[PATCH v2 0/2] arm64: Abstract syscallno manipulation

[PATCH v2 0/2] arm64: Abstract syscallno manipulation

[Dave Martin]

Changes since v1:

 * Change the argument of in_syscall() from int sycallno to a struct
   pt_regs *, since that's more natural for the current callsites.

 * Add missing #include of <asm/ptrace.h>.

 * Minor commit message clarifications.


Original blurb:

The upper 32 bits of the syscallno field in struct pt_regs are handled
inconsistently, being sometimes zero extended and sometimes
sign-extended.  The reason for this appears to be that they are not
intentionally significant at all.

This series attempts to clarify this situation by getting rid of the
spurious extra bits and adding a symbolic #define and helpers.

My current motivation for this series is SVE, which will makes use of
the in_syscall() condition to decide when the user SVE register state
for a thread can be discarded.


The refactoring done by this series also fixes a strange behaviour
observable in ftrace when a tracer cancels a syscall in a compat
binary.  Currently, ftrace traces the syscall number erroneously in
this case:

    cancel-sys32-2142  [001] ....  1740.510558: sys_enter: NR 4294967295 (1, 6a364, e, 0, 0, 85e)

instead of the expected:

    cancel-sys32-2142  [001] ....  1740.510558: sys_enter: NR -1 (1, 6a364, e, 0, 0, 85e)

(Native always says -1, as one might expect.)


The gdb testsuite reports no regressions resulting from this series.

For big-endian, I have only boot-tested, and checked by inspection that
the offset of syscallno in pt_regs is transformed in the correct way
(+4) compared with little-endian.


Dave Martin (2):
  arm64: syscallno is secretly an int, make it official
  arm64: Abstract syscallno manipulation

 arch/arm64/include/asm/processor.h |  2 +-
 arch/arm64/include/asm/ptrace.h    | 30 +++++++++++++++++++++++++++-
 arch/arm64/kernel/entry.S          | 40 ++++++++++++++++++--------------------
 arch/arm64/kernel/ptrace.c         |  2 +-
 arch/arm64/kernel/signal.c         | 10 +++++-----
 arch/arm64/kernel/signal32.c       |  2 +-
 arch/arm64/kernel/traps.c          |  2 +-
 7 files changed, 57 insertions(+), 31 deletions(-)

-- 
2.1.4

[PATCH v2 1/2] arm64: syscallno is secretly an int, make it official

[Dave Martin]

The upper 32 bits of the syscallno field in thread_struct are
handled inconsistently, being sometimes zero extended and sometimes
sign-extended.  In fact, only the lower 32 bits seem to have any
real significance for the behaviour of the code: it's been OK to
handle the upper bits inconsistently because they don't matter.

Currently, the only place I can find where those bits are
significant is in calling trace_sys_enter(), which may be
unintentional: for example, if a compat tracer attempts to cancel a
syscall by passing -1 to (COMPAT_)PTRACE_SET_SYSCALL at the
syscall-enter-stop, it will be traced as syscall 4294967295
rather than -1 as might be expected (and as occurs for a native
tracer doing the same thing).  Elsewhere, reads of syscallno cast
it to an int or truncate it.

There's also a conspicuous amount of code and casting to bodge
around the fact that although semantically an int, syscallno is
stored as a u64.

Let's not pretend any more.

In order to preserve the stp x instruction that stores the syscall
number in entry.S, this patch special-cases the layout of struct
pt_regs for big endian so that the newly 32-bit syscallno field
maps onto the low bits of the stored value.  This is not beautiful,
but benchmarking of the getpid syscall on Juno suggests indicates a
minor slowdown if the stp is split into an stp x and stp w.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/processor.h |  2 +-
 arch/arm64/include/asm/ptrace.h    |  9 ++++++++-
 arch/arm64/kernel/entry.S          | 34 +++++++++++++++++-----------------
 arch/arm64/kernel/ptrace.c         |  2 +-
 arch/arm64/kernel/signal.c         |  6 +++---
 arch/arm64/kernel/signal32.c       |  2 +-
 arch/arm64/kernel/traps.c          |  2 +-
 7 files changed, 32 insertions(+), 25 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 64c9e78..379def1 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -112,7 +112,7 @@ void tls_preserve_current_state(void);
 static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
 {
 	memset(regs, 0, sizeof(*regs));
-	regs->syscallno = ~0UL;
+	regs->syscallno = ~0;
 	regs->pc = pc;
 }
 
diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 11403fd..21c87dc 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -116,7 +116,14 @@ struct pt_regs {
 		};
 	};
 	u64 orig_x0;
-	u64 syscallno;
+#ifdef __AARCH64EB__
+	u32 unused2;
+	s32 syscallno;
+#else
+	s32 syscallno;
+	u32 unused2;
+#endif
+
 	u64 orig_addr_limit;
 	u64 unused;	// maintain 16 byte alignment
 };
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index b738880..3bf0bd7 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -142,8 +142,8 @@ alternative_else_nop_endif
 	 * Set syscallno to -1 by default (overridden later if real syscall).
 	 */
 	.if	\el == 0
-	mvn	x21, xzr
-	str	x21, [sp, #S_SYSCALLNO]
+	mvn	w21, wzr
+	str	w21, [sp, #S_SYSCALLNO]
 	.endif
 
