Re: [PATCH v3 09/12] man/man2/open_tree.2: document "new" mount API

[Askar Safin <safinaskar@xxxxxxxxxxxx>]

 ---- On Sat, 09 Aug 2025 00:39:53 +0400  Aleksa Sarai <cyphar@xxxxxxxxxx> wrote --- 
 > +If
 > +.I flags
 > +does not contain
 > +.BR \%OPEN_TREE_CLONE ,
 > +.BR open_tree ()
 > +returns a file descriptor
 > +that is exactly equivalent to
 > +one produced by
 > +.BR openat (2)

This is not true. They differ in handling of automounts.
open_tree follows them in final component (by default),
and openat - not.

See reproducer in the end of this letter.

I suggest merely adding this:
> that is exactly equivalent to one produced by openat(2) (modulo automounts)

--
Askar Safin
https://types.pl/@safinaskar


// Root in initial user namespace

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sched.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <linux/openat2.h>

#define MY_ASSERT(cond) do { \
    if (!(cond)) { \
        fprintf (stderr, "%s: assertion failed\n", #cond); \
        exit (1); \
    } \
} while (0)

bool
tracing_mounted (void)
{
    struct statx tracing;
    if (statx (AT_FDCWD, "/tmp/debugfs/tracing", AT_NO_AUTOMOUNT, 0, &tracing) != 0)
        {
            perror ("statx tracing");
            exit (1);
        }
    if (!(tracing.stx_attributes_mask & STATX_ATTR_MOUNT_ROOT))
        {
            fprintf (stderr, "???\n");
            exit (1);
        }
    return tracing.stx_attributes & STATX_ATTR_MOUNT_ROOT;
}

void
mount_debugfs (void)
{
    if (mount (NULL, "/tmp/debugfs", "debugfs", 0, NULL) != 0)
        {
            perror ("mount debugfs");
            exit (1);
        }
    MY_ASSERT (!tracing_mounted ());
}

void
umount_debugfs (void)
{
    umount ("/tmp/debugfs/tracing"); // Ignore errors
    if (umount ("/tmp/debugfs") != 0)
        {
            perror ("umount debugfs");
            exit (1);
        }
}

int
main (void)
{
    // Init
    {
        if (chdir ("/") != 0)
            {
                perror ("chdir /");
                exit (1);
            }
        if (unshare (CLONE_NEWNS) != 0)
            {
                perror ("unshare");
                exit (1);
            }
        if (mount (NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) != 0)
            {
                perror ("mount(NULL, /, NULL, MS_REC | MS_PRIVATE, NULL)");
                exit (1);
            }
        if (mount (NULL, "/tmp", "tmpfs", 0, NULL) != 0)
            {
                perror ("mount tmpfs");
                exit (1);
            }
    }
    if (mkdir ("/tmp/debugfs", 0777) != 0)
        {
            perror ("mkdir(/tmp/debugfs)");
            exit (1);
        }

    // open(O_PATH) doesn't follow automounts
    {
        mount_debugfs ();
        {
            int fd = open ("/tmp/debugfs/tracing", O_PATH);
            MY_ASSERT (fd >= 0);
            MY_ASSERT (close (fd) == 0);
        }
        MY_ASSERT (!tracing_mounted ());
        umount_debugfs ();
    }

    // open_tree does follow automounts (by default)
    {
        mount_debugfs ();
        {
            int fd = open_tree (AT_FDCWD, "/tmp/debugfs/tracing", 0);
            MY_ASSERT (fd >= 0);
            MY_ASSERT (close (fd) == 0);
        }
        MY_ASSERT (tracing_mounted ());
        umount_debugfs ();
    }

    // open (O_PATH | O_DIRECTORY)
    {
        mount_debugfs ();
        {
            int fd = open ("/tmp/debugfs/tracing", O_PATH | O_DIRECTORY);
            MY_ASSERT (fd >= 0);
            MY_ASSERT (close (fd) == 0);
        }
        MY_ASSERT (tracing_mounted ());
        umount_debugfs ();
    }

    // AT_NO_AUTOMOUNT
    {
        mount_debugfs ();
        {
            int fd = open_tree (AT_FDCWD, "/tmp/debugfs/tracing", AT_NO_AUTOMOUNT);
            MY_ASSERT (fd >= 0);
            MY_ASSERT (close (fd) == 0);
        }
        MY_ASSERT (!tracing_mounted ());
        umount_debugfs ();
    }

    printf ("All tests passed\n");
    exit (0);
}