
+			"%lu.%03lur %lu.%03luu %lu.%03lus",
+			rtime.tv_sec, rtime.tv_nsec / 1000000,
+			utime.tv_sec, utime.tv_usec / 1000,
+			stime.tv_sec, stime.tv_usec / 1000);
+	ret += psz;
+	msgp += psz;
+	size -= psz;
+
+	if (mm) {
+		psz = scnprintf(msgp, size,
+				" %luk/%luk",
+				get_mm_rss(mm) * PAGE_SIZE / 1024,
+				get_mm_hiwater_rss(mm) * PAGE_SIZE / 1024);
+		ret += psz;
+	}
+	return ret;
+}
+
+static int scnprint_state(char *msgp, ssize_t size,
+		struct task_struct *task, struct mm_struct *mm)
+{
+	char stat[8] = {0};
+	const char *state_descr = "";
+	struct task_struct *parent = NULL;
+	struct task_struct *real_parent = NULL;
+	unsigned long wchan = 0;
+	int psz = 0;
+	char symname[KSYM_NAME_LEN];
+
+	stat[psz++] = task_state_to_char(task);
+	if (task_nice(task) < 0)
+		stat[psz++] = '<';
+	else if (task_nice(task) > 0)
+		stat[psz++] = 'N';
+	if (mm && mm->locked_vm)
+		stat[psz++] = 'L';
+	if (get_nr_threads(task) > 1)
+		stat[psz++] = 'l';
+
+	switch (stat[0]) {
+	case 'R':
+		if (task_curr(task))
+			stat[psz++] = '!';
+		break;
+	case 'S':
+	case 'D':
+		wchan = get_wchan(task);
+		if (!wchan)
+			break;
+		if (!lookup_symbol_name(wchan, symname))
+			state_descr = symname;
+		else
+			state_descr = "*unknown*";
+		break;
+	case 'T':
+		state_descr = "stopped";
+		break;
+	case 't':
+		state_descr = "traced";
+		break;
+	case 'Z':
+		rcu_read_lock();
+		real_parent = rcu_dereference(task->real_parent);
+		parent = rcu_dereference(task->parent);
+		psz = sprintf(symname, "zombie; ppid=%d",
+			task_tgid_nr_ns(real_parent,
+				ns_of_pid(task_pid(task))));
+		if (parent != real_parent)
+			sprintf(symname + psz, " reaper=%d",
+				task_tgid_nr_ns(parent,
+					ns_of_pid(task_pid(task))));
+		rcu_read_unlock();
+		state_descr = symname;
+		break;
+	case 'I':
+		/* Can this even happen? */
+		state_descr = "idle";
+		break;
+	default:
+		state_descr = "unknown";
+	}
+
+	psz = scnprintf(msgp, size, "%s", stat);
+	msgp += psz;
+	size -= psz;
+	if (*state_descr)
+		psz += scnprintf(msgp, size, wchan ? " [%s]" : " (%s)", state_descr);
+
+	return psz;
+}
+
+/**
+ *	tty_sprint_status_line	-		produce kerninfo line
+ *	@tty: terminal device
+ *	@msg: preallocated memory buffer
+ *	@length: maximum line length
+ *
+ *	Reports state of foreground process group in a null-terminated string
+ *	located at @msg, @length bytes long. If @length is insufficient,
+ *	the line gets truncated.
+ */
+void tty_sprint_status_line(struct tty_struct *t, char *msg, size_t length)
+{
+	struct task_struct *tsk = NULL, *mit = NULL;
+	struct mm_struct *mm;
+	struct pid *pgrp = NULL;
+	char *msgp = msg;
+	int psz = 0;
+
+	if (!length)
+		return;
+	length--; /* Make room for trailing '\n' */
+
+	psz = scnprint_load(msgp, length);
+	if (psz > 0) {
+		msgp += psz;
