s3: smbd: Don't loop infinitely on bad-symlink resolution.

In the FILE_OPEN_IF case we have O_CREAT, but not
O_EXCL. Previously we went into a loop trying first
~(O_CREAT|O_EXCL), and if that returned ENOENT
try (O_CREAT|O_EXCL). We kept looping indefinately
until we got an error, or the file was created or
opened.

The big problem here is dangling symlinks. Opening
without O_NOFOLLOW means both bad symlink
and missing path return -1, ENOENT from open(). As POSIX
is pathname based it's not possible to tell
the difference between these two cases in a
non-racy way, so change to try only two attempts before
giving up.

We don't have this problem for the O_NOFOLLOW
case as we just return NT_STATUS_OBJECT_PATH_NOT_FOUND
mapped from the ELOOP POSIX error and immediately
returned.

Unroll the loop logic to two tries instead.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12572

Pair-programmed-with: Ralph Boehme <slow@samba.org>

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>

diff --git a/source3/smbd/open.c b/source3/smbd/open.c
index 931d76df44f22d35cb6b1e0feb5bd73b04c436e3..37c630b7e5abe4ae6b0a25a210583c73aadbc96a 100644 (file)
--- a/source3/smbd/open.c
+++ b/source3/smbd/open.c
@@ -640,7 +640,9 @@ static NTSTATUS fd_open_atomic(struct connection_struct *conn,
                        bool *file_created)
 {
        NTSTATUS status = NT_STATUS_UNSUCCESSFUL;
+       NTSTATUS retry_status;
        bool file_existed = VALID_STAT(fsp->fsp_name->st);
+       int curr_flags;
 
        *file_created = false;
 
@@ -672,59 +674,65 @@ static NTSTATUS fd_open_atomic(struct connection_struct *conn,
         * we can never call O_CREAT without O_EXCL. So if
         * we think the file existed, try without O_CREAT|O_EXCL.
         * If we think the file didn't exist, try with
-        * O_CREAT|O_EXCL. Keep bouncing between these two
-        * requests until either the file is created, or
-        * opened. Either way, we keep going until we get
-        * a returnable result (error, or open/create).
+        * O_CREAT|O_EXCL.
+        *
+        * The big problem here is dangling symlinks. Opening
+        * without O_NOFOLLOW means both bad symlink
+        * and missing path return -1, ENOENT from open(). As POSIX
+        * is pathname based it's not possible to tell
+        * the difference between these two cases in a
+        * non-racy way, so change to try only two attempts before
+        * giving up.
+        *
+        * We don't have this problem for the O_NOFOLLOW
+        * case as it just returns NT_STATUS_OBJECT_PATH_NOT_FOUND
+        * mapped from the ELOOP POSIX error.
         */
 
-       while(1) {
-               int curr_flags = flags;
+       curr_flags = flags;
 
-               if (file_existed) {
-                       /* Just try open, do not create. */
-                       curr_flags &= ~(O_CREAT);
-                       status = fd_open(conn, fsp, curr_flags, mode);
-                       if (NT_STATUS_EQUAL(status,
-                                       NT_STATUS_OBJECT_NAME_NOT_FOUND)) {
-                               /*
-                                * Someone deleted it in the meantime.
-                                * Retry with O_EXCL.
-                                */
-                               file_existed = false;
-                               DEBUG(10,("fd_open_atomic: file %s existed. "
-                                       "Retry.\n",
-                                       smb_fname_str_dbg(fsp->fsp_name)));
-                                       continue;
-                       }
-               } else {
-                       /* Try create exclusively, fail if it exists. */
-                       curr_flags |= O_EXCL;
-                       status = fd_open(conn, fsp, curr_flags, mode);
-                       if (NT_STATUS_EQUAL(status,
-                                       NT_STATUS_OBJECT_NAME_COLLISION)) {
-                               /*
-                                * Someone created it in the meantime.
-                                * Retry without O_CREAT.
-                                */
-                               file_existed = true;
-                               DEBUG(10,("fd_open_atomic: file %s "
-                                       "did not exist. Retry.\n",
-                                       smb_fname_str_dbg(fsp->fsp_name)));
-                               continue;
-                       }
-                       if (NT_STATUS_IS_OK(status)) {
-                               /*
-                                * Here we've opened with O_CREAT|O_EXCL
-                                * and got success. We *know* we created
-                                * this file.
-                                */
-                               *file_created = true;
-                       }
+       if (file_existed) {
+               curr_flags &= ~(O_CREAT);
+               retry_status = NT_STATUS_OBJECT_NAME_NOT_FOUND;
+       } else {
+               curr_flags |= O_EXCL;
+               retry_status = NT_STATUS_OBJECT_NAME_COLLISION;
+       }
+
+       status = fd_open(conn, fsp, curr_flags, mode);
+       if (NT_STATUS_IS_OK(status)) {
+               if (!file_existed) {
+                       *file_created = true;
                }
-               /* Create is done, or failed. */
-               break;
+               return NT_STATUS_OK;
        }
+       if (!NT_STATUS_EQUAL(status, retry_status)) {
+               return status;
+       }
+
+       curr_flags = flags;
+
+       /*
+        * Keep file_existed up to date for clarity.
+        */
+       if (NT_STATUS_EQUAL(status, NT_STATUS_OBJECT_NAME_NOT_FOUND)) {
+               file_existed = false;
+               curr_flags |= O_EXCL;
+               DBG_DEBUG("file %s did not exist. Retry.\n",
+                       smb_fname_str_dbg(fsp->fsp_name));
+       } else {
+               file_existed = true;
+               curr_flags &= ~(O_CREAT);
+               DBG_DEBUG("file %s existed. Retry.\n",
+                       smb_fname_str_dbg(fsp->fsp_name));
+       }
+
+       status = fd_open(conn, fsp, curr_flags, mode);
+
+       if (NT_STATUS_IS_OK(status) && (!file_existed)) {
+               *file_created = true;
+       }
+
        return status;
 }