 	/*
@@ -290,8 +290,9 @@ alternative_else_nop_endif
  *
  * x7 is reserved for the system call number in 32-bit mode.
  */
-sc_nr	.req	x25		// number of system calls
-scno	.req	x26		// syscall number
+wsc_nr	.req	w25		// number of system calls
+wscno	.req	w26		// syscall number
+xscno	.req	x26		// syscall number (zero-extended)
 stbl	.req	x27		// syscall table pointer
 tsk	.req	x28		// current thread_info
 
@@ -577,8 +578,8 @@ el0_svc_compat:
 	 * AArch32 syscall handling
 	 */
 	adrp	stbl, compat_sys_call_table	// load compat syscall table pointer
-	uxtw	scno, w7			// syscall number in w7 (r7)
-	mov     sc_nr, #__NR_compat_syscalls
+	mov	wscno, w7			// syscall number in w7 (r7)
+	mov     wsc_nr, #__NR_compat_syscalls
 	b	el0_svc_naked
 
 	.align	6
@@ -798,19 +799,19 @@ ENDPROC(ret_from_fork)
 	.align	6
 el0_svc:
 	adrp	stbl, sys_call_table		// load syscall table pointer
-	uxtw	scno, w8			// syscall number in w8
-	mov	sc_nr, #__NR_syscalls
+	mov	wscno, w8			// syscall number in w8
+	mov	wsc_nr, #__NR_syscalls
 el0_svc_naked:					// compat entry point
-	stp	x0, scno, [sp, #S_ORIG_X0]	// save the original x0 and syscall number
+	stp	x0, xscno, [sp, #S_ORIG_X0]	// save the original x0 and syscall number
 	enable_dbg_and_irq
 	ct_user_exit 1
 
 	ldr	x16, [tsk, #TSK_TI_FLAGS]	// check for syscall hooks
 	tst	x16, #_TIF_SYSCALL_WORK
 	b.ne	__sys_trace
-	cmp     scno, sc_nr                     // check upper syscall limit
+	cmp     wscno, wsc_nr			// check upper syscall limit
 	b.hs	ni_sys
-	ldr	x16, [stbl, scno, lsl #3]	// address in the syscall table
+	ldr	x16, [stbl, xscno, lsl #3]	// address in the syscall table
 	blr	x16				// call sys_* routine
 	b	ret_fast_syscall
 ni_sys:
@@ -824,24 +825,23 @@ ENDPROC(el0_svc)
 	 * switches, and waiting for our parent to respond.
 	 */
 __sys_trace:
-	mov	w0, #-1				// set default errno for
-	cmp     scno, x0			// user-issued syscall(-1)
+	cmp     wscno, #-1			// user-issued syscall(-1)?
 	b.ne	1f
-	mov	x0, #-ENOSYS
+	mov	x0, #-ENOSYS			// set default errno if so
 	str	x0, [sp, #S_X0]
 1:	mov	x0, sp
 	bl	syscall_trace_enter
 	cmp	w0, #-1				// skip the syscall?
 	b.eq	__sys_trace_return_skipped
-	uxtw	scno, w0			// syscall number (possibly new)
+	mov	wscno, w0			// syscall number (possibly new)
 	mov	x1, sp				// pointer to regs
-	cmp	scno, sc_nr			// check upper syscall limit
+	cmp	wscno, wsc_nr			// check upper syscall limit
 	b.hs	__ni_sys_trace
 	ldp	x0, x1, [sp]			// restore the syscall args
 	ldp	x2, x3, [sp, #S_X2]
 	ldp	x4, x5, [sp, #S_X4]
 	ldp	x6, x7, [sp, #S_X6]
-	ldr	x16, [stbl, scno, lsl #3]	// address in the syscall table
+	ldr	x16, [stbl, xscno, lsl #3]	// address in the syscall table
 	blr	x16				// call sys_* routine
 
