[PATCH] x86/signal: Remove pax argument from restore_sigcontext

[PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Brian Gerst]

The pax argument is unnecesary.  Instead, store the RAX value directly
in regs.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/ia32/ia32_signal.c        | 17 ++++++-----------
 arch/x86/include/asm/sighandling.h |  4 +---
 arch/x86/kernel/signal.c           | 22 ++++++++--------------
 3 files changed, 15 insertions(+), 28 deletions(-)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 1f5e2b0..c81d35e6 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -161,8 +161,7 @@ int copy_siginfo_from_user32(siginfo_t *to, compat_siginfo_t __user *from)
 }
 
 static int ia32_restore_sigcontext(struct pt_regs *regs,
-				   struct sigcontext_ia32 __user *sc,
-				   unsigned int *pax)
+				   struct sigcontext_ia32 __user *sc)
 {
 	unsigned int tmpflags, err = 0;
 	void __user *buf;
@@ -184,7 +183,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 		RELOAD_SEG(es);
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
-		COPY(dx); COPY(cx); COPY(ip);
+		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
 		/* Don't touch extended registers */
 
 		COPY_SEG_CPL3(cs);
@@ -197,8 +196,6 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 
 		get_user_ex(tmp, &sc->fpstate);
 		buf = compat_ptr(tmp);
-
-		get_user_ex(*pax, &sc->ax);
 	} get_user_catch(err);
 
 	err |= restore_xstate_sig(buf, 1);
@@ -213,7 +210,6 @@ asmlinkage long sys32_sigreturn(void)
 	struct pt_regs *regs = current_pt_regs();
 	struct sigframe_ia32 __user *frame = (struct sigframe_ia32 __user *)(regs->sp-8);
 	sigset_t set;
-	unsigned int ax;
 
 	if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
 		goto badframe;
@@ -226,9 +222,9 @@ asmlinkage long sys32_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (ia32_restore_sigcontext(regs, &frame->sc, &ax))
+	if (ia32_restore_sigcontext(regs, &frame->sc))
 		goto badframe;
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "32bit sigreturn");
@@ -240,7 +236,6 @@ asmlinkage long sys32_rt_sigreturn(void)
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe_ia32 __user *frame;
 	sigset_t set;
-	unsigned int ax;
 
 	frame = (struct rt_sigframe_ia32 __user *)(regs->sp - 4);
 
@@ -251,13 +246,13 @@ asmlinkage long sys32_rt_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext, &ax))
+	if (ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	if (compat_restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "32bit rt sigreturn");
diff --git a/arch/x86/include/asm/sighandling.h b/arch/x86/include/asm/sighandling.h
index 7a95816..89db467 100644
--- a/arch/x86/include/asm/sighandling.h
+++ b/arch/x86/include/asm/sighandling.h
@@ -13,9 +13,7 @@
 			 X86_EFLAGS_CF | X86_EFLAGS_RF)
 
 void signal_fault(struct pt_regs *regs, void __user *frame, char *where);
-
-int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
-		       unsigned long *pax);
+int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc);
 int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
 		     struct pt_regs *regs, unsigned long mask);
 
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index eaa2c5e..53cc408 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -61,8 +61,7 @@
 	regs->seg = GET_SEG(seg) | 3;			\
 } while (0)
 
-int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
-		       unsigned long *pax)
+int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
 {
 	void __user *buf;
 	unsigned int tmpflags;
@@ -81,7 +80,7 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
 #endif /* CONFIG_X86_32 */
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
-		COPY(dx); COPY(cx); COPY(ip);
+		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
 
 #ifdef CONFIG_X86_64
 		COPY(r8);
@@ -102,8 +101,6 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
 		regs->orig_ax = -1;		/* disable syscall checks */
 
 		get_user_ex(buf, &sc->fpstate);
-
-		get_user_ex(*pax, &sc->ax);
 	} get_user_catch(err);
 
 	err |= restore_xstate_sig(buf, config_enabled(CONFIG_X86_32));
@@ -545,7 +542,6 @@ asmlinkage unsigned long sys_sigreturn(void)
 {
 	struct pt_regs *regs = current_pt_regs();
 	struct sigframe __user *frame;
-	unsigned long ax;
 	sigset_t set;
 
