smbd: Fix crossing automounter mount points

We get ENOENT on /proc/self/fd/<fdnum> when we try to turn the
mountpoint's O_PATH handle into a real one. This does not trigger a
mount attempt, you have to use name-based calls.

This is not the real fix, because if the autofs mount triggers, the
inode number will change. For directories this is not a huge problem
as we don't touch the share mode database before we open the "real"
fd. We would only violate potential share modes with other pure
READ_ATTRIBUTES (i.e. stat-) opens that came before the mount
trigger.

As I don't think share modes on directories are really relevant, I
think we can live with this "fix". Once we do directory leases this
will potentially change.

As a quick remedy we could use our defer_open() mechanism that starts
path processing from scratch. But as long as this seems not really
required, we should not add users of this really bad way of going back
to square 1.

The "real" fix would be to go back to the point where we open the last
component with openat(). In the retry round we need to do this without
O_PATH to trigger the mount and only then do the initial fstat.

Right now I don't see an easy way to properly test this
behaviour. Intercepting with vfs_error_inject is certainly possible,
but nailing the exact call would clutter the main code path. So I have
just tested this manually.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Jun 27 11:59:05 UTC 2024 on atb-devel-224

diff --git a/source3/smbd/open.c b/source3/smbd/open.c
index 7d2227f9d9f229549a95417f6387da5bf203a868..20db4cf5e231ad1fb0ca2d23e2b961537c4a294c 100644 (file)
--- a/source3/smbd/open.c
+++ b/source3/smbd/open.c
@@ -43,6 +43,10 @@
 #include "lib/util/time_basic.h"
 #include "source3/smbd/dir.h"
 
+#if defined(HAVE_LINUX_MAGIC_H)
+#include <linux/magic.h>
+#endif
+
 extern const struct generic_mapping file_generic_mapping;
 
 struct deferred_open_record {
@@ -1180,6 +1184,27 @@ static NTSTATUS reopen_from_fsp(struct files_struct *dirfsp,
                                        fsp,
                                        how);
                if (new_fd == -1) {
+#if defined(HAVE_FSTATFS) && defined(HAVE_LINUX_MAGIC_H)
+                       if (S_ISDIR(fsp->fsp_name->st.st_ex_mode) &&
+                           (errno == ENOENT)) {
+                               struct statfs sbuf = {};
+                               int ret = fstatfs(old_fd, &sbuf);
+                               if (ret == -1) {
+                                       int saved_errno = errno;
+                                       DBG_ERR("fstatfs failed: %s\n",
+                                               strerror(errno));
+                                       errno = saved_errno;
+                               } else if (sbuf.f_type == AUTOFS_SUPER_MAGIC) {
+                                       /*
+                                        * When reopening an as-yet
+                                        * unmounted autofs mount
+                                        * point we get ENOENT. We
+                                        * have to retry pathbased.
+                                        */
+                                       goto namebased_open;
+                               }
+                       }
+#endif
                        status = map_nt_error_from_unix(errno);
                        fd_close(fsp);
                        return status;
@@ -1194,6 +1219,7 @@ static NTSTATUS reopen_from_fsp(struct files_struct *dirfsp,
                return NT_STATUS_OK;
        }
 
+namebased_open:
        /*
         * Close the existing pathref fd and set the fsp flag
         * is_pathref to false so we get a "normal" fd this time.

diff --git a/source3/wscript b/source3/wscript
index 4048ec5aad50c4be3958e53b7c6f4418e8af4301..d8f04646b0320accce2c3ee07b8408a15e163803 100644 (file)
--- a/source3/wscript
+++ b/source3/wscript
@@ -118,6 +118,7 @@ def configure(conf):
 
     conf.CHECK_HEADERS('netdb.h')
     conf.CHECK_HEADERS('linux/falloc.h linux/ioctl.h')
+    conf.CHECK_HEADERS('linux/magic.h')
 
     conf.CHECK_FUNCS('getcwd fchown chmod fchmod mknod mknodat')
     conf.CHECK_FUNCS('strtol strchr strupr chflags fchflags')
@@ -531,6 +532,10 @@ return acl_get_perm_np(permset_d, perm);
     if conf.CHECK_FUNCS('dirfd'):
         conf.DEFINE('HAVE_DIRFD_DECL', 1)
 
+    conf.CHECK_CODE('struct statfs fsd; int r = fstatfs(AT_FDCWD, &fsd);',
+                    'HAVE_FSTATFS',
+                    msg='Checking for fstatfs',
+                    headers='sys/statfs.h fcntl.h')
     conf.CHECK_CODE('struct statfs fsd; fsid_t fsid = fsd.f_fsid; return statfs(".", &fsd);',
                     'HAVE_STATFS_F_FSID',
                     msg="vfs_fileid checking for statfs() and struct statfs.f_fsid",