 __sys_trace_return:
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 1b38c01..de77480 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1363,7 +1363,7 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 	if (dir == PTRACE_SYSCALL_EXIT)
 		tracehook_report_syscall_exit(regs, 0);
 	else if (tracehook_report_syscall_entry(regs))
-		regs->syscallno = ~0UL;
+		regs->syscallno = ~0;
 
 	regs->regs[regno] = saved_reg;
 }
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 089c3747..4d04b89 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -387,7 +387,7 @@ static int restore_sigframe(struct pt_regs *regs,
 	/*
 	 * Avoid sys_rt_sigreturn() restarting.
 	 */
-	regs->syscallno = ~0UL;
+	regs->syscallno = ~0;
 
 	err |= !valid_user_regs(&regs->user_regs, current);
 	if (err == 0)
@@ -673,7 +673,7 @@ static void do_signal(struct pt_regs *regs)
 {
 	unsigned long continue_addr = 0, restart_addr = 0;
 	int retval = 0;
-	int syscall = (int)regs->syscallno;
+	int syscall = regs->syscallno;
 	struct ksignal ksig;
 
 	/*
@@ -687,7 +687,7 @@ static void do_signal(struct pt_regs *regs)
 		/*
 		 * Avoid additional syscall restarting via ret_to_user.
 		 */
-		regs->syscallno = ~0UL;
+		regs->syscallno = ~0;
 
 		/*
 		 * Prepare for system call restart. We do this here so that a
diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index c747a0f..d98ca76 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -354,7 +354,7 @@ static int compat_restore_sigframe(struct pt_regs *regs,
 	/*
 	 * Avoid compat_sys_sigreturn() restarting.
 	 */
-	regs->syscallno = ~0UL;
+	regs->syscallno = ~0;
 
 	err |= !valid_user_regs(&regs->user_regs, current);
 
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index d48f470..c8ef6da 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -593,7 +593,7 @@ asmlinkage long do_ni_syscall(struct pt_regs *regs)
 
 	if (show_unhandled_signals_ratelimited()) {
 		pr_info("%s[%d]: syscall %d\n", current->comm,
-			task_pid_nr(current), (int)regs->syscallno);
+			task_pid_nr(current), regs->syscallno);
 		dump_instr("", regs);
 		if (user_mode(regs))
 			__show_regs(regs);
-- 
2.1.4

[PATCH v2 2/2] arm64: Abstract syscallno manipulation

[Dave Martin]

The -1 "no syscall" value is written in various ways, shared with
the user ABI in some places, and generally obscure.

This patch attempts to make things a little more consistent and
readable by replacing all these uses with a single #define.  A
couple of symbolic helpers are provided to clarify the intent
further.

Because the in-syscall check in do_signal() is changed from >= 0 to
!= NO_SYSCALL by this patch, different behaviour may be observable
if syscallno is set to values less than -1 by a tracer.  However,
this is not different from the behaviour that is already observable
if a tracer sets syscallno to a value >= __NR_(compat_)syscalls.

It appears that this can cause spurious syscall restarting, but
that is not a new behaviour either, and does not appear harmful.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/processor.h |  2 +-
 arch/arm64/include/asm/ptrace.h    | 21 +++++++++++++++++++++
 arch/arm64/kernel/entry.S          | 10 ++++------
 arch/arm64/kernel/ptrace.c         |  2 +-
 arch/arm64/kernel/signal.c         | 10 +++++-----
 arch/arm64/kernel/signal32.c       |  2 +-
 6 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 379def1..b7334f1 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -112,7 +112,7 @@ void tls_preserve_current_state(void);
 static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
 {
 	memset(regs, 0, sizeof(*regs));
-	regs->syscallno = ~0;
+	forget_syscall(regs);
 	regs->pc = pc;
 }
 
diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 21c87dc..4f64373 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -72,8 +72,19 @@
 #define COMPAT_PT_TEXT_ADDR		0x10000
 #define COMPAT_PT_DATA_ADDR		0x10004
 #define COMPAT_PT_TEXT_END_ADDR		0x10008
+
+/*
+ * If pt_regs.syscallno == NO_SYSCALL, then the thread is not executing
+ * a syscall -- i.e., its most recent entry into the kernel from
+ * userspace was not via SVC, or otherwise a tracer cancelled the syscall.
+ *
+ * This must have the value -1, for ABI compatibility with ptrace etc.
+ */
+#define NO_SYSCALL (-1)
+
 #ifndef __ASSEMBLY__
 #include <linux/bug.h>
+#include <linux/types.h>
 
 /* sizeof(struct user) for AArch32 */
 #define COMPAT_USER_SZ	296
@@ -128,6 +139,16 @@ struct pt_regs {
 	u64 unused;	// maintain 16 byte alignment
 };
 
+static inline bool in_syscall(struct pt_regs const *regs)
+{
+	return regs->syscallno != NO_SYSCALL;
+}
+
+static inline void forget_syscall(struct pt_regs *regs)
+{
+	regs->syscallno = NO_SYSCALL;
+}
+
 #define MAX_REG_OFFSET offsetof(struct pt_regs, pstate)
 
