Re: [RFC PATCH] Implement /proc/pid/kill

[Daniel Colascione <dancol@google.com>]

On Tue, Oct 30, 2018 at 10:40 AM, Christian Brauner
<christian.brauner@canonical.com> wrote:
> On Tue, Oct 30, 2018 at 11:39:11AM +0100, Christian Brauner wrote:
>> On Tue, Oct 30, 2018 at 08:50:22AM +0000, Daniel Colascione wrote:
>> > On Tue, Oct 30, 2018 at 3:21 AM, Joel Fernandes <joelaf@google.com> wrote:
>> > > On Mon, Oct 29, 2018 at 3:11 PM Daniel Colascione <dancol@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

It's good to hear that others are thinking about this problem.

>> I
>> don't think this is the way to go.

Why not?

Does your proposed API allow for a race-free pkill, with arbitrary
selection criteria? This capability is a good litmus test for fixing
the long-standing Unix process API issues.