Re: [PATCH v3 08/10] mm/shmem_luo: Allow preserving tmpfs files that are still linked

[Pasha Tatashin <pasha.tatashin@soleen.com>]

> Allow preserving tmpfs files that are still linked. To preserve any
> file, require permission to write to it and the file to be opened for
> reading, and reject append-only files.
> 
> shmem_luo already accepts any shmem file without links, which includes
> unlinked tmpfs files and files created with O_TMPFILE. Accepting linked
> files means that userspace doesn't have to unlink files to preserve them
> and can keep using them by path until the live update.
> 
> The path of the file isn't preserved. Like other files from tmpfs
> mounts, a linked file is retrieved onto a tmpfs mount chosen by
> userspace, which can then link it at the path it wants.
> 
> While a file is preserved, its size is frozen, so truncating it,
> fallocate() and writes past its end fail with -EPERM. For a linked file,
> this affects everyone who opens the file by its path, so a caller that
> can only read the file, e.g. a world-readable file in /dev/shm, could
> use it to break the owner of the file. So require permission to write to
> the file to preserve it, for all files. Like FS_IOC_ENABLE_VERITY, don't
> require a file opened for writing, so that binaries that are being
> executed can still be preserved through read-only fds. Also don't
> require a writable mount, since preserving a file doesn't write to the
> filesystem. If the size of the file can't change anyway, because it is
> immutable or on a read-only superblock, also let its owner preserve it.
> Don't let others do that, since the size could change again once the
> file is made writable. Like FS_IOC_ENABLE_VERITY, reject append-only
> files, since their writers can only append to them, and freezing their
> size would stop all appends.
> 
> Also like FS_IOC_ENABLE_VERITY, require the file to be opened for
> reading. The retrieved file can be read, so otherwise a caller that can
> only write to a file could preserve it to read its contents.
> 
> Preserving a file also fills its holes, but don't require any permission
> for that. Anyone who can read a tmpfs file can already fill its holes by
> faulting them in through a read-only mapping, also on read-only mounts
> and for immutable files.
> 
> Document the frozen size and the permission requirements.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> Link: https://patch.msgid.link/20261103010216.3447716-9-dmatlack@google.com

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

-- 
Pasha