 	frame = (struct sigframe __user *)(regs->sp - 8);
@@ -559,9 +555,9 @@ asmlinkage unsigned long sys_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (restore_sigcontext(regs, &frame->sc, &ax))
+	if (restore_sigcontext(regs, &frame->sc))
 		goto badframe;
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "sigreturn");
@@ -574,7 +570,6 @@ asmlinkage long sys_rt_sigreturn(void)
 {
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe __user *frame;
-	unsigned long ax;
 	sigset_t set;
 
 	frame = (struct rt_sigframe __user *)(regs->sp - sizeof(long));
@@ -585,13 +580,13 @@ asmlinkage long sys_rt_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (restore_sigcontext(regs, &frame->uc.uc_mcontext, &ax))
+	if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	if (restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "rt_sigreturn");
@@ -786,7 +781,6 @@ asmlinkage long sys32_x32_rt_sigreturn(void)
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe_x32 __user *frame;
 	sigset_t set;
-	unsigned long ax;
 
 	frame = (struct rt_sigframe_x32 __user *)(regs->sp - 8);
 
@@ -797,13 +791,13 @@ asmlinkage long sys32_x32_rt_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (restore_sigcontext(regs, &frame->uc.uc_mcontext, &ax))
+	if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	if (compat_restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "x32 rt_sigreturn");
-- 
2.1.0

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Ingo Molnar]

* Brian Gerst <brgerst@gmail.com> wrote:

> The pax argument is unnecesary.  Instead, store the RAX value directly
> in regs.
> 
> Signed-off-by: Brian Gerst <brgerst@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/x86/ia32/ia32_signal.c        | 17 ++++++-----------
>  arch/x86/include/asm/sighandling.h |  4 +---
>  arch/x86/kernel/signal.c           | 22 ++++++++--------------
>  3 files changed, 15 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> index 1f5e2b0..c81d35e6 100644
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -161,8 +161,7 @@ int copy_siginfo_from_user32(siginfo_t *to, compat_siginfo_t __user *from)
>  }
>  
>  static int ia32_restore_sigcontext(struct pt_regs *regs,
> -				   struct sigcontext_ia32 __user *sc,
> -				   unsigned int *pax)
> +				   struct sigcontext_ia32 __user *sc)
>  {
>  	unsigned int tmpflags, err = 0;
>  	void __user *buf;
> @@ -184,7 +183,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>  		RELOAD_SEG(es);
>  
>  		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
> -		COPY(dx); COPY(cx); COPY(ip);
> +		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
>  		/* Don't touch extended registers */
>  
>  		COPY_SEG_CPL3(cs);
> @@ -197,8 +196,6 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>  
>  		get_user_ex(tmp, &sc->fpstate);
>  		buf = compat_ptr(tmp);
> -
> -		get_user_ex(*pax, &sc->ax);
>  	} get_user_catch(err);

Note that arch/x86/kernel/signal.c appears to have a similar pattern - 
and there it could be removed as well?

I'm wondering what the original reason for adding the extra handling 
of regs->ax was. Maybe something changed regs->ax - but I cannot find 
such code path anymore.

It would be nice to try to do a bit of Git archeology to figure out 
the origins of this complication - maybe it's something subtle - or 
it's something that has changed meanwhile.

Thanks,

	Ingo

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Brian Gerst]

On Sat, Apr 4, 2015 at 10:14 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
>> The pax argument is unnecesary.  Instead, store the RAX value directly
>> in regs.
>>
>> Signed-off-by: Brian Gerst <brgerst@gmail.com>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: x86@kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> ---
>>  arch/x86/ia32/ia32_signal.c        | 17 ++++++-----------
>>  arch/x86/include/asm/sighandling.h |  4 +---
>>  arch/x86/kernel/signal.c           | 22 ++++++++--------------
>>  3 files changed, 15 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
>> index 1f5e2b0..c81d35e6 100644
>> --- a/arch/x86/ia32/ia32_signal.c
>> +++ b/arch/x86/ia32/ia32_signal.c
>> @@ -161,8 +161,7 @@ int copy_siginfo_from_user32(siginfo_t *to, compat_siginfo_t __user *from)
>>  }
>>
>>  static int ia32_restore_sigcontext(struct pt_regs *regs,
>> -                                struct sigcontext_ia32 __user *sc,
>> -                                unsigned int *pax)
>> +                                struct sigcontext_ia32 __user *sc)
>>  {
>>       unsigned int tmpflags, err = 0;
>>       void __user *buf;
>> @@ -184,7 +183,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>>               RELOAD_SEG(es);
>>
>>               COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
>> -             COPY(dx); COPY(cx); COPY(ip);
>> +             COPY(dx); COPY(cx); COPY(ip); COPY(ax);
>>               /* Don't touch extended registers */
>>
>>               COPY_SEG_CPL3(cs);
>> @@ -197,8 +196,6 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>>
>>               get_user_ex(tmp, &sc->fpstate);
>>               buf = compat_ptr(tmp);
>> -
>> -             get_user_ex(*pax, &sc->ax);
>>       } get_user_catch(err);
>
> Note that arch/x86/kernel/signal.c appears to have a similar pattern -
> and there it could be removed as well?

