Re: [PATCH] rethook: Use tsk->on_cpu to check task execution state

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 8 Jun 2026 16:06:54 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Jun 08, 2026 at 10:08:11PM +0900, Masami Hiramatsu wrote:
> 
> > > > Anyway, I'm wondering what the purpose of this check here is, there is
> > > > no real comment, and commit 5120d167e21c ("rethook: Remove warning
> > > > messages printed for finding return address of a frame.") is just pure
> > > > voodoo as well.
> > > 
> > > FWIW, you should have had this discussion then.
> > 
> > Indeed. The rethook is making a shadow stack by list, thus caller must
> > guarantee the target process is blocked at least during this function.
> > 
> > The commit messages suggest that when BPF takes a backtrace, it also
> > includes other running tasks. Is that safe?
> 
> Well, you get to keep the pieces. At this point safe only pertains to
> 'doesn't-crash', all correctness is out the window.
> 
> I always forget the crazy BPF does ;-)
> 
> > > > Also, note the comment that goes with the usage of
> > > > task_on_another_cpu(); that thing is racy as all heck.
> > > > 
> > > > So it really comes down to what the purpose of this check is.
> > 
> > This check has been introduced when it is copied from
> > kretprobe_find_ret_addr(). It has the comment:
> > 
> >  * The @tsk must be 'current' or a task which is not running. @fp is a hint
> > 
> > IIRC, I added this check to explicitly verify this condition.
> 
> Right, but it is a prescriptive comment, not an explanatory one. That
> is, it doesn't explain the condition.
> 
> > > > I suspect the issue at hand is that tsk->rethook elements, such as
> > > > iterated by __rethook_find_ret_addr() are not safe to be accessed for a
> > > > running task.
> > > > 
> > > > Notably while rethook_recycle() has some RCU thing on, that objpool
> > > > thing (and the recycle name itself) seems to strongly suggest iterating
> > > > these things is not sound (you could start with things from this task,
> > > > hit a recycled entry and continue iterating rethooks from another task).
> > > > 
> > > > Also note that the current check is also racy, nothing really prevents a
> > > > wakeup from happening right after you observe task_is_running() being
> > > > false. The task can then get scheduled in on another CPU and tear down
> > > > its rethooks concurrent with __rethook_find_ret_addr().
> > 
> > Yeah, but is there any way to ensure the task is blocked? Even if it is
> > blocked, like TASK_UNINTERRUPTIBLE, unless holding the actual lock in
> > the rethook, it may not be possible to ensure it?
> > 
> > Of course, we could give up on checking within this function and leave
> > everything to the caller to guarantee - as kretprobe does.
> > 
> > BTW, the reason why we made it possible to pass tasks other than current
> >  is that the stack unwinding code itself supported unwinding tasks other
> > than current, so we had no choice but to create this interface.
> > 
> > However, it is a bad idea to check this in deep inside of unwinding.
> 
> This, you cannot take locks in unwinding. The only thing you can do is
> try to do the best you can without crashing.
> 
> Typically unwind only happens on self -- this is natural, a task crashes
> and unwinds itself, or a task does something (takes a lock, hits a
> tracepoint, etc) and takes a snapshot of its own stack, and this is
> safe.
> 
> Things like live-patch use task_call_func(), which ensures the callback
> function is done while holding sufficient locks for the task to not
> change state.

Hmm, is there any way to ensure the function is called from task_call_func()?
(Maybe checking p->pi_lock, but this is not sure the lock owner is this
context?) If not, I need to make this available only for current task
(anyway it just return kretprobe trampoline address, no critical issue)
or, introduce a spinlock.

Or, eventually it may be better to replace kretprobe/rethook with
fprobe return handler.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>