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

On Thu, Aug 12, 2021 at 6:14 PM Gabriel Krisman Bertazi
[off-list ref] wrote:

Jan Kara [off-list ref] writes:

quoted

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?

Jan,

Looking back at the email from Amir, I realize I misunderstood his
original suggestion.  Amir suggested it be FILEID_INVALID with 0-len OR
FILEID_INO32_GEN with zeroed fields.  I mixed the two suggestions.

That was from a discussion about UAPI and agree it doesn't make much sense
to report non-zero handle_bytes and invalid handle_type.

But specifically, Jan's question above was directly referring to the internal
representation of the event.

Since you are going to allocate a file handle buffer of size MAX_HANDLE_SZ
(right?), then there is no caveat in declaring the NULL_FH with positive size
internally, which I think, simplifies the implementation.
NULL_FH internal length could be 4 instead of 8 though.

You will not have to special case fanotify_fid_info_len() and the info record
hdr.len will include a 4 bytes zero padding after the fid info record
with NULL_FH.

You will only special case this line in copy_fid_info_to_user():
    handle.handle_type = fh->type;
    if (fh->type != FILEID_INVALID)
        handle.handle_bytes = fh_len;

Thanks,
Amir.