Ignored return value of __clear_user in fs/binfmt_elf.c?

Ignored return value of __clear_user in fs/binfmt_elf.c?

[Horst von Brand]

Machine is sparc64, bk of today, gcc-3.4.2-6.fc3 (Aurora Corona). First 2.6
I try to build here, so it might be something known.

Build fails due to -Werror with:

include/asm/uaccess.h: In function `load_elf_binary':
arch/sparc64/kernel/../../../fs/binfmt_elf.c:811: warning: ignoring return value of `__clear_user', declared with attribute warn_unused_result

Around line 811 of fs/binfmt_elf.c I see:

                             /*
                              * This bss-zeroing can fail if the ELF file
                              * specifies odd protections.  So we don't check                                * the return value
                              */
                              (void)clear_user((void __user *)elf_bss +
                                                      load_bias, nbyte);

so presumably this discarding is OK here...

I wonder why an explicit (void) cast is not considered "use" by the
compiler. But then again, explicitly throwing away isn't really "use"...
-- 
Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513

Re: Ignored return value of __clear_user in fs/binfmt_elf.c?

[Andrew Morton]

Horst von Brand <vonbrand@inf.utfsm.cl> wrote:
>
> Machine is sparc64, bk of today, gcc-3.4.2-6.fc3 (Aurora Corona). First 2.6
> I try to build here, so it might be something known.
> 
> Build fails due to -Werror with:
> 
> include/asm/uaccess.h: In function `load_elf_binary':
> arch/sparc64/kernel/../../../fs/binfmt_elf.c:811: warning: ignoring return value of `__clear_user', declared with attribute warn_unused_result

Oh bugger.

> Around line 811 of fs/binfmt_elf.c I see:
> 
>                              /*
>                               * This bss-zeroing can fail if the ELF file
>                               * specifies odd protections.  So we don't check                                * the return value
>                               */
>                               (void)clear_user((void __user *)elf_bss +
>                                                       load_bias, nbyte);
> 
> so presumably this discarding is OK here...
> 
> I wonder why an explicit (void) cast is not considered "use" by the
> compiler. But then again, explicitly throwing away isn't really "use"...

I'd assumed that it would work.  How about this?

--- 25/fs/binfmt_elf.c~binfmt_elf-build-fix	Wed Feb 23 16:52:48 2005
+++ 25-akpm/fs/binfmt_elf.c	Wed Feb 23 16:53:40 2005
@@ -821,13 +821,14 @@ static int load_elf_binary(struct linux_
 				nbyte = ELF_MIN_ALIGN - nbyte;
 				if (nbyte > elf_brk - elf_bss)
 					nbyte = elf_brk - elf_bss;
-				/*
-				 * This bss-zeroing can fail if the ELF file
-				 * specifies odd protections.  So we don't check
-				 * the return value
-				 */
-				(void)clear_user((void __user *)elf_bss +
-							load_bias, nbyte);
+				if (clear_user((void __user *)elf_bss +
+							load_bias, nbyte)) {
+					/*
+					 * This bss-zeroing can fail if the ELF
+					 * file specifies odd protections.  So
+					 * we don't check the return value
+					 */
+				}
 			}
 		}
 
_