+		length -= psz;
+	}
+	if (!length)
+		goto finalize_message;
+
+	/* Not sure if session pid is protected by ctrl_lock
+	 * or tasklist_lock...
+	 */
+	pgrp = t->session;
+	if (pgrp == NULL) {
+		psz = scnprintf(msgp, length, "not a controlling tty");
+		if (psz > 0)
+			msgp += psz;
+		goto finalize_message;
+	}
+	pgrp = tty_get_pgrp(t);
+	if (pgrp == NULL) {
+		psz = scnprintf(msgp, length, "no foreground process group");
+		if (psz > 0)
+			msgp += psz;
+		goto finalize_message;
+	}
+	/* empty foreground pgid check? */
+
+	/* If TTYSL_ENUMERATE_PGRP is defined,
+	 * just enumerate all processes in foreground process group
+	 * for now for debugging purposes.
+	 */
+	read_lock(&tasklist_lock);
+	do_each_pid_task(pgrp, PIDTYPE_PGID, tsk)
+	{
+		/* Select the most interesting task. */
+		if (tsk == better_proc(mit, tsk))
+			mit = tsk;
+#ifdef TTYSL_ENUMERATE_PGRP
+		psz = sprintf(msgp, "%d, ", task_pid_vinr(tsk));
+		if (psz > 0)
+			msgp += psz;
+#endif
+	} while_each_pid_task(pgrp, PIDTYPE_PGID, tsk);
+	read_unlock(&tasklist_lock);
+#ifdef TTYSL_ENUMERATE_PGRP
+	msgp -= 2;
+	*msgp++ = ';';
+	*msgp++ = ' ';
+#endif
+
+	if (!pid_alive(mit))
+		goto finalize_message;
+
+	/* Gather intel on most interesting task. */
+	/* Can the mm of a foreground process turn out to be NULL?
+	 * Definitely; for example, if it is a zombie.
+	 */
+	mm = get_task_mm(mit);
+
+	psz = scnprint_task(msgp, length, mit);
+	if (psz > 0) {
+		msgp += psz;
+		length -= psz;
+	}
+	if (!length)
+		goto finalize_message;
+	*msgp++ = ' ';
+	length--;
+
+	psz = scnprint_state(msgp, length, mit, mm);
+	if (psz > 0) {
+		msgp += psz;
+		length -= psz;
+	}
+	if (!length)
+		goto finalize_message;
+	*msgp++ = ' ';
+	length--;
+
+	psz = scnprint_rusage(msgp, length, mit, mm);
+	if (psz > 0) {
+		msgp += psz;
+		length -= psz;
+	}
+	if (!length)
+		goto finalize_message;
+	*msgp++ = ' ';
+	length--;
+
+	if (!mm)
+		goto finalize_message;
+
+	mmput(mm);
+
+finalize_message:
+	*msgp++ = '\n';
+	if (pgrp)
+		put_pid(pgrp);
+}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 11837410690f..21bcb7fb0888 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1229,6 +1229,8 @@ static inline struct pid *task_pid(struct task_struct *task)
  * task_xid_nr()     : global id, i.e. the id seen from the init namespace;
  * task_xid_vnr()    : virtual id, i.e. the id seen from the pid namespace of
  *                     current.
+ * task_xid_vinr()   : virtual inner id, i.e. the id seen from the namespace of
+ *                     the task itself;
  * task_xid_nr_ns()  : id seen from the ns specified;
  *
  * see also pid_nr() etc in include/linux/pid.h
@@ -1250,6 +1252,11 @@ static inline pid_t task_pid_vnr(struct task_struct *tsk)
 	return __task_pid_nr_ns(tsk, PIDTYPE_PID, NULL);
 }
 
