Re: [RFC v3 03/23] sysdeps/wait: Use waitid if avaliable

[Rich Felker <dalias at libc dot org>]

On Tue, Jul 16, 2019 at 05:08:48PM -0700, Alistair Francis wrote:
> If the waitid syscall is avaliable let's use that as waitpid
> and wait4 aren't always avaliable (they aren't avaliable on RV32).
> 
> Unfortunately waitid is substantially differnt to waitpid and wait4, so
> the conversion ends up being complex.
> 
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>  ChangeLog                                  |  3 ++
>  sysdeps/unix/sysv/linux/wait.c             | 39 ++++++++++++++++--
>  sysdeps/unix/sysv/linux/waitpid.c          | 46 ++++++++++++++++++++++
>  sysdeps/unix/sysv/linux/waitpid_nocancel.c | 45 +++++++++++++++++++++
>  4 files changed, 130 insertions(+), 3 deletions(-)
> [...]
>  
>  weak_alias (__libc_wait, __wait)
> diff --git a/sysdeps/unix/sysv/linux/waitpid.c b/sysdeps/unix/sysv/linux/waitpid.c
> index f0897574c0..7d4e0bb77d 100644
> --- a/sysdeps/unix/sysv/linux/waitpid.c
> +++ b/sysdeps/unix/sysv/linux/waitpid.c
> @@ -20,12 +20,58 @@
>  #include <sysdep-cancel.h>
>  #include <stdlib.h>
>  #include <sys/wait.h>
> +#include <unistd.h>
>  
>  __pid_t
>  __waitpid (__pid_t pid, int *stat_loc, int options)
>  {
>  #ifdef __NR_waitpid
>    return SYSCALL_CANCEL (waitpid, pid, stat_loc, options);
> +#elif defined(__NR_waitid)
> +  __pid_t ret;
> +  idtype_t idtype = P_PID;
> +  siginfo_t infop;
> +
> +  if (pid < -1) {
> +    idtype = P_PGID;
> +    pid *= -1;
> +  } else if (pid == -1) {
> +    idtype = P_ALL;
> +  } else if (pid == 0) {
> +    idtype = P_PGID;
> +    pid = getpgrp();
> +  }
> +
> +  options |= WEXITED;
> +
> +  ret = SYSCALL_CANCEL (waitid, idtype, pid, &infop, options, NULL);

This emulation has a fundamental race condition. Between getpgrp and
waitid, a signal handler may perform setpgrp, setsid, and/or fork in
ways that cause the wrong pgid to be passed to the waitid syscall.
There is no way around this because you cannot block signals for the
interval, since signals must be able to interrupt the waitid syscall.

Unless there's some trick I'm missing here, the kernel folks' removal
of the wait4 syscall is just a bug in the kernel that they need to
fix. It also makes it impossible to implement the wait4 function,
since there's no way to get rusage for the exited process.

Rich