On Wed, May 17, 2023 at 09:17:36AM -0700, Alexei Starovoitov wrote: > On Wed, May 17, 2023 at 5:05 AM Christoph Hellwig <hch@xxxxxx> wrote: > > > > On Wed, May 17, 2023 at 11:11:24AM +0200, Christian Brauner wrote: > > > Adding fsdevel so we're aware of this quirk. > > > > > > So I'm not sure whether this was ever discussed on fsdevel when you took > > > the decision to treat fd 0 as AT_FDCWD or in general treat fd 0 as an > > > invalid value. > > > > I've never heard of this before, and I think it is compltely > > unacceptable. 0 ist just a normal FD, although one that happens to > > have specific meaning in userspace as stdin. > > > > > > > > If it was discussed then great but if not then I would like to make it > > > very clear that if in the future you decide to introduce custom > > > semantics for vfs provided infrastructure - especially when exposed to > > > userspace - that you please Cc us. > > > > I don't think it's just the future. We really need to undo this ASAP. > > Christian is not correct in stating that treatment of fd==0 as invalid > bpf object applies to vfs fd-s. > The path_fd addition in this patch is really the very first one of this kind. > At the same time bpf anon fd-s (progs, maps, links, btfs) with fd == 0 > are invalid and this is not going to change. It's been uapi for a long time. > > More so fd-s 0,1,2 are not "normal FDs". > Unix has made two mistakes: > 1. fd==0 being valid fd > 2. establishing convention that fd-s 0,1,2 are stdin, stdout, stderr. > > The first mistake makes it hard to pass FD without an extra flag. > The 2nd mistake is just awful. > We've seen plenty of severe datacenter wide issues because some > library or piece of software assumes stdin/out/err. > Various services have been hurt badly by this "convention". > In libbpf we added ensure_good_fd() to make sure none of bpf objects > (progs, maps, etc) are ever seen with fd=0,1,2. > Other pieces of datacenter software enforce the same. > > In other words fds=0,1,2 are taken. They must not be anything but > stdin/out/err or gutted to /dev/null. > Otherwise expect horrible bugs and multi day debugging. > > Because of that, several years ago, we've decided to fix unix mistake #1 > when it comes to bpf objects and started reserving fd=0 as invalid. > This patch is proposing to do the same for path_fd (normal vfs fd) when It isn't as you now realized but I'm glad we cleared that up off-list. > it is passed to bpf syscall. I think it's a good trade-off and fits > the rest of bpf uapi. > > Everyone who's hiding behind statements: but POSIX is a standard.. > or this is how we've been doing things... are ignoring the practical > situation at hand. fd-s 0,1,2 are taken. Make sure your sw never produces them. (Prefix: Imagine me calmly writing this and in a relaxed tone.) Just to clarify. I do think that deciding that 0 is an invalid file descriptor number is weird and I wish you'd have discussed this with us before you took that decision. You've seen the reaction that other low-level userspace people you talked to had to these news... I'm not sure what to make of the POSIX excursion. I think that it is a complete sideshow to the issue here in a way. But fwiw... We don't follow arbitrary conventions such as 0, 1, and 2 because we all have sworn allegiance to The Secret Order of the POSIX but because the alternative is that one subsystem finds it neat to use fd 0 to refer to AT_FDCWD and another one to AT_MY_CUSTOM_FD0_MEANING. Which is exactly what would've happened if this patch would have made it unnoticed. This doesn't scale and our interfaces aren't designed around Shakespeare's dictum "What's in a name?". This will quickly devolve into a situation similar to letting a step on a staircase be off by a few millimeters. See those users falling. I'm glad we cleared this up. My main issue is indeed that fd 0 now isn't just forbidden it would be given an entirely different meaning which is not acceptable from the vfs perspective.