+static inline pid_t task_pid_vinr(struct task_struct *tsk)
+{
+	return __task_pid_nr_ns(tsk, PIDTYPE_PID, ns_of_pid(task_pid(tsk)));
+}
+
 
 static inline pid_t task_tgid_nr(struct task_struct *tsk)
 {
diff --git a/include/linux/tty.h b/include/linux/tty.h
index ddb97704956d..a8e4df6d64b4 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -725,6 +725,9 @@ extern void __init n_tty_init(void);
 static inline void n_tty_init(void) { }
 #endif
 
+/* n_tty_status.c */
+extern void tty_sprint_status_line(struct tty_struct *tty, char *msg, size_t size);
+
 /* tty_audit.c */
 #ifdef CONFIG_AUDIT
 extern void tty_audit_add_data(struct tty_struct *tty, const void *data,
-- 
2.20.1

Re: [PATCH 0/7] TTY Keyboard Status Request

[Pavel Machek]

Hi!

> This patch series introduces TTY keyboard status request, a feature of
> the n_tty line discipline that reserves a character in struct termios
> (^T by default) and reacts to it by printing a short informational line
> to the terminal and sending a Unix signal to the tty's foreground
> process group. The processes may, in response to the signal, output a
> textual description of what they're doing.
> 
> The feature has been present in a similar form at least in
> Free/Open/NetBSD; it would be nice to have something like this in Linux
> as well. There is an LKML thread[1] where users have previously
> expressed the rationale for this.
> 
> The current implementation does not break existing kernel API in any
> way, since, fortunately, all the architectures supported by the kernel
> happen to have at least 1 free byte in the termios control character
> array.

I like the idea... I was often wondering "how long will this dd take". (And in
case of dd, SIGUSR1 does the job).

I assume this will off by default, so that applications using ^T today will not
get surprise signals?
									Pavel

Re: [PATCH 0/7] TTY Keyboard Status Request

[Arseny Maslennikov]

[-- Attachment #1: Type: text/plain, Size: 1652 bytes --]

On Sun, Jun 09, 2019 at 07:41:39PM +0200, Pavel Machek wrote:
> Hi!
> 
> > This patch series introduces TTY keyboard status request, a feature of
> > the n_tty line discipline that reserves a character in struct termios
> > (^T by default) and reacts to it by printing a short informational line
> > to the terminal and sending a Unix signal to the tty's foreground
> > process group. The processes may, in response to the signal, output a
> > textual description of what they're doing.
> > 
> > The feature has been present in a similar form at least in
> > Free/Open/NetBSD; it would be nice to have something like this in Linux
> > as well. There is an LKML thread[1] where users have previously
> > expressed the rationale for this.
> > 
> > The current implementation does not break existing kernel API in any
> > way, since, fortunately, all the architectures supported by the kernel
> > happen to have at least 1 free byte in the termios control character
> > array.
> 
> I like the idea... I was often wondering "how long will this dd take". (And in
> case of dd, SIGUSR1 does the job).
> 
> I assume this will off by default, so that applications using ^T today will not
> get surprise signals?
> 									Pavel

If any of isig, icanon and iexten is disabled on the tty, the signal is
not sent.

Any application that wants to handle raw terminal input events itself,
e.g. vim, mutt, libreadline, anything ncurses-based, etc., has to turn
off the tty's cooked mode, i.e. at least icanon. This means those
applications are unaffected.

The commit messages of individual patches try to explain this in detail.

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Re: [PATCH 0/7] TTY Keyboard Status Request

[Pavel Machek]

[-- Attachment #1: Type: text/plain, Size: 1941 bytes --]

Hi!

> > > This patch series introduces TTY keyboard status request, a feature of
> > > the n_tty line discipline that reserves a character in struct termios
> > > (^T by default) and reacts to it by printing a short informational line
> > > to the terminal and sending a Unix signal to the tty's foreground
> > > process group. The processes may, in response to the signal, output a
> > > textual description of what they're doing.
> > > 
> > > The feature has been present in a similar form at least in
> > > Free/Open/NetBSD; it would be nice to have something like this in Linux
> > > as well. There is an LKML thread[1] where users have previously
> > > expressed the rationale for this.
> > > 
> > > The current implementation does not break existing kernel API in any
> > > way, since, fortunately, all the architectures supported by the kernel
> > > happen to have at least 1 free byte in the termios control character
> > > array.
> > 
> > I like the idea... I was often wondering "how long will this dd take". (And in
> > case of dd, SIGUSR1 does the job).
> > 
> > I assume this will off by default, so that applications using ^T today will not
> > get surprise signals?
> 
> If any of isig, icanon and iexten is disabled on the tty, the signal is
> not sent.

