[PATCH 0/2] x86/boot/64: a couple of start_cpu() cleanups

[PATCH 0/2] x86/boot/64: a couple of start_cpu() cleanups

[Josh Poimboeuf]

Josh Poimboeuf (2):
  x86/boot/64: use 'push' instead of 'call' in start_cpu()
  x86/boot/64: push correct start_cpu() return address

 arch/x86/kernel/head_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

-- 
2.7.4

[PATCH 1/2] x86/boot/64: use 'push' instead of 'call' in start_cpu()

[Josh Poimboeuf]

start_cpu() pushes a text address on the stack so that stack traces from
idle tasks will show start_cpu() at the end.  But it uses a call
instruction to do that, which is rather obtuse.  Use a straightforward
push instead.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/kernel/head_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 90de288..1facaf4 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -298,7 +298,7 @@ ENTRY(start_cpu)
 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
 	 *		address given in m16:64.
 	 */
-	call	1f		# put return address on stack for unwinder
+	pushq	$1f		# put return address on stack for unwinder
 1:	xorq	%rbp, %rbp	# clear frame pointer
 	movq	initial_code(%rip), %rax
 	pushq	$__KERNEL_CS	# set correct cs
-- 
2.7.4

[PATCH 2/2] x86/boot/64: push correct start_cpu() return address

[Josh Poimboeuf]

start_cpu() pushes a text address on the stack so that stack traces from
idle tasks will show start_cpu() at the end.  But it currently shows the
wrong function offset.  It's more correct to show the address
immediately after the 'lretq' instruction.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/kernel/head_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 1facaf4..b467b14 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -298,12 +298,13 @@ ENTRY(start_cpu)
 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
 	 *		address given in m16:64.
 	 */
-	pushq	$1f		# put return address on stack for unwinder
-1:	xorq	%rbp, %rbp	# clear frame pointer
+	pushq	$.Lafter_lret	# put return address on stack for unwinder
+	xorq	%rbp, %rbp	# clear frame pointer
 	movq	initial_code(%rip), %rax
 	pushq	$__KERNEL_CS	# set correct cs
 	pushq	%rax		# target address in negative space
 	lretq
+.Lafter_lret:
 ENDPROC(start_cpu)
 
 #include "verify_cpu.S"
-- 
2.7.4

[tip:x86/urgent] x86/boot/64: Use 'push' instead of 'call' in start_cpu()

[tip-bot for Josh Poimboeuf]

Commit-ID:  ec2d86a9b646d93f1948569f368e2c6f5449e6c7
Gitweb:     http://git.kernel.org/tip/ec2d86a9b646d93f1948569f368e2c6f5449e6c7
Author:     Josh Poimboeuf <jpoimboe@redhat.com>
AuthorDate: Tue, 13 Dec 2016 21:25:35 -0600
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 14 Dec 2016 08:48:05 +0100

x86/boot/64: Use 'push' instead of 'call' in start_cpu()

start_cpu() pushes a text address on the stack so that stack traces from
idle tasks will show start_cpu() at the end.  But it uses a call
instruction to do that, which is rather obtuse.  Use a straightforward
push instead.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/4d8a1952759721d42d1e62ba9e4a7e3ac5df8574.1481685203.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/head_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 90de288..1facaf4 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -298,7 +298,7 @@ ENTRY(start_cpu)
 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
 	 *		address given in m16:64.
 	 */
-	call	1f		# put return address on stack for unwinder
+	pushq	$1f		# put return address on stack for unwinder
 1:	xorq	%rbp, %rbp	# clear frame pointer
 	movq	initial_code(%rip), %rax
 	pushq	$__KERNEL_CS	# set correct cs

[tip:x86/urgent] x86/boot/64: Push correct start_cpu() return address

[tip-bot for Josh Poimboeuf]

Commit-ID:  31dcfec11f827e9a5d8720fe4728f1305894884f
Gitweb:     http://git.kernel.org/tip/31dcfec11f827e9a5d8720fe4728f1305894884f
Author:     Josh Poimboeuf <jpoimboe@redhat.com>
AuthorDate: Tue, 13 Dec 2016 21:25:36 -0600
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 14 Dec 2016 08:48:05 +0100

x86/boot/64: Push correct start_cpu() return address

start_cpu() pushes a text address on the stack so that stack traces from
idle tasks will show start_cpu() at the end.  But it currently shows the
wrong function offset.  It's more correct to show the address
immediately after the 'lretq' instruction.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/2cadd9f16c77da7ee7957bfc5e1c67928c23ca48.1481685203.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/head_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 1facaf4..b467b14 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -298,12 +298,13 @@ ENTRY(start_cpu)
 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
 	 *		address given in m16:64.
 	 */
-	pushq	$1f		# put return address on stack for unwinder
-1:	xorq	%rbp, %rbp	# clear frame pointer
+	pushq	$.Lafter_lret	# put return address on stack for unwinder
+	xorq	%rbp, %rbp	# clear frame pointer
 	movq	initial_code(%rip), %rax
 	pushq	$__KERNEL_CS	# set correct cs
 	pushq	%rax		# target address in negative space
 	lretq
+.Lafter_lret:
 ENDPROC(start_cpu)
 
