Re: [PATCH v5 14/23] fanotify: Encode invalid file handler when no inode is provided

On Thu, Aug 12, 2021 at 5:20 PM Jan Kara [off-list ref] wrote:

On Wed 11-08-21 17:12:05, Gabriel Krisman Bertazi wrote:

quoted

Jan Kara [off-list ref] writes:

quoted

quoted

@@ -376,14 +371,24 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
           fh->flags |= FANOTIFY_FH_FLAG_EXT_BUF;
   }

-  dwords = fh_len >> 2;
-  type = exportfs_encode_inode_fh(inode, buf, &dwords, NULL);
-  err = -EINVAL;
-  if (!type || type == FILEID_INVALID || fh_len != dwords << 2)
-          goto out_err;
-
-  fh->type = type;
-  fh->len = fh_len;
+  if (inode) {
+          dwords = fh_len >> 2;
+          type = exportfs_encode_inode_fh(inode, buf, &dwords, NULL);
+          err = -EINVAL;
+          if (!type || type == FILEID_INVALID || fh_len != dwords << 2)
+                  goto out_err;
+          fh->type = type;
+          fh->len = fh_len;
+  } else {
+          /*
+           * Invalid FHs are used on FAN_FS_ERROR for errors not
+           * linked to any inode. Caller needs to guarantee the fh
+           * has at least FANOTIFY_NULL_FH_LEN bytes of space.
+           */
+          fh->type = FILEID_INVALID;
+          fh->len = FANOTIFY_NULL_FH_LEN;
+          memset(buf, 0, FANOTIFY_NULL_FH_LEN);
+  }

Maybe it will become clearer later during the series but why do you set
fh->len to FANOTIFY_NULL_FH_LEN and not 0?

Jan,

That is how we encode a NULL file handle (i.e. superblock error).  Amir
suggested it would be an invalid FILEID_INVALID, with a zeroed handle of
size 8.  I will improve the comment on the next iteration.

Thanks for info. Then I have a question for Amir I guess :) Amir, what's
the advantage of zeroed handle of size 8 instead of just 0 length file
handle?

With current code, zero fh->len means we are not reporting an FID info
record (e.g. due to encode error), see copy_info_records_to_user().

This is because fh->len plays a dual role for indicating the length of the
file handle and the existence of FID info.

I figured that keeping a positive length for the special NULL_FH is an
easy way to workaround this ambiguity and keep the code simpler.
We don't really need to pay any cost for keeping the 8 bytes zero buffer.

Thanks,
Amir.