[PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

[PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

[Dave Anderson]

One of the changes in commit a4928cffe6435caf427ae673131a633c1329dbf3 
made the venosys_1() system call static, which causes it to be 
optimized out of the kernel:

  Author: Ingo Molnar <mingo@elte.hu>
  Date:   Wed Apr 23 13:20:56 2008 +0200

  "make namespacecheck" fixes

  Signed-off-by: Ingo Molnar <mingo@elte.hu>

Commit 2e8ad43ec07545780ce7992cb18e2d82c7abd24c had originally
changed all vsyscalls from being static for the same reason:

  Author: Andi Kleen <ak@suse.de>
  Date:   Mon Sep 12 18:49:24 2005 +0200

  [PATCH] x86-64: Prevent gcc 4 from optimizing away vsyscalls
    
  They were previously static.
    
  Signed-off-by: Andi Kleen <ak@suse.de>
  Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Dave Anderson <anderson@redhat.com>

---

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 3e68218..60a34af 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -222,7 +222,7 @@ vgetcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache)
 	return 0;
 }
 
-static long __vsyscall(3) venosys_1(void)
+long __vsyscall(3) venosys_1(void)
 {
 	return -ENOSYS;
 }

Re: [PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

[Andi Kleen]

>  	return 0;
>  }
>  
> -static long __vsyscall(3) venosys_1(void)
> +long __vsyscall(3) venosys_1(void)

Better add a __used too. Otherwise it can be optimized out again
in some builds e.g. when someone enables gc-sections for the linker.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.

Re: [PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

[Dave Anderson]

----- Original Message -----
> >  	return 0;
> >  }
> >
> > -static long __vsyscall(3) venosys_1(void)
> > +long __vsyscall(3) venosys_1(void)
> 
> Better add a __used too. Otherwise it can be optimized out again
> in some builds e.g. when someone enables gc-sections for the linker.

Considering that all of the vsyscall prototypes have this attribute:

#define __vsyscall(nr) \
                __attribute__ ((unused, __section__(".vsyscall_" #nr))) notrace

How would any of them work?

Dave

> 
> -Andi
> --
> ak@linux.intel.com -- Speaking for myself only.

Re: [PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

[Andi Kleen]

On Fri, Jun 03, 2011 at 05:32:06PM -0400, Dave Anderson wrote:
> 
> 
> ----- Original Message -----
> > >  	return 0;
> > >  }
> > >
> > > -static long __vsyscall(3) venosys_1(void)
> > > +long __vsyscall(3) venosys_1(void)
> > 
> > Better add a __used too. Otherwise it can be optimized out again
> > in some builds e.g. when someone enables gc-sections for the linker.
> 
> Considering that all of the vsyscall prototypes have this attribute:
> 
> #define __vsyscall(nr) \
>                 __attribute__ ((unused, __section__(".vsyscall_" #nr))) notrace
> 
> How would any of them work?

unused is just to prevent warnings I think. used stops the optimizer
from throwing it away.

It would make more sense to drop the unused and add an used.  I think
used prevents the warnings too.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.

Re: [PATCH] x86-64: Prevent gcc from optimizing away venosys_1()

[Ingo Molnar]

* Dave Anderson <anderson@redhat.com> wrote:

> One of the changes in commit a4928cffe6435caf427ae673131a633c1329dbf3 
> made the venosys_1() system call static, which causes it to be 
> optimized out of the kernel:
> 
>   Author: Ingo Molnar <mingo@elte.hu>
>   Date:   Wed Apr 23 13:20:56 2008 +0200
> 
>   "make namespacecheck" fixes
> 
>   Signed-off-by: Ingo Molnar <mingo@elte.hu>

Andy noticed this recently, so we got rid of that broken (and unused) 
syscall in:

  bb5fe2f78ead: x86-64: Remove vsyscall number 3 (venosys)

which is in the tip:x86/vdso branch at:

   http://people.redhat.com/mingo/tip.git/README

Thanks,

	Ingo