As expected.

> Any application that wants to handle raw terminal input events itself,
> e.g. vim, mutt, libreadline, anything ncurses-based, etc., has to turn
> off the tty's cooked mode, i.e. at least icanon. This means those
> applications are unaffected.

Agreed, those are unaffected.

But if I have an application doing read() from console (without
manipulating tty), am I going to get surprise signal when user types
^T?

	     	      	      	     	      	     	       	     Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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Re: [PATCH 0/7] TTY Keyboard Status Request

[Arseny Maslennikov]

[-- Attachment #1: Type: text/plain, Size: 2422 bytes --]

On Sun, Jun 09, 2019 at 09:51:32PM +0200, Pavel Machek wrote:
> Hi!
> 
> > > > This patch series introduces TTY keyboard status request, a feature of
> > > > the n_tty line discipline that reserves a character in struct termios
> > > > (^T by default) and reacts to it by printing a short informational line
> > > > to the terminal and sending a Unix signal to the tty's foreground
> > > > process group. The processes may, in response to the signal, output a
> > > > textual description of what they're doing.
> > > > 
> > > > The feature has been present in a similar form at least in
> > > > Free/Open/NetBSD; it would be nice to have something like this in Linux
> > > > as well. There is an LKML thread[1] where users have previously
> > > > expressed the rationale for this.
> > > > 
> > > > The current implementation does not break existing kernel API in any
> > > > way, since, fortunately, all the architectures supported by the kernel
> > > > happen to have at least 1 free byte in the termios control character
> > > > array.
> > > 
> > > I like the idea... I was often wondering "how long will this dd take". (And in
> > > case of dd, SIGUSR1 does the job).
> > > 
> > > I assume this will off by default, so that applications using ^T today will not
> > > get surprise signals?
> > 
> > If any of isig, icanon and iexten is disabled on the tty, the signal is
> > not sent.
> 
> As expected.
> 
> > Any application that wants to handle raw terminal input events itself,
> > e.g. vim, mutt, libreadline, anything ncurses-based, etc., has to turn
> > off the tty's cooked mode, i.e. at least icanon. This means those
> > applications are unaffected.
> 
> Agreed, those are unaffected.
> 
> But if I have an application doing read() from console (without
> manipulating tty), am I going to get surprise signal when user types
> ^T?
> 
> 	     	      	      	     	      	     	       	     Pavel

As of now, that application will indeed receive a signal that is
guaranteed to be ignored by default.

This is similar to SIGWINCH, which is default-ignored as well: if the
terminal width/height changes (like when a terminal emulator window is
resized), its foreground pgrp gets a surprise signal as well, and the
processes that don't care about WINCH (and thus have default
disposition) do not get confused.
E.g. 'strace cat' demonstrates this quite clearly.


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Re: [PATCH 0/7] TTY Keyboard Status Request

[Rob Landley]

On 6/5/19 3:18 AM, Arseny Maslennikov wrote:
> This patch series introduces TTY keyboard status request, a feature of
> the n_tty line discipline that reserves a character in struct termios
> (^T by default) and reacts to it by printing a short informational line
> to the terminal and sending a Unix signal to the tty's foreground
> process group. The processes may, in response to the signal, output a
> textual description of what they're doing.

I had a long twitter thread about this with some BSD developers,
https://twitter.com/landley/status/1127148250430152704
asked on the toybox list for opinions,
http://lists.landley.net/pipermail/toybox-landley.net/2019-May/010461.html
and became aware of this patch set when the android bionic maintainer pointed me
at news coverage of it
http://lists.landley.net/pipermail/toybox-landley.net/2019-June/010536.html

So there would appear to at least be interest in the concept.

(The conclusion I came to looking at it last month is is it can't be done
without kernel support, but if such support _does_ arrive I want to add it to
toybox.)

Rob

Re: [PATCH 1/7] signal.h: Define SIGINFO on all architectures

[Rob Landley]

On 6/5/19 3:19 AM, Arseny Maslennikov wrote:
> This complementary patch defines SIGINFO as a synonym for SIGPWR
> on every architecture supported by the kernel.
> The particular signal number chosen does not really matter and is only
> required for the related tty functionality to work properly,
> so if it does not suite expectations, any suggestions are warmly
> welcome.

