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

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella via Libc-alpha:

>> So I think we have to do this:
>> 
>> * If there are more flags than just the two, fail with EINVAL.
>> 
>> * To handle AT_EMPTY_PATH, do not open a new file descriptor (using
>>   openat) if AT_EMPTY_PATH is specified *and* the file name is "".
>> 
>> * To handle AT_SYMLINK_NOFOLLOW, openat needs to be called with
>>   O_NOFOLLOW in that case (in addition to O_CLOEXEC).
>
> These will surely need to be on the testcase.

Yes, these permutations need to be tested.

>> The behavior with AT_EMPTY_PATH/"" and AT_SYMLINK_NOFOLLOW at the same
>> time is not immedately obvious from the kernel code, so I wrote a small
>> test program (/bin/sh is a symbolic link to /bin/bash on this system):
>> 
>> #include <err.h>
>> #include <fcntl.h>
>> #include <sys/syscall.h>
>> #include <unistd.h>
>> 
>> int
>> main (void)
>> {
>>   int fd = open ("/bin/sh", O_PATH | O_NOFOLLOW);
>>   if (fd < 0)
>>     err (1, "open");
>>   static char *const argv[] = { "sh", "-c", "exit 0", NULL };
>>   static char *const envp[] = { NULL };
>>   syscall (SYS_execveat, fd, "", argv, envp,
>>            AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
>>   err (1, "execveat");
>> }
>> 
>> This fails:
>> 
>> openat(AT_FDCWD, "/bin/sh", O_RDONLY|O_NOFOLLOW|O_PATH) = 3
>> execveat(3, "", ["sh", "-c", "exit 0"], 0x402040 /* 0 vars */, AT_SYMLINK_NOFOLLOW|AT_EMPTY_PATH) = -1 ELOOP (Too many levels of symbolic links)
>> […]
>> execveat-opath-symlink: execveat: Too many levels of symbolic links
>> 
>> So I think for the AT_EMPTY_PATH/"" and AT_SYMLINK_NOFOLLOW case, we
>> need to call fstatat64 with AT_EMPTY_PATH and see if st_mode indicates
>> that the descriptor refers to a symbolic link.  If it does, the function
>> needs to fail with ELOOP.
>
> I think execve would handle it:
>
> openat(AT_FDCWD, "/bin/sh", O_RDONLY|O_NOFOLLOW|O_PATH) = 3
> execve("/proc/self/fd/3", ["sh", "-c", "echo test"], 0x556815e580a8 /* 0 vars */) = -1 ELOOP (Too many levels of symbolic   links)

And execveat fails even without AT_SYMLINK_NOFOLLOW:

openat(AT_FDCWD, "/bin/sh", O_RDONLY|O_NOFOLLOW|O_PATH) = 3
execveat(3, "", ["sh", "-c", "exit 0"], 0x402040 /* 0 vars */, AT_EMPTY_PATH) = -1 ELOOP (Too many levels of symbolic links)

Does this mean we do not need a special case for an O_PATH|O_NOFOLLOW
open of a symbolic link?

Thanks,
Florian