[PATCH] linux: Fix a non-constant expression in _Static_assert

[Fāng-ruì Sòng]

On 2021-09-27, Andrew Pinski wrote:
>On Mon, Sep 27, 2021 at 10:18 AM Fāng-ruì Sòng via Libc-alpha
><libc-alpha@sourceware.org> wrote:
>>
>> On Mon, Sep 27, 2021 at 6:09 AM Florian Weimer <fweimer@redhat.com> wrote:
>> >
>> > * Fangrui Song via Libc-alpha:
>> >
>> > > The description needs some adjustment
>> > > https://www.iso-9899.info/n1570.html#6.6p10
>> > >
>> > > 10   An implementation may accept other forms of constant expressions.
>> >
>> > Yeah, but the change itself is clearly correct.  Thanks.
>> >
>> > Florian
>> >
>>
>> Ugh, you may have missed my point that GCC -O0 and -O2 are not
>> consistent on whether a `const int` operand can be used in a constant
>> expression.
>
>Yes and that was fixed for GCC 8 by r8-4755.
>Since you were not specific on which version of GCC, I am assuming it
>was GCC7.x.
>
>Thanks,
>Andrew Pinski

Sorry for the imprecise description.
I think the issue is different from your mentioned GCC 8 bugfix.

I commented out these lines in include/libc-symbols.h

-#if !defined __ASSEMBLER__ && !defined __OPTIMIZE__
-# error "glibc cannot be compiled without optimization"
-#endif
+//#if !defined __ASSEMBLER__ && !defined __OPTIMIZE__
+//# error "glibc cannot be compiled without optimization"
+//#endif

Next,

a=(../sysdeps/unix/sysv/linux/dl-opendir.c -std=gnu11 -fgnu89-inline -g -Wall -Wwrite-strings -Wundef -fmerge-all-constants -frounding-math -fno-stack-protector -fno-common -Wstrict-prototypes -Wold-style-definition -fmath-errno -fPIC -fno-stack-protector -DSTACK_PROTECTOR_LEVEL=0 -mno-mmx -ftls-model=initial-exec -I../include -I$HOME/Dev/glibc/out/clang/elf -I$HOME/Dev/glibc/out/clang -I../sysdeps/unix/sysv/linux/x86_64/64 -I../sysdeps/unix/sysv/linux/x86_64 -I../sysdeps/unix/sysv/linux/x86/include -I../sysdeps/unix/sysv/linux/x86 -I../sysdeps/x86/nptl -I../sysdeps/unix/sysv/linux/wordsize-64 -I../sysdeps/x86_64/nptl -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux -I../sysdeps/nptl -I../sysdeps/pthread -I../sysdeps/gnu -I../sysdeps/unix/inet -I../sysdeps/unix/sysv -I../sysdeps/unix/x86_64 -I../sysdeps/unix -I../sysdeps/posix -I../sysdeps/x86_64/64 -I../sysdeps/x86_64/fpu/multiarch -I../sysdeps/x86_64/fpu -I../sysdeps/x86/fpu -I../sysdeps/x86_64/multiarch -I../sysdeps/x86_64 -I../sysdeps/x86/include -I../sysdeps/x86 -I../sysdeps/ieee754/float128 -I../sysdeps/ieee754/ldbl-96/include -I../sysdeps/ieee754/ldbl-96 -I../sysdeps/ieee754/dbl-64 -I../sysdeps/ieee754/flt-32 -I../sysdeps/wordsize-64 -I../sysdeps/ieee754 -I../sysdeps/generic -I.. -I../libio -I. -D_LIBC_REENTRANT -include $HOME/Dev/glibc/out/clang/libc-modules.h -include ../include/libc-symbols.h -DPIC -DSHARED -DTOP_NAMESPACE=glibc -fsyntax-only)

% gcc-11 $=a -O2  # good
% gcc-11 $=a -O0  # -O1 is similar
In file included from ../include/features.h:488,
                  from ../posix/sys/types.h:25,
                  from ../include/sys/types.h:1,
                  from ../sysdeps/unix/sysv/linux/dirstream.h:21,
                  from ../include/dirent.h:3,
                  from ../sysdeps/unix/sysv/linux/opendir.c:18,
                  from ../sysdeps/unix/sysv/linux/dl-opendir.c:1:
../sysdeps/unix/sysv/linux/opendir.c: In function ‘__alloc_dir’:
../sysdeps/unix/sysv/linux/opendir.c:107:35: error: expression in static assertion is not constant
   107 |   _Static_assert (allocation_size >= sizeof (struct dirent64),
       |                   ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
../include/sys/cdefs.h:7:59: note: in definition of macro ‘_Static_assert’
     7 | # define _Static_assert(expr, diagnostic) _Static_assert (expr, diagnostic)
       |                                                           ^~~~

Then I tested gcc-8, gcc-9, gcc-10 on Debian testing, they have the same behavior as gcc-11.

>>
>> Updated description:
>>
>> ---
>>
>> linux: Fix a non-constant expression in _Static_assert
>>
>> According to C11 6.6p6, `const int` as an operand may not make up a
>> constant expression. GCC's -O0 and -O2 optimization levels are not
>> consistent on whether this operand can be used in a constant expression:
>>
>> ../sysdeps/unix/sysv/linux/opendir.c:107:19: error: static_assert
>> expression is not an integral constant expression
>>   _Static_assert (allocation_size >= sizeof (struct dirent64),
>>
>> Use enum which is guaranteed to be a constant expression.
>> This fixes an error when compiling with GCC -O0 and Clang.
>>
>> Fixes: 4b962c9e859de23b461d61f860dbd3f21311e83a ("linux: Simplify
>> opendir buffer allocation")