 #define arch_has_single_step()	(1)
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 3bf0bd7..cace76d 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -138,11 +138,9 @@ alternative_else_nop_endif
 
 	stp	x22, x23, [sp, #S_PC]
 
-	/*
-	 * Set syscallno to -1 by default (overridden later if real syscall).
-	 */
+	/* Not in a syscall by default (el0_svc overwrites for real syscall) */
 	.if	\el == 0
-	mvn	w21, wzr
+	mov	w21, #NO_SYSCALL
 	str	w21, [sp, #S_SYSCALLNO]
 	.endif
 
@@ -825,13 +823,13 @@ ENDPROC(el0_svc)
 	 * switches, and waiting for our parent to respond.
 	 */
 __sys_trace:
-	cmp     wscno, #-1			// user-issued syscall(-1)?
+	cmp     wscno, #NO_SYSCALL		// user-issued syscall(-1)?
 	b.ne	1f
 	mov	x0, #-ENOSYS			// set default errno if so
 	str	x0, [sp, #S_X0]
 1:	mov	x0, sp
 	bl	syscall_trace_enter
-	cmp	w0, #-1				// skip the syscall?
+	cmp	w0, #NO_SYSCALL			// skip the syscall?
 	b.eq	__sys_trace_return_skipped
 	mov	wscno, w0			// syscall number (possibly new)
 	mov	x1, sp				// pointer to regs
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index de77480..28619b5 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1363,7 +1363,7 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 	if (dir == PTRACE_SYSCALL_EXIT)
 		tracehook_report_syscall_exit(regs, 0);
 	else if (tracehook_report_syscall_entry(regs))
-		regs->syscallno = ~0;
+		forget_syscall(regs);
 
 	regs->regs[regno] = saved_reg;
 }
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 4d04b89..4991e87 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -36,6 +36,7 @@
 #include <asm/ucontext.h>
 #include <asm/unistd.h>
 #include <asm/fpsimd.h>
+#include <asm/ptrace.h>
 #include <asm/signal32.h>
 #include <asm/vdso.h>
 
@@ -387,7 +388,7 @@ static int restore_sigframe(struct pt_regs *regs,
 	/*
 	 * Avoid sys_rt_sigreturn() restarting.
 	 */
-	regs->syscallno = ~0;
+	forget_syscall(regs);
 
 	err |= !valid_user_regs(&regs->user_regs, current);
 	if (err == 0)
@@ -673,13 +674,12 @@ static void do_signal(struct pt_regs *regs)
 {
 	unsigned long continue_addr = 0, restart_addr = 0;
 	int retval = 0;
-	int syscall = regs->syscallno;
 	struct ksignal ksig;
 
 	/*
 	 * If we were from a system call, check for system call restarting...
 	 */
-	if (syscall >= 0) {
+	if (in_syscall(regs)) {
 		continue_addr = regs->pc;
 		restart_addr = continue_addr - (compat_thumb_mode(regs) ? 2 : 4);
 		retval = regs->regs[0];
@@ -687,7 +687,7 @@ static void do_signal(struct pt_regs *regs)
 		/*
 		 * Avoid additional syscall restarting via ret_to_user.
 		 */
-		regs->syscallno = ~0;
+		forget_syscall(regs);
 
 		/*
 		 * Prepare for system call restart. We do this here so that a
@@ -731,7 +731,7 @@ static void do_signal(struct pt_regs *regs)
 	 * Handle restarting a different system call. As above, if a debugger
 	 * has chosen to restart at a different PC, ignore the restart.
 	 */
-	if (syscall >= 0 && regs->pc == restart_addr) {
+	if (in_syscall(regs) && regs->pc == restart_addr) {
 		if (retval == -ERESTART_RESTARTBLOCK)
 			setup_restart_syscall(regs);
 		user_rewind_single_step(current);
diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index d98ca76..4e5a664 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -354,7 +354,7 @@ static int compat_restore_sigframe(struct pt_regs *regs,
 	/*
 	 * Avoid compat_sys_sigreturn() restarting.
 	 */
-	regs->syscallno = ~0;
+	forget_syscall(regs);
 
 	err |= !valid_user_regs(&regs->user_regs, current);
 
-- 
2.1.4

[PATCH v2 0/2] arm64: Abstract syscallno manipulation

[Will Deacon]

On Tue, Aug 01, 2017 at 02:54:12PM +0100, Dave Martin wrote:
> Changes since v1:
> 
>  * Change the argument of in_syscall() from int sycallno to a struct
>    pt_regs *, since that's more natural for the current callsites.
> 
>  * Add missing #include of <asm/ptrace.h>.
> 
>  * Minor commit message clarifications.

Please add my ack to these two now:

Acked-by: Will Deacon <will.deacon@arm.com>

Will