This was the problem I saw last month: 32 bits worth of signal numbers already
defined, gotta alias something.

> SIGPWR looks like a nice candidate for this role, because it is
> defined on every supported arch; it is currently only used to inform
> PID 1 of power failures, and daemons that care about low-level
> events do not tend to have a controlling terminal.

/dev/console isn't a controlling tty so ctrl-T wouldn't send SIGPWR to PID 1 anyway.

> However, on sparcs SIGPWR is a synonym for SIGLOST, a signal unique
> to that architecture, with a narrow set of intended uses that do not
> combine well with interactively requesting status.
> SIGLOST is not used by any kernel code at the moment.
> I'm not sure there is a more reasonable alternative right now.

The fact it's already _been_ aliased once says it's a good candidate for it. The
easy solution is don't support SIGINFO on sparc until the sparc guys figure out
what to do there and add sparc support in a follow-up patch.

Rob

Re: [PATCH 0/7] TTY Keyboard Status Request

[Rob Landley]

On 6/9/19 3:56 PM, Arseny Maslennikov wrote:
> This is similar to SIGWINCH, which is default-ignored as well: if the
> terminal width/height changes (like when a terminal emulator window is
> resized), its foreground pgrp gets a surprise signal as well, and the
> processes that don't care about WINCH (and thus have default
> disposition) do not get confused.
> E.g. 'strace cat' demonstrates this quite clearly.

Once upon a time suspending pipelines with ctrl-z broke stuff all the time due
to zero length reads being interpreted as EOF. These days I don't see that so
much anymore, I think SA_RESTART is the default now?

Rob

Re: [PATCH 1/7] signal.h: Define SIGINFO on all architectures

[Eric W. Biederman]

Arseny Maslennikov <ar@cs.msu.ru> writes:

> This complementary patch defines SIGINFO as a synonym for SIGPWR
> on every architecture supported by the kernel.
> The particular signal number chosen does not really matter and is only
> required for the related tty functionality to work properly,
> so if it does not suite expectations, any suggestions are warmly
> welcome.
>
> SIGPWR looks like a nice candidate for this role, because it is
> defined on every supported arch; it is currently only used to inform
> PID 1 of power failures, and daemons that care about low-level
> events do not tend to have a controlling terminal.
>
> However, on sparcs SIGPWR is a synonym for SIGLOST, a signal unique
> to that architecture, with a narrow set of intended uses that do not
> combine well with interactively requesting status.
> SIGLOST is not used by any kernel code at the moment.
> I'm not sure there is a more reasonable alternative right now.

Is the name SIGINFO already well established.

It just is a little bit confusing with struct siginfo.

At least on x86 it looks like we have signals 32 and 33 that are
reserved and not used for anything.  Is there a reason you have
not picked one of those?

Also should this be a realtime signal with signal information
or a non-realtime signal?

I don't expect there is much to encode except that the user is asking
for information.  I half wonder if it could be done as a different
si_code to SIGWINCH.  But of course that doesn't work because it is
not a real time signal so does not queue more than one siginfo. (Sigh).

I just would like to see that we have a clear concept of how this new
signal plays into all of the signal handling bits.

Added linux-api because this is fundamentally extending the linux-api,
and we probably want man-page updates etc.

