linux-kernel - Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

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Message-ID: <20241030061043.eo2vuqgsoqmjytjr@treble.attlocal.net>
Date: Tue, 29 Oct 2024 23:10:43 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kerne.org,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Tue, Oct 29, 2024 at 04:32:46PM -0700, Andrii Nakryiko wrote:
> >  struct audit_context;
> > @@ -1592,6 +1593,10 @@ struct task_struct {
> >         struct user_event_mm            *user_event_mm;
> >  #endif
> >
> > +#ifdef CONFIG_UNWIND_USER
> > +       struct unwind_task_info         unwind_task_info;
> 
> this is quite a lot of memory to pay on each task, a lot of which a)
> might not have sframe

Frame pointers are also supported.

> and b) might not need stack unwinding during their lifetime.

Right, I'm not super happy about that.

> It can be a pointer and allocated in copy_process(), no?
> Though ideally this should be initialized lazily, if possible.

Problem is, the unwinder doesn't know in advance which tasks will be
unwound.

Its first clue is unwind_user_register(), would it make sense for the
caller to clarify whether all tasks need to be unwound or only a
specific subset?

Its second clue is unwind_user_deferred(), which is called for the task
itself.  But by then it's too late because it needs to access the
per-task data from (potentially) irq context so it can't do a lazy
allocation.

I'm definitely open to ideas...

> > +       if (!info->entries) {
> > +               info->entries = kmalloc(UNWIND_MAX_ENTRIES * sizeof(long),
> > +                                       GFP_KERNEL);
> > +               if (!info->entries)
> > +                       return;
> 
> uhm... can we notify callbacks that stack capture failed? otherwise
> we'd need some extra timeouts and other complications if we are
> *waiting* for this callback to be called

Hm, do you actually plan to wait for the callback?

I assume this is BPF, can you give some high-level detail about how it
will use these interfaces?

-- 
Josh