 #include "verify_cpu.S"

Re: [tip:x86/urgent] x86/boot/64: Use 'push' instead of 'call' in start_cpu()

[hpa]

On December 14, 2016 12:36:58 AM PST, tip-bot for Josh Poimboeuf <tipbot@zytor.com> wrote:
>Commit-ID:  ec2d86a9b646d93f1948569f368e2c6f5449e6c7
>Gitweb:    
>http://git.kernel.org/tip/ec2d86a9b646d93f1948569f368e2c6f5449e6c7
>Author:     Josh Poimboeuf <jpoimboe@redhat.com>
>AuthorDate: Tue, 13 Dec 2016 21:25:35 -0600
>Committer:  Ingo Molnar <mingo@kernel.org>
>CommitDate: Wed, 14 Dec 2016 08:48:05 +0100
>
>x86/boot/64: Use 'push' instead of 'call' in start_cpu()
>
>start_cpu() pushes a text address on the stack so that stack traces
>from
>idle tasks will show start_cpu() at the end.  But it uses a call
>instruction to do that, which is rather obtuse.  Use a straightforward
>push instead.
>
>Suggested-by: Borislav Petkov <bp@alien8.de>
>Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
>Cc: Andy Lutomirski <luto@kernel.org>
>Cc: Brian Gerst <brgerst@gmail.com>
>Cc: Denys Vlasenko <dvlasenk@redhat.com>
>Cc: H. Peter Anvin <hpa@zytor.com>
>Cc: Linus Torvalds <torvalds@linux-foundation.org>
>Cc: Peter Zijlstra <peterz@infradead.org>
>Cc: Thomas Gleixner <tglx@linutronix.de>
>Link:
>http://lkml.kernel.org/r/4d8a1952759721d42d1e62ba9e4a7e3ac5df8574.1481685203.git.jpoimboe@redhat.com
>Signed-off-by: Ingo Molnar <mingo@kernel.org>
>---
> arch/x86/kernel/head_64.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>index 90de288..1facaf4 100644
>--- a/arch/x86/kernel/head_64.S
>+++ b/arch/x86/kernel/head_64.S
>@@ -298,7 +298,7 @@ ENTRY(start_cpu)
> 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
> 	 *		address given in m16:64.
> 	 */
>-	call	1f		# put return address on stack for unwinder
>+	pushq	$1f		# put return address on stack for unwinder
> 1:	xorq	%rbp, %rbp	# clear frame pointer
> 	movq	initial_code(%rip), %rax
> 	pushq	$__KERNEL_CS	# set correct cs

This adds another relocation to the kernel.  I hope this is safe at this point in the code?
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

Re: [tip:x86/urgent] x86/boot/64: Use 'push' instead of 'call' in start_cpu()

[Josh Poimboeuf]

On Wed, Dec 14, 2016 at 11:24:19AM -0800, hpa@zytor.com wrote:
> On December 14, 2016 12:36:58 AM PST, tip-bot for Josh Poimboeuf <tipbot@zytor.com> wrote:
> >diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> >index 90de288..1facaf4 100644
> >--- a/arch/x86/kernel/head_64.S
> >+++ b/arch/x86/kernel/head_64.S
> >@@ -298,7 +298,7 @@ ENTRY(start_cpu)
> > 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
> > 	 *		address given in m16:64.
> > 	 */
> >-	call	1f		# put return address on stack for unwinder
> >+	pushq	$1f		# put return address on stack for unwinder
> > 1:	xorq	%rbp, %rbp	# clear frame pointer
> > 	movq	initial_code(%rip), %rax
> > 	pushq	$__KERNEL_CS	# set correct cs
> 
> This adds another relocation to the kernel.  I hope this is safe at this point in the code?

AFAIK, it should be fine.  All relocations were either applied at build
time, or for KASLR, in the compressed boot code which extracts and
copies this code.

Also there are already a bunch of relocations in the rest of the code in
this file, all of which runs before this code does.

(And even if that weren't the case, this address is only used for
displaying stack traces, so pushing a zero or some garbage here wouldn't
really break anything.)

-- 
Josh