Eric

>
> Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
> ---
>  arch/arm/include/uapi/asm/signal.h     | 1 +
>  arch/h8300/include/uapi/asm/signal.h   | 1 +
>  arch/ia64/include/uapi/asm/signal.h    | 1 +
>  arch/m68k/include/uapi/asm/signal.h    | 1 +
>  arch/mips/include/uapi/asm/signal.h    | 1 +
>  arch/parisc/include/uapi/asm/signal.h  | 1 +
>  arch/powerpc/include/uapi/asm/signal.h | 1 +
>  arch/s390/include/uapi/asm/signal.h    | 1 +
>  arch/sparc/include/uapi/asm/signal.h   | 2 ++
>  arch/x86/include/uapi/asm/signal.h     | 1 +
>  arch/xtensa/include/uapi/asm/signal.h  | 1 +
>  include/uapi/asm-generic/signal.h      | 1 +
>  12 files changed, 13 insertions(+)
>
> diff --git a/arch/arm/include/uapi/asm/signal.h b/arch/arm/include/uapi/asm/signal.h
> index 9b4185ba4f8a..b80b53a17267 100644
> --- a/arch/arm/include/uapi/asm/signal.h
> +++ b/arch/arm/include/uapi/asm/signal.h
> @@ -50,6 +50,7 @@ typedef unsigned long sigset_t;
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31
>  
> diff --git a/arch/h8300/include/uapi/asm/signal.h b/arch/h8300/include/uapi/asm/signal.h
> index e15521037348..7a2b783af22b 100644
> --- a/arch/h8300/include/uapi/asm/signal.h
> +++ b/arch/h8300/include/uapi/asm/signal.h
> @@ -50,6 +50,7 @@ typedef unsigned long sigset_t;
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31
>  
> diff --git a/arch/ia64/include/uapi/asm/signal.h b/arch/ia64/include/uapi/asm/signal.h
> index aa98ff1b9e22..b4c98cb17165 100644
> --- a/arch/ia64/include/uapi/asm/signal.h
> +++ b/arch/ia64/include/uapi/asm/signal.h
> @@ -45,6 +45,7 @@
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  /* signal 31 is no longer "unused", but the SIGUNUSED macro remains for backwards compatibility */
>  #define	SIGUNUSED	31
> diff --git a/arch/m68k/include/uapi/asm/signal.h b/arch/m68k/include/uapi/asm/signal.h
> index 915cc755a184..a0b4e4108cb8 100644
> --- a/arch/m68k/include/uapi/asm/signal.h
> +++ b/arch/m68k/include/uapi/asm/signal.h
> @@ -50,6 +50,7 @@ typedef unsigned long sigset_t;
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31
>  
> diff --git a/arch/mips/include/uapi/asm/signal.h b/arch/mips/include/uapi/asm/signal.h
> index 53104b10aae2..975a6f0d3b0b 100644
> --- a/arch/mips/include/uapi/asm/signal.h
> +++ b/arch/mips/include/uapi/asm/signal.h
> @@ -43,6 +43,7 @@ typedef unsigned long old_sigset_t;		/* at least 32 bits */
>  #define SIGCHLD		18	/* Child status has changed (POSIX).  */
>  #define SIGCLD		SIGCHLD /* Same as SIGCHLD (System V).	*/
>  #define SIGPWR		19	/* Power failure restart (System V).  */
> +#define SIGINFO		SIGPWR	/* Keyboard status request (4.2 BSD). */
>  #define SIGWINCH	20	/* Window size change (4.3 BSD, Sun).  */
>  #define SIGURG		21	/* Urgent condition on socket (4.2 BSD).  */
>  #define SIGIO		22	/* I/O now possible (4.2 BSD).	*/
> diff --git a/arch/parisc/include/uapi/asm/signal.h b/arch/parisc/include/uapi/asm/signal.h
> index d38563a394f2..fe2e00d590ac 100644
> --- a/arch/parisc/include/uapi/asm/signal.h
> +++ b/arch/parisc/include/uapi/asm/signal.h
> @@ -22,6 +22,7 @@
>  #define SIGUSR2		17
>  #define SIGCHLD		18
>  #define SIGPWR		19
> +#define SIGINFO		SIGPWR
>  #define SIGVTALRM	20
>  #define SIGPROF		21
>  #define SIGIO		22
> diff --git a/arch/powerpc/include/uapi/asm/signal.h b/arch/powerpc/include/uapi/asm/signal.h
> index 85b0a7aa43e7..e7f3885905b4 100644
> --- a/arch/powerpc/include/uapi/asm/signal.h
> +++ b/arch/powerpc/include/uapi/asm/signal.h
> @@ -53,6 +53,7 @@ typedef struct {
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31
>  
> diff --git a/arch/s390/include/uapi/asm/signal.h b/arch/s390/include/uapi/asm/signal.h
> index 9a14a611ed82..12ee62987971 100644
> --- a/arch/s390/include/uapi/asm/signal.h
> +++ b/arch/s390/include/uapi/asm/signal.h
> @@ -58,6 +58,7 @@ typedef unsigned long sigset_t;
>  #define SIGLOST         29
>  */
>  #define SIGPWR          30
> +#define SIGINFO         SIGPWR
>  #define SIGSYS		31
>  #define SIGUNUSED       31
>  
> diff --git a/arch/sparc/include/uapi/asm/signal.h b/arch/sparc/include/uapi/asm/signal.h
> index ff9505923b9a..b655163198bb 100644
> --- a/arch/sparc/include/uapi/asm/signal.h
> +++ b/arch/sparc/include/uapi/asm/signal.h
> @@ -71,6 +71,8 @@
>  #define SIGWINCH	28
>  #define SIGLOST		29
>  #define SIGPWR		SIGLOST
> +/* XXX: is it OK for SIGINFO to collide with LOST? */
> +#define SIGINFO		SIGPWR
>  #define SIGUSR1		30
>  #define SIGUSR2		31
>  
> diff --git a/arch/x86/include/uapi/asm/signal.h b/arch/x86/include/uapi/asm/signal.h
> index e5745d593dc7..1539bb28826c 100644
> --- a/arch/x86/include/uapi/asm/signal.h
> +++ b/arch/x86/include/uapi/asm/signal.h
> @@ -55,6 +55,7 @@ typedef unsigned long sigset_t;
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31
>  
> diff --git a/arch/xtensa/include/uapi/asm/signal.h b/arch/xtensa/include/uapi/asm/signal.h
> index 005dec5bfde4..d644234305de 100644
> --- a/arch/xtensa/include/uapi/asm/signal.h
> +++ b/arch/xtensa/include/uapi/asm/signal.h
> @@ -65,6 +65,7 @@ typedef struct {
>  #define SIGPOLL		SIGIO
>  /* #define SIGLOST		29 */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31
>  
> diff --git a/include/uapi/asm-generic/signal.h b/include/uapi/asm-generic/signal.h
> index 5c716a952cbe..9f9a1db0d43c 100644
> --- a/include/uapi/asm-generic/signal.h
> +++ b/include/uapi/asm-generic/signal.h
> @@ -43,6 +43,7 @@
>  #define SIGLOST		29
>  */
>  #define SIGPWR		30
> +#define SIGINFO		SIGPWR
>  #define SIGSYS		31
>  #define	SIGUNUSED	31

