Messages in this thread | | | From | Andrew Lutomirski <> | Date | Mon, 13 Jun 2011 10:18:12 -0400 | Subject | Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall_64.c |
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On Mon, Jun 13, 2011 at 10:14 AM, Ingo Molnar <mingo@elte.hu> wrote: > > * 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.
Fair enough.
Unfortunately there doesn't seem to be an EKERNELBUG error code, and initializing to EFAULT seems silly. 0 is probably harmless.
I'll wait awhile longer for that GCC version, since there might be a better fix. In any case, it would be nice for the changelog entry to say which version has a warning that's being worked around.
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