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

[Daniel Colascione]

On Tue, Oct 30, 2018 at 3:21 AM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
> On Mon, Oct 29, 2018 at 3:11 PM Daniel Colascione <dancol@xxxxxxxxxx> 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.