Re: [PATCH 1/7] signal.h: Define SIGINFO on all architectures

[Arseny Maslennikov]

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On Tue, Jun 11, 2019 at 03:36:00PM -0500, Eric W. Biederman wrote:
> Arseny Maslennikov <ar@cs.msu.ru> writes:
> 
> > This complementary patch defines SIGINFO as a synonym for SIGPWR
> > on every architecture supported by the kernel.
> > The particular signal number chosen does not really matter and is only
> > required for the related tty functionality to work properly,
> > so if it does not suite expectations, any suggestions are warmly
> > welcome.
> >
> > SIGPWR looks like a nice candidate for this role, because it is
> > defined on every supported arch; it is currently only used to inform
> > PID 1 of power failures, and daemons that care about low-level
> > events do not tend to have a controlling terminal.
> >
> > However, on sparcs SIGPWR is a synonym for SIGLOST, a signal unique
> > to that architecture, with a narrow set of intended uses that do not
> > combine well with interactively requesting status.
> > SIGLOST is not used by any kernel code at the moment.
> > I'm not sure there is a more reasonable alternative right now.
> 
> Is the name SIGINFO already well established.
> 
> It just is a little bit confusing with struct siginfo.
> 

I thought about this, but the alternatives are clunkier.
SIGKBINFO? Implies there actually is a keyboard, most ttys are
virtual/software nowadays. Misplaced emphasis.
SIGSTATUS? Too generic.
SIGTINFO? This could work, like conveys the nature and all, but many
applications already do #ifdef SIGINFO; if we reuse the name they will
suddenly just work, and this helps adoption.
SIGTINFO does decipher to "terminal info", this sounds too ambiguous.

