Messages in this thread | | | Date | Mon, 13 Jun 2011 16:14:08 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall_64.c |
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* Andrew Lutomirski <luto@mit.edu> wrote:
> On Mon, Jun 13, 2011 at 5:29 AM, Ingo Molnar <mingo@elte.hu> wrote: > > > > * Andrew Lutomirski <luto@mit.edu> wrote: > > > >> On Sat, Jun 11, 2011 at 3:31 AM, Rakib Mullick <rakib.mullick@gmail.com> wrote: > >> > Due to commit 5cec93c216db77 (x86-64: Emulate legacy vsyscalls), we get the following warning: > >> > > >> > arch/x86/kernel/vsyscall_64.c: In function ‘do_emulate_vsyscall’: > >> > arch/x86/kernel/vsyscall_64.c:111:7: warning: ‘ret’ may be used uninitialized in this function > >> > >> What's the code path that uses ret without initializing it? > > > > If the code is correct but GCC got confused then please use the > > simplest possible patch to help GCC find its way around the code. > > The simplest patch is to mark ret as uninitialized_var.
No - that primitive really sucks as it might hide *future* debug warnings and silently break code.
The problem with uninitialized_var() is that such code:
int test(void) { int uninitialized_var(ret);
return ret; }
Builds without a single warning but it is very broken code.
So if we use uninitialized_var() and the code is changed in the future to have the above broken sequence, we'll have a silent runtime failure ...
So we try to avoid using uninitialized_var() in arch/x86/ and use explicit initialization instead.
That way GCC that can see through the flow will optimize away the superfluous initialization - GCC versions that are older will generate one more instruction but that's OK.
Thanks,
Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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