On Tue, 2024-12-10 at 12:02 -0500, James Bottomley wrote: > Even though this fixes the bug that a create either not followed by a > write or followed by a write that errored would leave a remnant file > for the variable, the file will appear momentarily globally visible > until the close of the fd deletes it. This is safe because the > normal filesystem operations will mediate any races; however, it is > still possible for a directory listing at that instant between create > and close contain a variable that doesn't exist in the EFI table. Systemd doesn't like 0 length files appearing in efivarfs, even if only momentarily, so I think this needs updating to prevent even momentary instances of zero length files: https://github.com/systemd/systemd/issues/34304 These occur for two reasons 1. The system has hibernated and resumed and the dcache entries are now out of sync with the original variables 2. between the create and a successful write of a variable being created in efivarfs 1. can only really be fixed by adding a hibernation hook to the filesystem code, which would be a separate patch set (which I'll work on after we get this upstream); but 2. can be fixed by ensuring that all variables returned from .create aren't visible in the directory listing until a successful write. Since we need the file to be visible to lookups but not the directory, the only two ways of doing this are either to mark the directory in a way that libfs.c:dcache_readdir() won't see it ... I think this would have to be marking it as a cursor (we'd remove the cursor mark on successful write); or to implement our own .iterate_shared function and hijack the actor to skip newly created files (this is similar to what overlayfs does to merge directories) which would be identified as having zero size. Do the fs people have a preference? The cursor mark is simpler to implement but depends on internal libfs.c magic. The actor hijack is at least something that already exists, so would be less prone to breaking due to internal changes. Regards, James