Fortunately, from UNIX tradition signal names are defined in caps, and
given there is much more related written discussion than spoken, this
stands out enough to my taste. I'd say, it stands out a lot.

> At least on x86 it looks like we have signals 32 and 33 that are
> reserved and not used for anything.  Is there a reason you have
> not picked one of those?

Quoting signal(7) from man-pages:
       The Linux kernel supports a range of 33 different real-time
       signals, numbered 32  to  64.   However,  the  glibc  POSIX
       threads  implementation  internally  uses two (for NPTL) or
       three   (for   LinuxThreads)   real-time    signals    (see
       pthreads(7)),  and  adjusts  the value of SIGRTMIN suitably
       (to 34 or 35).

This excerpt gives the cue that signals 32 and 33 are not really unused,
and I did not want to cause further adjustments to SIGRTMIN/SIGRTMAX,
since this means we have to recompile the world with the new, changed
SIGRTMIN/MAX...

If we'd change RTMIN, this makes all the rt signal numbers shift to the
right and can cause havoc if we do not recompile the world.

We could probably go with decrementing RTMAX and reusing the value
(effectively reserving a signal number from the end), but aliasing PWR
seems much easier and less harmful, and easier for apps to adopt.
If we find a strong reason not to reuse PWR, this may be considered as a
plan B, but the idea to allocate even more precious signals seems too
wasteful to me for now.

> 
> Also should this be a realtime signal with signal information
> or a non-realtime signal?

I see no requirement for this to be a realtime signal.

Non-realtime signals like PWR or INT also have at least some signal
information which can be accessed with handlers installed with
sigaction(2). [1] has a small example.
The most obvious piece of info is that signals sent by n_tty
(si_code==SI_KERNEL) are easy to distinguish from signals sent by
processes (si_code==SI_USER)

> 
> I don't expect there is much to encode except that the user is asking
> for information.  I half wonder if it could be done as a different
> si_code to SIGWINCH.  But of course that doesn't work because it is
> not a real time signal so does not queue more than one siginfo. (Sigh).
> 

Signals are just a huge mess (not that this is news...)  :)

> I just would like to see that we have a clear concept of how this new
> signal plays into all of the signal handling bits.
> 
> Added linux-api because this is fundamentally extending the linux-api,
> and we probably want man-page updates etc.
> 
> Eric
> 

[1]https://github.com/porrided/tty-kb-status-userspace/blob/d32028056c85b279cf8d5f43478b5419d090637c/siginfo-test/siginfo.c

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Re: [PATCH 0/7] TTY Keyboard Status Request

[Pavel Machek]

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Hi!

> > > If any of isig, icanon and iexten is disabled on the tty, the signal is
> > > not sent.
> > 
> > As expected.
> > 
> > > Any application that wants to handle raw terminal input events itself,
> > > e.g. vim, mutt, libreadline, anything ncurses-based, etc., has to turn
> > > off the tty's cooked mode, i.e. at least icanon. This means those
> > > applications are unaffected.
> > 
> > Agreed, those are unaffected.
> > 
> > But if I have an application doing read() from console (without
> > manipulating tty), am I going to get surprise signal when user types
> > ^T?
> > 
> > 	     	      	      	     	      	     	       	     Pavel
> 
> As of now, that application will indeed receive a signal that is
> guaranteed to be ignored by default.
> 
> This is similar to SIGWINCH, which is default-ignored as well: if the
> terminal width/height changes (like when a terminal emulator window is
> resized), its foreground pgrp gets a surprise signal as well, and the
> processes that don't care about WINCH (and thus have default
> disposition) do not get confused.
> E.g. 'strace cat' demonstrates this quite clearly.

Ok, so this should be safe as long as noone listens for SIGPWR. So
... I guess we are ok :-).
								Pavel


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