Am Do., 9. Nov. 2023 um 23:00 Uhr schrieb Al Viro <viro@xxxxxxxxxxxxxxxxxx>: > On Thu, Nov 09, 2023 at 08:08:44PM +0100, Andreas Gruenbacher wrote: > > Jens, > > > > since your commit 99668f618062, applications can request cached lookups > > with the RESOLVE_CACHED openat2() flag. When adding support for that in > > gfs2, we found that this causes the ->permission inode operation to be > > called with the MAY_NOT_BLOCK flag set for directories along the path, > > which is good, but the ->permission check on the final path component is > > missing that flag. The filesystem will then sleep when it needs to read > > in the ACL, for example. > > > > This doesn't look like the intended RESOLVE_CACHED behavior. > > > > The file permission checks in path_openat() happen as follows: > > > > (1) link_path_walk() -> may_lookup() -> inode_permission() is called for > > each but the final path component. If the LOOKUP_RCU nameidata flag is > > set, may_lookup() passes the MAY_NOT_BLOCK flag on to > > inode_permission(), which passes it on to the permission inode > > operation. > > > > (2) do_open() -> may_open() -> inode_permission() is called for the > > final path component. The MAY_* flags passed to inode_permission() are > > computed by build_open_flags(), outside of do_open(), and passed down > > from there. The MAY_NOT_BLOCK flag doesn't get set. > > > > I think we can fix this in build_open_flags(), by setting the > > MAY_NOT_BLOCK flag when a RESOLVE_CACHED lookup is requested, right > > where RESOLVE_CACHED is mapped to LOOKUP_CACHED as well. > > No. This will expose ->permission() instances to previously impossible > cases of MAY_NOT_BLOCK lookups, and we already have enough trouble > in that area. True, lockdep wouldn't be happy. > See RCU pathwalk patches I posted last cycle; Do you have a pointer? Thanks. > I'm > planning to rebase what still needs to be rebased and feed the > fixes into mainline, but that won't happen until the end of this > week *AND* ->permission()-related part of code audit will need > to be repeated and extended. > > Until then - no, with the side of fuck, no. >
