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

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

On 21/04/2021 15:11, Alexandra Hájková via Libc-alpha wrote:
> From: Alexandra Hájková <ahajkova@redhat.com>
> 
>  Also add the test for the new wrapper.

Patch looks ok in general, but there are minor issues. Since it is being
back and forth for some time, I took the liberty to fix them and I will
push it upstream.

> 
> diff --git a/NEWS b/NEWS
> index aa0f10a891..0765db4543 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -18,6 +18,11 @@ Major new features:
>    a dump of information related to IFUNC resolver operation and
>    glibc-hwcaps subdirectory selection.
>  
> +* The function execveat has been added and it operates similar to execve.
> +  The syscall is already used to implement fexecve without requiring /proc to
> +  be mounted.  Similar to fexecve, if the syscall is not supported ENOSYS is
> +  returned.
> +

With the fallback removal, execveat does not act similar to fexecve.  I changed
the NEWS entry to:

  * On Linux, the function execveat has been added.  It operates similar to
    execve and it is is already used to implement fexecve without requiring
    /proc to be mounted.  However, different than fexecve, if the syscall is not
    supported by the kernel an error is returned instead of trying a fallback.


> +int
> +call_execveat (int fd, const char *pathname, int flags, int expected_fail,
> +               int num)
> +{
> +  char *envp[] = { (char *) "FOO=3", NULL };
> +  char *argv[] = { (char *) "sh", (char *) "-c", (char *) "exit $FOO", NULL };
> +  pid_t pid;
> +  int status;
> +
> +  if (test_verbose > 0)
> +    printf ("call line number: %d\n", num);
> +
> +  pid = xfork ();
> +  if (pid == 0)
> +    {
> +      TEST_COMPARE (execveat (fd, pathname, argv, envp, flags), -1);
> +      if (errno == ENOSYS)
> +        FAIL_UNSUPPORTED ("execveat is unimplemented");

This will print 'execveat is unimplemented', since the WEXITSTATUS below
will already handle it.  Instead I replace it with:

  exit (EXIT_UNSUPPORTED); 


> +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 ((flags & ~(AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW)) != 0)
> +    return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
> +
> +  if (errno != ENOSYS)
> +    return -1;
> +  else
> +    return ENOSYS;
> +#endif
> +
> +  return -1;
> +}

Since there is no fallback anymore, there is no need to check for flag
support neither handle the errno value.  It can be simple as:

  int
  execveat (int dirfd, const char *path, char *const argv[], char *const envp[],
            int flags)
  {
    /* Avoid implicit array coercion in syscall macros.  */
    return INLINE_SYSCALL_CALL (execveat, dirfd, path, &argv[0], &envp[0],
                                flags);
  }