Re: [PATCH] Linux: Add execveat system call wrapper

[Florian Weimer <fw@deneb.enyo.de>]

* Alexandra Hájková via Libc-alpha:

> diff --git a/sysdeps/mach/hurd/i386/libc.abilist b/sysdeps/mach/hurd/i386/libc.abilist
> index 60696d827f..571c281b06 100644
> --- a/sysdeps/mach/hurd/i386/libc.abilist
> +++ b/sysdeps/mach/hurd/i386/libc.abilist
> @@ -2182,6 +2182,7 @@ GLIBC_2.3.4 xdr_quad_t F
>  GLIBC_2.3.4 xdr_u_quad_t F
>  GLIBC_2.30 twalk_r F
>  GLIBC_2.32 mach_print F
> +GLIBC_2.32 execveat F
>  GLIBC_2.32 thrd_current F
>  GLIBC_2.32 thrd_equal F
>  GLIBC_2.32 thrd_sleep F
> diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile

This is not correct because you are not adding execveat for Hurd.  You
can just remove that change.

> diff --git a/sysdeps/unix/sysv/linux/bits/unistd_ext.h b/sysdeps/unix/sysv/linux/bits/unistd_ext.h
> index c315cc5cb8..55225b5447 100644
> --- a/sysdeps/unix/sysv/linux/bits/unistd_ext.h
> +++ b/sysdeps/unix/sysv/linux/bits/unistd_ext.h
> @@ -17,7 +17,7 @@
>     <https://www.gnu.org/licenses/>.  */
>  
>  #ifndef _UNISTD_H
> -# error "Never include <bits/unistd_ext.h> directly; use <unistd.h> instead."
> +#error "Never include <bits/unistd_ext.h> directly; use <unistd.h> instead."
>  #endif

Spurious change, please remove it.

> diff --git a/sysdeps/unix/sysv/linux/execveat.c b/sysdeps/unix/sysv/linux/execveat.c
> new file mode 100644
> index 0000000000..7ebf397091
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/execveat.c
> @@ -0,0 +1,67 @@
> +/* Copyright (C) 1994-2020 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 <errno.h>
> +#include <stddef.h>
> +#include <stdio.h>
> +#include <unistd.h>
> +#include <fcntl.h>
> +#include <sys/stat.h>
> +
> +#include <sysdep.h>
> +#include <sys/syscall.h>
> +#include <kernel-features.h>
> +#include <fd_to_filename.h>
> +
> +
> +/* Execute the file FD refers to, overlaying the running program image.
> +   ARGV and ENVP are passed to the new program, as for `execve'.  */
> +int
> +execveat (int dirfd, const char *path, char *const argv[], char *const envp[],
> +          int flags)
> +{
> +  /* Avoid implicit array coercion in syscall macros.  */
> +  INLINE_SYSCALL_CALL (execveat, dirfd, path, &argv[0], &envp[0], flags);
> +#ifndef __ASSUME_EXECVEAT
> +  if (errno != ENOSYS)
> +    return -1;
> +
> +  int fd = openat (dirfd, path, flags | O_CLOEXEC);
> +  if (fd < 0)
> +    return -1;
> +
> +  struct fd_to_filename fdfilename;
> +  const char *gfilename
> +    = path != NULL ? path : __fd_to_filename (fd, &fdfilename);
> +
> +  /* We do not need the return value.  */
> +  __execve (gfilename, argv, envp);
> +
> +  int save = errno;
> +
> +  /* We come here only if the 'execve' call fails.  Determine whether
> +     /proc is mounted.  If not we return ENOSYS.  */
> +  struct stat st;
> +  if (stat ("/proc/self/fd", &st) != 0 && errno == ENOENT)
> +    save = ENOSYS;
> +
> +  close (fd);
> +  __set_errno (save);
> +#endif
> +
> +  return -1;
> +}

I think you should use __openat_nocancel and __close_nocancel (from
<not-cancel.h>) to avoid introducing cancellation points.  It will
also fix the elf/check-localplt failures.

> diff --git a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> index 6418ace78a..8f4388bf44 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> @@ -2061,6 +2061,7 @@ GLIBC_2.30 gettid F
>  GLIBC_2.30 tgkill F
>  GLIBC_2.30 twalk_r F
>  GLIBC_2.32 pthread_sigmask F
> +GLIBC_2.32 execveat F
>  GLIBC_2.4 __confstr_chk F
>  GLIBC_2.4 __fgets_chk F
>  GLIBC_2.4 __fgets_unlocked_chk F

This is quite odd.  Did you update the ABI list manually?  The order
of pthread_sigmask and execveat is wrong.  This is what causes
elf/check-abi-libc failures.