[PATCH] linux: Enforce zero fill of siginfo_t

[Adhemerval Zanella Netto]

On 19/01/26 14:08, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> The glibc siginfo struct is larger than the kABI one, and the kernel
>> enforces that for an unknown si_code, all the extra bytes should be 0
>> (post_copy_siginfo_from_user).
>>
>> For armhf, gcc-15 is fully zero-filling the struct (it seems to be a
>> compiler issue, although I haven't found any already registered on
>> gcc bugzilla).
>>
>> In any case, to avoid false positives, force zero-filling on the
>> structure.
>>
>> Checked on arm-linux-gnueabihf.
>> ---
>>  sysdeps/unix/sysv/linux/tst-pidfd.c | 14 ++++++--------
>>  1 file changed, 6 insertions(+), 8 deletions(-)
>>
>> diff --git a/sysdeps/unix/sysv/linux/tst-pidfd.c b/sysdeps/unix/sysv/linux/tst-pidfd.c
>> index d50e36b18a..d65e3960ac 100644
>> --- a/sysdeps/unix/sysv/linux/tst-pidfd.c
>> +++ b/sysdeps/unix/sysv/linux/tst-pidfd.c
>> @@ -210,14 +210,12 @@ do_test (void)
>>    /* Wait for second sigtimedwait.  */
>>    support_process_state_wait (pid, support_process_state_sleeping);
>>    {
>> -    siginfo_t info =
>> -      {
>> -	.si_signo = SIGUSR2,
>> -	.si_errno = EAGAIN,
>> -	.si_code = -10,
>> -	.si_pid = ppid,
>> -	.si_uid = puid
>> -      };
>> +    siginfo_t info = { 0 };
>> +    info.si_signo = SIGUSR2;
>> +    info.si_errno = EAGAIN;
>> +    info.si_code = -10;
>> +    info.si_pid = ppid;
>> +    info.si_uid = puid;
>>      TEST_COMPARE (pidfd_send_signal (pidfd, SIGUSR2, &info, 0), 0);
>>    }
> 
> Aren't those two equivalent?  Is the issue about padding?

That is my understanding, but for some reason gcc-15 on armhf is *not*
zero-filling the remaining fields. That's why I stated in commit message
that this seems a compiler issue, although I haven't yet create a
more simple reproducer and open a bug report for this.

I still think it would be valuable to avoid some false-positives on different
environments, from 2.43 release wiki [1] it seems that tst-pidfd also fails
for powerpc32 (PMacG4), openrisc, and  x86 (thinkpad t60).

[1] https://sourceware.org/glibc/wiki/Release/2.43