I'm guessing you didn't read the whole patch, because I did change it.

> I'm wondering what the original reason for adding the extra handling
> of regs->ax was. Maybe something changed regs->ax - but I cannot find
> such code path anymore.
>
> It would be nice to try to do a bit of Git archeology to figure out
> the origins of this complication - maybe it's something subtle - or
> it's something that has changed meanwhile.

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Brian Gerst]

On Sat, Apr 4, 2015 at 10:14 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
>> The pax argument is unnecesary.  Instead, store the RAX value directly
>> in regs.
>>
>> Signed-off-by: Brian Gerst <brgerst@gmail.com>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: x86@kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> ---
>>  arch/x86/ia32/ia32_signal.c        | 17 ++++++-----------
>>  arch/x86/include/asm/sighandling.h |  4 +---
>>  arch/x86/kernel/signal.c           | 22 ++++++++--------------
>>  3 files changed, 15 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
>> index 1f5e2b0..c81d35e6 100644
>> --- a/arch/x86/ia32/ia32_signal.c
>> +++ b/arch/x86/ia32/ia32_signal.c
>> @@ -161,8 +161,7 @@ int copy_siginfo_from_user32(siginfo_t *to, compat_siginfo_t __user *from)
>>  }
>>
>>  static int ia32_restore_sigcontext(struct pt_regs *regs,
>> -                                struct sigcontext_ia32 __user *sc,
>> -                                unsigned int *pax)
>> +                                struct sigcontext_ia32 __user *sc)
>>  {
>>       unsigned int tmpflags, err = 0;
>>       void __user *buf;
>> @@ -184,7 +183,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>>               RELOAD_SEG(es);
>>
>>               COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
>> -             COPY(dx); COPY(cx); COPY(ip);
>> +             COPY(dx); COPY(cx); COPY(ip); COPY(ax);
>>               /* Don't touch extended registers */
>>
>>               COPY_SEG_CPL3(cs);
>> @@ -197,8 +196,6 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
>>
>>               get_user_ex(tmp, &sc->fpstate);
>>               buf = compat_ptr(tmp);
>> -
>> -             get_user_ex(*pax, &sc->ax);
>>       } get_user_catch(err);
>
> Note that arch/x86/kernel/signal.c appears to have a similar pattern -
> and there it could be removed as well?
>
> I'm wondering what the original reason for adding the extra handling
> of regs->ax was. Maybe something changed regs->ax - but I cannot find
> such code path anymore.
>
> It would be nice to try to do a bit of Git archeology to figure out
> the origins of this complication - maybe it's something subtle - or
> it's something that has changed meanwhile.

It goes all the way back to 2.1.106pre1, when restore_sigcontext() was
changed to return an error code instead of EAX directly.

https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/arch/i386/kernel/signal.c?id=9a8f8b7ca3f319bd668298d447bdf32730e51174

--
Brian Gerst

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Ingo Molnar]

* Brian Gerst <brgerst@gmail.com> wrote:

> On Sat, Apr 4, 2015 at 10:14 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Brian Gerst <brgerst@gmail.com> wrote:
> >
> >> The pax argument is unnecesary.  Instead, store the RAX value directly
> >> in regs.
> >>
> >> Signed-off-by: Brian Gerst <brgerst@gmail.com>
> >> Cc: Ingo Molnar <mingo@kernel.org>
> >> Cc: "H. Peter Anvin" <hpa@zytor.com>
> >> Cc: Andy Lutomirski <luto@amacapital.net>
> >> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> >> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> >> Cc: Borislav Petkov <bp@alien8.de>
> >> Cc: x86@kernel.org
> >> Cc: linux-kernel@vger.kernel.org
> >> ---
> >>  arch/x86/ia32/ia32_signal.c        | 17 ++++++-----------
> >>  arch/x86/include/asm/sighandling.h |  4 +---
> >>  arch/x86/kernel/signal.c           | 22 ++++++++--------------
> >>  3 files changed, 15 insertions(+), 28 deletions(-)
> >>
> >> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> >> index 1f5e2b0..c81d35e6 100644
> >> --- a/arch/x86/ia32/ia32_signal.c
> >> +++ b/arch/x86/ia32/ia32_signal.c
> >> @@ -161,8 +161,7 @@ int copy_siginfo_from_user32(siginfo_t *to, compat_siginfo_t __user *from)
> >>  }
> >>
> >>  static int ia32_restore_sigcontext(struct pt_regs *regs,
> >> -                                struct sigcontext_ia32 __user *sc,
> >> -                                unsigned int *pax)
> >> +                                struct sigcontext_ia32 __user *sc)
> >>  {
> >>       unsigned int tmpflags, err = 0;
> >>       void __user *buf;
> >> @@ -184,7 +183,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
> >>               RELOAD_SEG(es);
> >>
> >>               COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
> >> -             COPY(dx); COPY(cx); COPY(ip);
> >> +             COPY(dx); COPY(cx); COPY(ip); COPY(ax);
> >>               /* Don't touch extended registers */
> >>
> >>               COPY_SEG_CPL3(cs);
> >> @@ -197,8 +196,6 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
> >>
> >>               get_user_ex(tmp, &sc->fpstate);
> >>               buf = compat_ptr(tmp);
> >> -
> >> -             get_user_ex(*pax, &sc->ax);
> >>       } get_user_catch(err);
> >
> > Note that arch/x86/kernel/signal.c appears to have a similar pattern -
> > and there it could be removed as well?
> 
> I'm guessing you didn't read the whole patch, because I did change it.

Yes :-/

	Ingo

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Ingo Molnar]

* Brian Gerst <brgerst@gmail.com> wrote:

> > I'm wondering what the original reason for adding the extra 
> > handling of regs->ax was. Maybe something changed regs->ax - but I 
> > cannot find such code path anymore.
> >
> > It would be nice to try to do a bit of Git archeology to figure 
> > out the origins of this complication - maybe it's something subtle 
> > - or it's something that has changed meanwhile.
> 
> It goes all the way back to 2.1.106pre1, when restore_sigcontext() 
> was changed to return an error code instead of EAX directly.
> 
> https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/arch/i386/kernel/signal.c?id=9a8f8b7ca3f319bd668298d447bdf32730e51174

Indeed: restore_sigcontext() used to return eax as a return value, 
without copying it into regs->ax.

Then in 2007 sigaltstack syscall support was added, where the return 
value of restore_sigcontext() was changed to carry the memory-copying 
failure code. But instead of putting 'ax' into regs->ax, it was 
carried in via a pointer and then returned, where the generic syscall 
return code copied it to regs->ax.

So there was never any deeper reason for this suboptimal pattern, it 
was simply never noticed after being introduced.

(Btw., the regs->ax we return will be copied back to regs->ax after 
the syscall straight away once again - but I guess this cannot be 
helped.)

I've added this information to the changelog.

Thanks,

	Ingo

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Brian Gerst]

On Mon, Apr 6, 2015 at 3:03 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
>> > I'm wondering what the original reason for adding the extra
>> > handling of regs->ax was. Maybe something changed regs->ax - but I
>> > cannot find such code path anymore.
>> >
>> > It would be nice to try to do a bit of Git archeology to figure
>> > out the origins of this complication - maybe it's something subtle
>> > - or it's something that has changed meanwhile.
>>
>> It goes all the way back to 2.1.106pre1, when restore_sigcontext()
>> was changed to return an error code instead of EAX directly.
>>
>> https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/arch/i386/kernel/signal.c?id=9a8f8b7ca3f319bd668298d447bdf32730e51174
>
> Indeed: restore_sigcontext() used to return eax as a return value,
> without copying it into regs->ax.
>
> Then in 2007

Version 2.1.106 was released on Jun 13, 1998.

> sigaltstack syscall support was added, where the return
> value of restore_sigcontext() was changed to carry the memory-copying
> failure code. But instead of putting 'ax' into regs->ax, it was
> carried in via a pointer and then returned, where the generic syscall
> return code copied it to regs->ax.
>
> So there was never any deeper reason for this suboptimal pattern, it
> was simply never noticed after being introduced.
>
> (Btw., the regs->ax we return will be copied back to regs->ax after
> the syscall straight away once again - but I guess this cannot be
> helped.)

