[PATCH] Linux: Add execveat system call wrapper

[Florian Weimer]

* Adhemerval Zanella via Libc-alpha:

> On 28/04/2020 09:20, Alexandra Hájková via Libc-alpha wrote:
>
>> +/* 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;
>> +
>
> Is this subject to a TOCTOU where a thread might close the fd by explicitly
> reading /proc/self/fd/?

Sure, but that's as undefined as closing a file descriptor that the
code has not opened.  This impacts any code in glibc which opens file
descriptors.  I don't think it's a problem.