On Tue, Dec 10, 2024 at 05:48:40AM +0100, Mateusz Guzik wrote: > Oh huh. I had seen that code before, did not mentally register there > may be repeat file alloc/free calls due to repeat path_openat. > > Indeed it would be nice if someone(tm) sorted it out, but I don't see > how this has any relation to installing the file early and thus having > fget worry about it. Other than the former being an obvious prereq for the latter? Not much... > Suppose the "embryo"/"larval" file pointer is to be installed early > and populated later. I don't see a benefit but do see a downside: this > requires protection against close() on the fd (on top of dup2 needed > now). > The options that I see are: > - install the file with a refcount of 2, let dup2/close whack it, do a > fput in open to bring back to 1 or get rid of it if it raced (yuck) > (freebsd is doing this) > - dup2 is already special casing to not mess with it, add that to > close as well (also yuck imo) As a possibility (again, I'm not sold on the benefits of that scheme, just looking into feasibility): dup2() when evicting an embryo: mark it evicted remove from descriptor table do nothing to refcount (in effect, transfer it to open()) then proceed as if it hadn't been there [== pretend that dup2() always loses the race] close() when running into an embryo return -EBADF [== pretend that close() always loses the race] open() when it's done setting file up: if opening failed if not marked evicted remove from descriptor table fput() return whatever error we've got else if marked evicted fput() return the descriptor [== pretend that open() always wins the race] "open" in the above stands for everything that opens a descriptor - socket(2), pipe(2), eventfd(2), whatever. > >From userspace side the only programs which can ever see EBUSY are > buggy or trying to screw the kernel, so not a concern on that front. Agreed. I'm not saying we should go that way.
