On Tue, Oct 30, 2018 at 08:50:22AM +0000, Daniel Colascione wrote: > On Tue, Oct 30, 2018 at 3:21 AM, Joel Fernandes <joe...@google.com> wrote: > > On Mon, Oct 29, 2018 at 3:11 PM Daniel Colascione <dan...@google.com> wrote: > >> > >> Add a simple proc-based kill interface. To use /proc/pid/kill, just > >> write the signal number in base-10 ASCII to the kill file of the > >> process to be killed: for example, 'echo 9 > /proc/$$/kill'. > >> > >> Semantically, /proc/pid/kill works like kill(2), except that the > >> process ID comes from the proc filesystem context instead of from an > >> explicit system call parameter. This way, it's possible to avoid races > >> between inspecting some aspect of a process and that process's PID > >> being reused for some other process. > >> > >> With /proc/pid/kill, it's possible to write a proper race-free and > >> safe pkill(1). An approximation follows. A real program might use > >> openat(2), having opened a process's /proc/pid directory explicitly, > >> with the directory file descriptor serving as a sort of "process > >> handle". > > > > How long does the 'inspection' procedure take? If its a short > > duration, then is PID reuse really an issue, I mean the PIDs are not > > reused until wrap around and the only reason this can be a problem is > > if you have the wrap around while the 'inspecting some aspect' > > procedure takes really long. > > It's a race. Would you make similar statements about a similar fix for > a race condition involving a mutex and a double-free just because the > race didn't crash most of the time? The issue I'm trying to fix here > is the same problem, one level higher up in the abstraction hierarchy. > > > Also the proc fs is typically not the right place for this. Some > > entries in proc are writeable, but those are for changing values of > > kernel data structures. The title of man proc(5) is "proc - process > > information pseudo-filesystem". So its "information" right? > > Why should userspace care whether a particular operation is "changing > [a] value[] of [a] kernel data structure" or something else? That > something in /proc is a struct field is an implementation detail. It's > the interface semantics that matters, and whether a particular > operation is achieved by changing a struct field or by making a > function call is irrelevant to userspace. Proc is a filesystem about > processes. Why shouldn't you be able to send a signal to a process via > proc? It's an operation involving processes. > > It's already possible to do things *to* processes via proc, e.g., > adjust OOM killer scores. Proc filesystem file descriptors are > userspace references to kernel-side struct pid instances, and as such, > make good process handles. There are already "verb" files in procfs, > such as /proc/sys/vm/drop_caches and /proc/sysrq-trigger. Why not add > a kill "verb", especially if it closes a race that can't be closed > some other way? > > You could implement this interface as a system call that took a procfs > directory file descriptor, but relative to this proposal, it would be > all downside. Such a thing would act just the same way as > /pric/pid/kill, and wouldn't be usable from the shell or from programs > that didn't want to use syscall(2). (Since glibc isn't adding new > system call wrappers.) AFAIK, the only downside of having a "kill" > file is the need for a string-to-integer conversion, but compared to > process killing, integer parsing is insignificant. > > > IMO without a really good reason for this, it could really be a hard > > sell but the RFC was worth it anyway to discuss it ;-) > > The traditional unix process API is down there at level -10 of Rusty > Russel's old bad API scale: "It's impossible to get right". The races > in the current API are unavoidable. That most programs don't hit these > races most of the time doesn't mean that the race isn't present. > > We've moved to a model where we identify other system resources, like > DRM fences, locks, sockets, and everything else via file descriptors. > This change is a step toward using procfs file descriptors to work > with processes, which makes the system more regular and easier to > reason about. A clean API that's possible to use correctly is a > worthwhile project.
So I have been disucssing a new process API With David Howells, Kees Cook and a few others and I am working on an RFC/proposal for this. It is partially inspired by the new mount API. So I would like to block this patch until then. I would like to get this right very much and I don't think this is the way to go. I hope to have a more detailed proposal out soon(ish). David and I were also thinking about an adhoc session at the kernel summit but we aren't clear whether there's still a slot.
