Re: [PATCH 5/7] linux: Use waitid on wait4 if __NR_wait4 is not defined

[Alistair Francis <alistair23@gmail.com>]

On Thu, Nov 14, 2019 at 9:47 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> If the wait4 syscall is not avaliable (such as y2038 safe 32-bit
> systems) waitid should be used instead.  However prior Linux 5.4
> waitid is not a fully superset of other wait instruction, since it
> does not include support for waiting for the current process group.
>
> It would be possible to emulate to issue an extra syscall to get
> the current process group, but it is inherent racy: after the current
> process group is received and before it is passed to waitid a signal
> could arrive causing the current process group to change
>
> So waitid is used if wait4 is not defined iff the build is
> enabled with a minimum kernel if 5.4+.  The new assume
> __ASSUME_WAITID_PID0_P_PGID is added and an error is issued if waitid
> can not be implemented by either __NR_wait4 or
> __NR_waitid && __ASSUME_WAITID_PID0_P_PGID.
>
> Checked on x86_64-linux-gnu and i686-linux-gnu.
>
> Co-authored-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>  sysdeps/unix/sysv/linux/kernel-features.h |  6 ++
>  sysdeps/unix/sysv/linux/syscalls.list     |  1 -
>  sysdeps/unix/sysv/linux/wait4.c           | 88 +++++++++++++++++++++++
>  3 files changed, 94 insertions(+), 1 deletion(-)
>  create mode 100644 sysdeps/unix/sysv/linux/wait4.c
>
> diff --git a/sysdeps/unix/sysv/linux/kernel-features.h b/sysdeps/unix/sysv/linux/kernel-features.h
> index e6be76ff46..43faaa3f9d 100644
> --- a/sysdeps/unix/sysv/linux/kernel-features.h
> +++ b/sysdeps/unix/sysv/linux/kernel-features.h
> @@ -208,4 +208,10 @@
>  # define __ASSUME_TIME64_SYSCALLS 1
>  #endif
>
> +/* Linux waitid prior kernel 5.4 does not support waiting for the current
> +   process group.  */
> +#if __LINUX_KERNEL_VERSION >= 0x050400
> +# define __ASSUME_WAITID_PID0_P_PGID
> +#endif
> +
>  #endif /* kernel-features.h */
> diff --git a/sysdeps/unix/sysv/linux/syscalls.list b/sysdeps/unix/sysv/linux/syscalls.list
> index 603e517ca6..5f1352ad43 100644
> --- a/sysdeps/unix/sysv/linux/syscalls.list
> +++ b/sysdeps/unix/sysv/linux/syscalls.list
> @@ -67,7 +67,6 @@ swapoff               -       swapoff         i:s     __swapoff       swapoff
>  unshare                EXTRA   unshare         i:i     unshare
>  uselib         EXTRA   uselib          i:s     __compat_uselib uselib@GLIBC_2.0:GLIBC_2.23
>  utime          -       utime           i:sP    utime
> -wait4          -       wait4           i:iWiP  __wait4         wait4
>
>  chown          -       chown           i:sii   __libc_chown    __chown chown
>
> diff --git a/sysdeps/unix/sysv/linux/wait4.c b/sysdeps/unix/sysv/linux/wait4.c
> new file mode 100644
> index 0000000000..d09917c7ff
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/wait4.c
> @@ -0,0 +1,88 @@
> +/* Wait for process to change state.  Linux version.
> +   Copyright (C) 2019 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <sys/wait.h>
> +#include <sys/resource.h>
> +#include <sysdep-cancel.h>
> +
> +pid_t
> +__wait4 (pid_t pid, int *stat_loc, int options, struct rusage *usage)
> +{
> +#if __NR_wait4

This needs to be #ifdef as __NR_wait4 doesn't exist on RV32.

Alistair

> +   return SYSCALL_CANCEL (wait4, pid, stat_loc, options, usage);
> +#elif defined (__ASSUME_WAITID_PID0_P_PGID)
> +  idtype_t idtype = P_PID;
> +
> +  if (pid < -1)
> +    {
> +      idtype = P_PGID;
> +      pid *= -1;
> +    }
> +  else if (pid == -1)
> +    {
> +      idtype = P_ALL;
> +    }
> +  else if (pid == 0)
> +    {
> +      idtype = P_PGID;
> +    }
> +
> +  options |= WEXITED;
> +
> +  siginfo_t infop;
> +  if (SYSCALL_CANCEL (waitid, idtype, pid, &infop, options, usage) < 0)
> +    return -1;
> +
> +  if (stat_loc)
> +    {
> +      switch (infop.si_code)
> +        {
> +        case CLD_EXITED:
> +          *stat_loc = W_EXITCODE (infop.si_status, 0);
> +          break;
> +        case CLD_DUMPED:
> +          *stat_loc = WCOREFLAG | infop.si_status;
> +         break;
> +        case CLD_KILLED:
> +          *stat_loc = infop.si_status;
> +          break;
> +        case CLD_TRAPPED:
> +        case CLD_STOPPED:
> +          *stat_loc = W_STOPCODE (infop.si_status);
> +          break;
> +        case CLD_CONTINUED:
> +          *stat_loc = __W_CONTINUED;
> +          break;
> +       default:
> +         *stat_loc = 0;
> +         break;
> +        }
> +    }
> +
> +  return infop.si_pid;
> +# else
> +/* Linux waitid prior kernel 5.4 does not support waiting for the current
> +   process.  It would be possible to emulate it by calling getpgid for pid 0,
> +   however, it would require an additional syscall and it is inherent racy:
> +   after the current process group is received and before it is passed
> +   to waitid a signal could arrive causing the current process group to
> +   change.  */
> +# error "The kernel ABI does not provide a way to implement wait4"
> +#endif
> +}
> +weak_alias (__wait4, wait4)
> --
> 2.17.1
>