Re: [PATCH v3 07/10] mm/shmem_luo: Retrieve tmpfs files onto a tmpfs mount chosen by userspace

[Pasha Tatashin]

> Retrieve files that were preserved from a tmpfs mount onto a tmpfs mount
> chosen by userspace, instead of as memfds on the internal tmpfs mount.
> Add struct liveupdate_shmem_retrieve_arg, which userspace passes as the
> retrieve argument of LIVEUPDATE_SESSION_RETRIEVE_FD to choose the mount.
> Record which mount a file was on with a new SHMEM_LUO_FLAG_TMPFS flag in
> struct shmem_luo_ser, based on the superblock of the file.
> 
> shmem_luo already accepts unlinked tmpfs files, such as files created
> with O_TMPFILE, and a following commit also accepts linked ones. The
> mount such a file was on doesn't exist in the new kernel, and userspace
> may want to put the file elsewhere anyway.
> 
> Model struct liveupdate_shmem_retrieve_arg on open(): The kernel creates
> a new file as if by openat(dirfd, ".", O_TMPFILE | flags, mode), so the
> file gets the owner, permission checks and security labels of any new
> file in that directory, and is charged to that mount. The kernel then
> fills the file with the preserved contents, size, position and seals
> before returning it, so nothing else can access the file before it is
> complete. Userspace links the file at the path it wants with linkat(),
> or passes O_EXCL to keep it unlinked. Neither the path nor any other
> inode attribute of the file is part of the preserved state, which keeps
> them out of the KHO ABI.
> 
> Pass the flags from userspace to file_open_root(), so that the VFS
> checks them like for open(). file_open_root() silently ignores unknown
> flags and mode bits, so reject those like openat2() does. Also reject
> O_PATH, with which file_open_root() would ignore O_TMPFILE and open the
> directory.
> 
> Keep the type of the file: memfds can only be retrieved as memfds, and
> files on tmpfs mounts only onto tmpfs mounts, so that e.g. a file
> can't be moved off the mount whose size limit and quotas it was subject
> to. Passing the argument when retrieving a memfd, or not passing it when
> retrieving a file from a tmpfs mount, fails with -EINVAL. Unlinked tmpfs
> files were retrieved as memfds until now, but nothing relies on that, as
> the LUO uAPI is not stable yet.
> 
> Like the other failures before the folios are restored, problems with
> the argument and with creating the file can be retried.
> 
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> Link: https://patch.msgid.link/20261103010216.3447716-8-dmatlack@xxxxxxxxxx

Acked-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>

-- 
Pasha