Messages in this thread | | | Date | Wed, 28 Apr 2021 14:41:56 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH] powerpc: Avoid clang uninitialized warning in __get_user_size_allowed |
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On Tue, Apr 27, 2021 at 07:05:12AM +0200, Christophe Leroy wrote: > > > Le 26/04/2021 à 22:35, Nathan Chancellor a écrit : > > Commit 9975f852ce1b ("powerpc/uaccess: Remove calls to __get_user_bad() > > and __put_user_bad()") switch to BUILD_BUG() in the default case, which > > leaves x uninitialized. This will not be an issue because the build will > > be broken in that case but clang does static analysis before it realizes > > the default case will be done so it warns about x being uninitialized > > (trimmed for brevity): > > > > In file included from mm/mprotect.c:13: > > In file included from ./include/linux/hugetlb.h:28: > > In file included from ./include/linux/mempolicy.h:16: > > ./include/linux/pagemap.h:772:16: warning: variable '__gu_val' is used > > uninitialized whenever switch default is taken [-Wsometimes-uninitialized] > > if (unlikely(__get_user(c, uaddr) != 0)) > > ^~~~~~~~~~~~~~~~~~~~ > > ./arch/powerpc/include/asm/uaccess.h:266:2: note: expanded from macro '__get_user' > > __get_user_size_allowed(__gu_val, __gu_addr, __gu_size, __gu_err); \ > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > ./arch/powerpc/include/asm/uaccess.h:235:2: note: expanded from macro > > '__get_user_size_allowed' > > default: BUILD_BUG(); \ > > ^~~~~~~ > > > > Commit 5cd29b1fd3e8 ("powerpc/uaccess: Use asm goto for get_user when > > compiler supports it") added an initialization for x because of the same > > reason. Do the same thing here so there is no warning across all > > versions of clang. > > Ah yes, I tested with Clang 11 which has CONFIG_CC_HAS_ASM_GOTO_OUTPUT, > that's the reason why I hit that warning only in the > CONFIG_CC_HAS_ASM_GOTO_OUTPUT branch. > > But regardless, is that normal that Clang warns that on a never taken branch ? That's puzzling.
It seems to be related to the fact that the value of sizeof is assigned to a variable. At this point in the pipeline, clang does not realize that the default branch is never taken because __gu_size has not actually been evaluated. If you stuck a numeric constant in there, it would not fire.
A simple example: https://godbolt.org/z/jbrqEbh1j
It is possible that could be improved in clang but I am not sure.
> > > > Link: https://github.com/ClangBuiltLinux/linux/issues/1359 > > Signed-off-by: Nathan Chancellor <nathan@kernel.org> > > Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Thanks for taking a look!
Cheers, Nathan
> > --- > > arch/powerpc/include/asm/uaccess.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h > > index a4e791bcd3fe..a09e4240c5b1 100644 > > --- a/arch/powerpc/include/asm/uaccess.h > > +++ b/arch/powerpc/include/asm/uaccess.h > > @@ -232,7 +232,7 @@ do { \ > > case 2: __get_user_asm(x, (u16 __user *)ptr, retval, "lhz"); break; \ > > case 4: __get_user_asm(x, (u32 __user *)ptr, retval, "lwz"); break; \ > > case 8: __get_user_asm2(x, (u64 __user *)ptr, retval); break; \ > > - default: BUILD_BUG(); \ > > + default: x = 0; BUILD_BUG(); \ > > } \ > > } while (0) > > > > base-commit: ee6b25fa7c037e42cc5f3b5c024b2a779edab6dd > >
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