On Mon, Nov 19, 2018 at 02:49:22PM -0800, Daniel Colascione wrote: > On Mon, Nov 19, 2018 at 2:40 PM Tycho Andersen <ty...@tycho.ws> wrote: > > Can I just register an objection here that I think using a syscall > > just for this is silly? > > Yes, you can argue that the bikeshed should be ioctl-colored and not > syscall-colored. > > > My understanding is that the concern is that some code might do: > > > > unknown_fd = recv_fd(); > > ioctl(unknown_fd, SOME_IOCTL, NULL); // where SOME_IOCTL == PROC_FD_KILL > > // whoops, unknown_fd was a procfd and we killed a task! > > > > In my experience when writing fd sending/receiving code, the sender and > > receiver are fairly tightly coupled. Has anyone ever actually fixed a > > bug where they had an fd that they lost track of what "type" it was > > and screwed up like this? It seems completely theoretical to me. > > Yes, I have fixed bugs of this form. > > > The ioctl() approach has the benefit of being extensible. > > The system call table is also extensible.
But not infinitely so. The x32 ABI starts at 512, and right now I see 334 syscalls on x86_64. So the next 178 people can say "let's just define a syscall", and after that? I suppose we could move to setting BIT(10), but how much userspace assumes > 512 => compat syscall and blocks it via seccomp or whatever? Contrast that with the ioctl space, which is an unsigned long and fairly sparse still (Documentation/ioctl/ioctl-number.txt). > ioctl is for when a given piece of functionality *can't* > realistically get its own system call because it's separated from > the main kernel somehow. procfs is a core part of the kernel, so we > can and should expose interfaces to it using system calls. I suppose it's obvious, but I disagree. > > Adding a > > syscall means that everyone has to do all the boilerplate for each new > > pid op in the kernel, arches, libc, strace, etc. > > These tools also care about ioctls. Adding a system call is a pain, > but the solution is to make adding system calls less of a pain, not to > permanently make the Linux ABI worse. For user-defined values of "worse" :) Tycho
