Re: [PATCH v2] signal: add procfd_signal() syscall

[Aleksa Sarai]

On 2018-12-25, Lai Jiangshan <jiangshanlai+lkml@xxxxxxxxx> wrote:
> On Tue, Dec 25, 2018 at 1:32 PM Lai Jiangshan
> <jiangshanlai+lkml@xxxxxxxxx> wrote:
> >
> > Is it possible to avoid adding any syscall?
> >
> > Since holding /proc/pid/reg_file can also hold the pid.
> > With this guarantee, /proc/pid/uuid (universally unique identifier ) can be
> > introduced to identify tasks, the kernel generates
> > a uuid for every task when created.
> >
> > save_pid_uuid_pair_for_later_kill(int pid) {
> >   /* save via /proc/$pid/uuid */
> >   /* don't need to keep any fd after save */
> > }
> >
> > safe_kill(pid, uuid, sig) {
> >     fd = open(/proc/$pid/uuid); /* also hold the pid until close() if
> > open() successes */
> >     if (open successes and read uuid from fd and if it equals to uuid)
> >         kill(pid, sig)
> >     close(fd)
> > }
> >
> > All things needed to be done is to implement /proc/pid/uuid. And if pid can't
> > be recycled within 1 ticket, or the user can ensure it. The user can use
> > starttime(in /proc/pid/stat) instead.
> >
> > save_pid_starttime_pair_for_later_kill(int pid) {
> >   /* save via /proc/$pid/stat */
> >   /* don't need to keep any fd after save or keep it for 1 ticket at most */
> > }
> >
> > safe_kill(pid, starttime, sig) {
> >     fd = open(/proc/$pid/stat); /* also hold the pid until close() if
> > open() successes */
> >     if (open successes and read starttime from fd and if it equals to starttime)
> >         kill(pid, sig)
> >     close(fd)
> > }
> >
> > In this case, zero LOC is added in the kernel. All of it depends on
> > the guarantee that holding /proc/pid/reg_file also holds the pid,
> > one of which I haven't checked carefully either.
> >
> 
> Oh, Sorry, I was wrong, the pid isn't reserved even when
> the fd is kept in the user space. And I'm sorry that I had
> replied to an "old" email thread.

Don't worry, this was a common point of confusion during this (and
sister) threads. All the fd ensures is that access through that fd will
give you -ESRCH if the process is gone (and if the PID is reused it will
still give you -ESRCH).


-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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