Hi Askar, Carlos,

On Sun, Feb 16, 2025 at 06:18:28AM +0000, Askar Safin wrote:

I verified using expirement that modern glibc wrapper getcwd actually never returns "(unreachable)".
Also I have read modern glibc sources for all 3 functions documented here.
All they don't return "(unreachable)".
Also I changed "pathname" to "pathnames".

Now let me describe my expirement. I compiled this C source:

===
#include <sys/syscall.h>
#include <unistd.h>
#include <stdio.h>

int
main (void)
{
  char buf[1000];
  if(syscall(SYS_getcwd, buf, sizeof(buf)) == -1)
    {
      perror ("SYS_getcwd");
    }
  else
    {
      printf ("SYS_getcwd: %s\n", buf);
    }
  if(getcwd(buf, sizeof(buf)) == NULL)
    {
      perror ("getcwd");
    }
  else
    {
      printf ("getcwd: %s\n", buf);
    }
  return 0;
}
===

to a binary /tmp/a and run the following command:

$ sudo unshare --mount bash -c 'set -e; mkdir /tmp/dir; mount -t tmpfs tmpfs /tmp/dir; cd /tmp/dir; umount -l .; /tmp/a'

The output was:

SYS_getcwd: (unreachable)/
getcwd: No such file or directory

Thanks!  I prefer if we show a continuous shell session, which is easier
to follow than code mixed with explanations.  So I would add the
following (and indented, to show it's code; that also makes it easier to
keep the #include lines in git(1)):

	alx@devuan:~/tmp$ cat getcwd.c;
	#include <unistd.h>
	#include <stdio.h>
	#include <sys/syscall.h>

	int
	main(void)
	{
		char  buf[1000];

		if (syscall(SYS_getcwd, buf, sizeof(buf)) == -1)
			perror("SYS_getcwd");
		else
			printf("SYS_getcwd: %s\n", buf);

		if (getcwd(buf, sizeof(buf)) == NULL)
			perror("getcwd");
		else
			printf("getcwd: %s\n", buf);

		return 0;
	}
	alx@devuan:~/tmp$ gcc -Wall -Wextra -o /tmp/getcwd getcwd.c;
	alx@devuan:~/tmp$ sudo unshare --mount bash;
	root@devuan:/home/alx/tmp# set -e;
	root@devuan:/home/alx/tmp# mkdir /tmp/dir;
	root@devuan:/home/alx/tmp# mount -t tmpfs tmpfd /tmp/dir/;
	root@devuan:/home/alx/tmp# cd /tmp/dir/;
	root@devuan:/tmp/dir# umount -l .;
	root@devuan:/tmp/dir# /tmp/getcwd;
	SYS_getcwd: (unreachable)/
	getcwd: No such file or directory
	root@devuan:/tmp/dir# exit;
	exit

I have also adapted the example program to the C coding style of the
project:
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING.d/style/c>

Signed-off-by: Askar Safin <redacted>

Please add a line for Carlos:

	Cc: Carlos O'Donell [off-list ref]

As Carlos said, I prefer if this patch doesn't include the change
s/pathname/&s/.  That would reduce the diff.

Thanks for the help with this, Carlos!

+functions described in this page (until glibc 2.27)
+or raw

I think it would be more appropriate to say 's/or raw/or the raw/'.

+.BR getcwd ()
+system call

Other than those minor comments, I think this LGTM.  Please address
those, and I'll take the patch.

Carlos do you have any further comments?


Have a lovely day!
Alex

 should consider checking whether the returned pathname starts
 with '/' or '(' to avoid misinterpreting an unreachable path
 as a relative pathname.
-- 
2.39.5

-- 
<https://www.alejandro-colomar.es/>