Re: [PATCH 1/5] perf jitdump: Fix PID namespace detection

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Nov 14, 2025 at 01:44:30PM +0100, Ilya Leoshkevich wrote:
> On Fri, 2025-11-14 at 00:07 -0800, Namhyung Kim wrote:
> > Hello,
> > 
> > Sorry for the delay.
> > 
> > On Fri, Nov 07, 2025 at 09:19:47AM +0100, Ilya Leoshkevich wrote:
> > > On Thu, 2025-11-06 at 18:16 -0800, Namhyung Kim wrote:
> > > > On Wed, Nov 05, 2025 at 08:10:24PM +0100, Ilya Leoshkevich wrote:
> > > > > perf inject fails to detect jitdump file produced by a process
> > > > > running in a different PID namespace if this process has not
> > > > > exited
> > > > > yet.
> > > > > 
> > > > > The PID namespace heuristic in jit_detect() compares two PIDs:
> > > > > 
> > > > > * pid: outermost NStgid of mmap(jitdump) caller from perf's
> > > > > PoV.
> > > > > * nsinfo__nstgid(nsi): innermost NStgid of mmap(jitdump) caller
> > > > > from
> > > > >                        perf's PoV.
> > > > > 
> > > > > The semantics of the in_pidns variable can be seen in, e.g.,
> > > > > nsinfo__get_nspid(): it's true if and only if perf and the
> > > > > profiled
> > > > > process are in different PID namespaces.
> > > > > 
> > > > > The current logic is clearly inverted: if pid and
> > > > > nsinfo__nstgid(nsi)
> > > > > are different, then the profiled process must be in a different
> > > > > PID
> > > > > namespace. This, of course, ignores that fact that they may end
> > > > > up
> > > > > being equal by accident, but that's not the point here.
> > > > > 
> > > > > Fix by flipping the comparison.
> > > > > 
> > > > > Changing just that, however, breaks the case when the process
> > > > > has
> > > > > exited. Add explicit support for that by adding "synthesized"
> > > > > field
> > > > > to
> > > > > nsinfo, which tracks whether NStgid was obtained from a running
> > > > > process (ignoring considerations of PID reuse or running inject
> > > > > on
> > > > > a different machine). When the namespace information is
> > > > > synthesized,
> > > > > assume the process ran in a different PID namespace.
> > > > 
> > > > I'm not sure I'm following.  It'd be great if anyone understand
> > > > the
> > > > topic well could review.
> > > 
> > > Perhaps some data from the testcase from [5/5] can make it more
> > > clear.
> > > Here are the PIDs that exist in reality:
> > > 
> > >              unshare[a] perf-record unshare[b] jshell
> > > Host PIDNS:  1000       1001        1002       1003
> > > PIDNS[a]:       -          1           2          3
> > > PIDNS[b]:       -          -           -          1
> > > 
> > > In jit_detect() we deal with 2 of them.
> > > 
> > > - pid is jshell@PIDNS[a].
> > >   It is taken from the MMAP2 event, this is how perf sees jshell.
> > > 
> > > - pid2 is jshell@PIDNS[b].
> > >   It is taken from "jit-1.dump", this is how jshell sees itself.
> > > 
> > > - nsinfo__nstgid(nsi) ideally should be jshell@PIDNS[b].
> > >   This is jshell's innermost NStgid.
> > >   But perf can see it differently. This is the core of the problem
> > > this
> > >   series deals with.
> > 
> > Thanks a lot for the example and explanation!  I'm trying to
> > understand.
> > :)
> 
> Thank you for the patience!
> 
> > > Why does nsinfo__nstgid(nsi) vary? Because the kernel does not
> > > record
> > > it, and perf has to guess it. I have a WIP patch to fix that [1],
> > > but
> > > it needs a lot more work at this point.
> > > 
> > > How does perf guess it? It looks into /proc/$PID/status. This is
> > > quite
> > > unreliable, but this is the best perf can do under circumstances.
> > > As a
> > > result we have 3 possibilities:
> > > 
> > > - The original process is still around. This is the buggy case. In
> > > this
> > >   case nsinfo__nstgid(nsi) == jshell@PIDNS[b]. IMHO this is a very
> > >   clear indication of namespacing, and that's why the condition
> > > should
> > >   be flipped.
> > 
> > So perf would look at /proc/3/status and the file would have the
> > below
> > 
> >   NStgid:  1003  3  1
> > 
> > and *in_pidns should be true, right?
> 
> It should be
> 
>     NStgid: 3  1
> 
> because perf itself is namespaced too - in this testcase nobody sees
> the root PID namespace. I wrote it this way to make sure there are
> no accidental leaks from the root PID namespace, but it's not very
> important and perhaps obscures things somewhat unnecessarily.

Ah, ok.  Thanks for the correction.

> 
> But regardless, I believe that in this case setting *in_pidns to true
> is the right thing.

Yep, I agree.

> 
> > > - The original process has exited and PID was not reused. I believe
> > >   this is the use case the current code has been extensively tested
> > >   with. In this case perf assumes there was no namespacing and
> > >   nsinfo__nstgid(nsi) == pid. That's why I need the "synthesized"
> > >   field: to indicate that NStgid is just an assumption and didn't
> > > come
> > >   from any real data source.
> > 
> > Ok, can you please put short comments on each boolean field so that
> > we
> > can see the meaning of them clearly?
> 
> Sure, I can add this in v2.

Perhaps it's better to split the part adding "synthesize" field to
nsinfo in a separate commit and this one focuses on setting in_pidns.

Thanks,
Namhyung