The 64-bit stub could skip saving it back to regs.  However 32-bit
does not have a special stub so sys_rt_sigreturn() still needs to
return regs->ax.

--
Brian Gerst

[tip:x86/asm] x86/signal: Remove pax argument from restore_sigcontext

[tip-bot for Brian Gerst]

Commit-ID:  6a3713f001b3b53587e411ab0d3036ae9b0fb93b
Gitweb:     http://git.kernel.org/tip/6a3713f001b3b53587e411ab0d3036ae9b0fb93b
Author:     Brian Gerst <brgerst@gmail.com>
AuthorDate: Sat, 4 Apr 2015 08:58:23 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 6 Apr 2015 09:06:39 +0200

x86/signal: Remove pax argument from restore_sigcontext

The 'pax' argument is unnecesary.  Instead, store the RAX value
directly in regs.

This pattern goes all the way back to 2.1.106pre1, when restore_sigcontext()
was changed to return an error code instead of EAX directly:

  https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/arch/i386/kernel/signal.c?id=9a8f8b7ca3f319bd668298d447bdf32730e51174

In 2007 sigaltstack syscall support was added, where the return
value of restore_sigcontext() was changed to carry the memory-copying
failure code.

But instead of putting 'ax' into regs->ax directly, it was carried
in via a pointer and then returned, where the generic syscall return
code copied it to regs->ax.

So there was never any deeper reason for this suboptimal pattern, it
was simply never noticed after being introduced.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1428152303-17154-1-git-send-email-brgerst@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/ia32/ia32_signal.c        | 17 ++++++-----------
 arch/x86/include/asm/sighandling.h |  4 +---
 arch/x86/kernel/signal.c           | 22 ++++++++--------------
 3 files changed, 15 insertions(+), 28 deletions(-)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 1f5e2b0..c81d35e6 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -161,8 +161,7 @@ int copy_siginfo_from_user32(siginfo_t *to, compat_siginfo_t __user *from)
 }
 
 static int ia32_restore_sigcontext(struct pt_regs *regs,
-				   struct sigcontext_ia32 __user *sc,
-				   unsigned int *pax)
+				   struct sigcontext_ia32 __user *sc)
 {
 	unsigned int tmpflags, err = 0;
 	void __user *buf;
@@ -184,7 +183,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 		RELOAD_SEG(es);
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
-		COPY(dx); COPY(cx); COPY(ip);
+		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
 		/* Don't touch extended registers */
 
 		COPY_SEG_CPL3(cs);
@@ -197,8 +196,6 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 
 		get_user_ex(tmp, &sc->fpstate);
 		buf = compat_ptr(tmp);
-
-		get_user_ex(*pax, &sc->ax);
 	} get_user_catch(err);
 
 	err |= restore_xstate_sig(buf, 1);
@@ -213,7 +210,6 @@ asmlinkage long sys32_sigreturn(void)
 	struct pt_regs *regs = current_pt_regs();
 	struct sigframe_ia32 __user *frame = (struct sigframe_ia32 __user *)(regs->sp-8);
 	sigset_t set;
-	unsigned int ax;
 
 	if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
 		goto badframe;
@@ -226,9 +222,9 @@ asmlinkage long sys32_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (ia32_restore_sigcontext(regs, &frame->sc, &ax))
+	if (ia32_restore_sigcontext(regs, &frame->sc))
 		goto badframe;
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "32bit sigreturn");
@@ -240,7 +236,6 @@ asmlinkage long sys32_rt_sigreturn(void)
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe_ia32 __user *frame;
 	sigset_t set;
-	unsigned int ax;
 
 	frame = (struct rt_sigframe_ia32 __user *)(regs->sp - 4);
 
@@ -251,13 +246,13 @@ asmlinkage long sys32_rt_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext, &ax))
+	if (ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	if (compat_restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "32bit rt sigreturn");
diff --git a/arch/x86/include/asm/sighandling.h b/arch/x86/include/asm/sighandling.h
index 7a95816..89db467 100644
--- a/arch/x86/include/asm/sighandling.h
+++ b/arch/x86/include/asm/sighandling.h
@@ -13,9 +13,7 @@
 			 X86_EFLAGS_CF | X86_EFLAGS_RF)
 
 void signal_fault(struct pt_regs *regs, void __user *frame, char *where);
-
-int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
-		       unsigned long *pax);
+int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc);
 int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
 		     struct pt_regs *regs, unsigned long mask);
 
