Re: [PATCH] linux-user/syscall: Do not ignore info.si_pid == 0 in waitid()

[Laurent Vivier]

Le 13/01/2022 à 10:37, Serge Belyshev a écrit :
> When called with WNOHANG and no child has exited, waitid returns with
> info.si_pid set to zero and thus check for info.si_pid != 0 will cause
> target siginfo structure to be uninitialized.  Fixed by removing the check.
>
> Signed-off-by: Serge Belyshev <belyshev@depni.sinp.msu.ru>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/817
> ---
>   linux-user/syscall.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 5950222a77..b80531ac4c 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8724,9 +8724,8 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> abi_long arg1,
>       case TARGET_NR_waitid:
>           {
>               siginfo_t info;
> -            info.si_pid = 0;
>               ret = get_errno(safe_waitid(arg1, arg2, &info, arg4, NULL));
> -            if (!is_error(ret) && arg3 && info.si_pid != 0) {
> +            if (!is_error(ret) && arg3) {
>                   if (!(p = lock_user(VERIFY_WRITE, arg3, 
> sizeof(target_siginfo_t), 0)))
>                       return -TARGET_EFAULT;
>                   host_to_target_siginfo(p, &info);

According to wait(2), it sounds a little bit more complicated than that.

       If WNOHANG was specified in options and there were no children in a 
waitable state, then
       waitid() returns 0 immediately and the state of the siginfo_t  structure 
 pointed  to  by
       infop  depends  on  the  implementation.   To (portably) distinguish 
this case from that
       where a child was in a waitable state, zero out the si_pid field before 
the call and check
       for a nonzero value in this field after the call returns.

       POSIX.1-2008  Technical  Corrigendum  1 (2013) adds the requirement that 
when WNOHANG is
       specified in options and there were no children in a waitable state, 
then waitid() should
       zero out the si_pid and si_signo fields of the structure.  On Linux and 
other implementations
       that adhere to this requirement, it is not necessary to zero out the 
si_pid field before
       calling waitid().  However, not all implementations follow the POSIX.1  
specification  on
       this point.

Perhaps the best approach would be to copy the caller target siginfo to the host one, call host 
waitpid(), remove the "info.si_pid != 0" and copy back the host siginfo to target one?

Thanks,
Laurent