On Sat, 22 Jun 2024 at 14:25, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote: > > +cc Linus Thanks. > To sum up the problem: stat and statx met with "" + AT_EMPTY_PATH have > more work to do than fstat and its hypotethical statx counterpart: > - buf alloc/free for the path > - userspace access (very painful on x86_64 + SMAP) > - lockref acquire/release Yes. That LOOKUP_EMPTY_NOCHECK is *not* the fix. I do think that we should make AT_EMPTY_PATH with a NULL path "JustWork(tm)", because the stupid "look if the pathname is empty" is horrible. But moving that check into getname() is *NOT* the right answer, because by the time you get to getname(), you have already lost. There's a very real reason why vfs_fstatat() catches this empty case early, and never goes to filename lookup at all. You don't want to generate a 'struct path' from the 'int fd', because you want to never get anywhere close to that path, and instead only ever need a 'struct fd' that can be looked up much more cheaply (particularly if not in a threaded environment). So the short-cut in vfs_fstatat() to never get a pathname is disgusting - people should have used 'fstat()' - but it's _important_ disgusting. This thing that tries to short-circuit things at the path level is too late. Linus