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index eaa2c5e..53cc408 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -61,8 +61,7 @@
 	regs->seg = GET_SEG(seg) | 3;			\
 } while (0)
 
-int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
-		       unsigned long *pax)
+int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
 {
 	void __user *buf;
 	unsigned int tmpflags;
@@ -81,7 +80,7 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
 #endif /* CONFIG_X86_32 */
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
-		COPY(dx); COPY(cx); COPY(ip);
+		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
 
 #ifdef CONFIG_X86_64
 		COPY(r8);
@@ -102,8 +101,6 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
 		regs->orig_ax = -1;		/* disable syscall checks */
 
 		get_user_ex(buf, &sc->fpstate);
-
-		get_user_ex(*pax, &sc->ax);
 	} get_user_catch(err);
 
 	err |= restore_xstate_sig(buf, config_enabled(CONFIG_X86_32));
@@ -545,7 +542,6 @@ asmlinkage unsigned long sys_sigreturn(void)
 {
 	struct pt_regs *regs = current_pt_regs();
 	struct sigframe __user *frame;
-	unsigned long ax;
 	sigset_t set;
 
 	frame = (struct sigframe __user *)(regs->sp - 8);
@@ -559,9 +555,9 @@ asmlinkage unsigned long sys_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (restore_sigcontext(regs, &frame->sc, &ax))
+	if (restore_sigcontext(regs, &frame->sc))
 		goto badframe;
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "sigreturn");
@@ -574,7 +570,6 @@ asmlinkage long sys_rt_sigreturn(void)
 {
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe __user *frame;
-	unsigned long ax;
 	sigset_t set;
 
 	frame = (struct rt_sigframe __user *)(regs->sp - sizeof(long));
@@ -585,13 +580,13 @@ asmlinkage long sys_rt_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (restore_sigcontext(regs, &frame->uc.uc_mcontext, &ax))
+	if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	if (restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "rt_sigreturn");
@@ -786,7 +781,6 @@ asmlinkage long sys32_x32_rt_sigreturn(void)
 	struct pt_regs *regs = current_pt_regs();
 	struct rt_sigframe_x32 __user *frame;
 	sigset_t set;
-	unsigned long ax;
 
 	frame = (struct rt_sigframe_x32 __user *)(regs->sp - 8);
 
@@ -797,13 +791,13 @@ asmlinkage long sys32_x32_rt_sigreturn(void)
 
 	set_current_blocked(&set);
 
-	if (restore_sigcontext(regs, &frame->uc.uc_mcontext, &ax))
+	if (restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	if (compat_restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	return ax;
+	return regs->ax;
 
 badframe:
 	signal_fault(regs, frame, "x32 rt_sigreturn");

Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Ingo Molnar]

* Brian Gerst <brgerst@gmail.com> wrote:

> On Mon, Apr 6, 2015 at 3:03 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Brian Gerst <brgerst@gmail.com> wrote:
> >
> >> > I'm wondering what the original reason for adding the extra
> >> > handling of regs->ax was. Maybe something changed regs->ax - but I
> >> > cannot find such code path anymore.
> >> >
> >> > It would be nice to try to do a bit of Git archeology to figure
> >> > out the origins of this complication - maybe it's something subtle
> >> > - or it's something that has changed meanwhile.
> >>
> >> It goes all the way back to 2.1.106pre1, when restore_sigcontext()
> >> was changed to return an error code instead of EAX directly.
> >>
> >> https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/arch/i386/kernel/signal.c?id=9a8f8b7ca3f319bd668298d447bdf32730e51174
> >
> > Indeed: restore_sigcontext() used to return eax as a return value,
> > without copying it into regs->ax.
> >
> > Then in 2007
> 
> Version 2.1.106 was released on Jun 13, 1998.

Sigh, the Git timestamp of the historic tree threw me off :-)

> > sigaltstack syscall support was added, where the return value of 
> > restore_sigcontext() was changed to carry the memory-copying 
> > failure code. But instead of putting 'ax' into regs->ax, it was 
> > carried in via a pointer and then returned, where the generic 
> > syscall return code copied it to regs->ax.
> >
> > So there was never any deeper reason for this suboptimal pattern, 
> > it was simply never noticed after being introduced.
> >
> > (Btw., the regs->ax we return will be copied back to regs->ax 
> > after the syscall straight away once again - but I guess this 
> > cannot be helped.)
> 
> The 64-bit stub could skip saving it back to regs.

Yeah, but at the cost of having a duplicated entry stub, right?

Thanks,

	Ingo