Re: [RFC PATCH 0/3] tracing: Introduce relative stacktrace

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

On Tue, 28 Jan 2025 20:09:38 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 29 Jan 2025 09:58:19 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > Hmm, that also works if we only consider the kallsyms access. But that
> > means to export KASLR information in the trace buffer. We need to check
> > it is OK.
> 
> If they say we can't have KASLR information in the ring buffer then
> that is pretty much a brick wall, and we are done with this. The best
> we can do is to prevent reading the current trace buffer. But honestly,
> we want that too. Heck, already get kernel stack traces from perfetto
> right? That has KASLR information doesn't it?

I read the perfetto callstack feature, but it seems to support user
space callstack.

https://perfetto.dev/docs/quickstart/callstack-sampling

> 
> > 
> > My another concern is how to handle this stacktrace on live system. The
> > stacktrace has to be handled in both crash and live trace, but in both case
> > we need to consider not leaking KASLR offset.
> 
> I don't think we do.

I meant that my [PATCH 3/3] can do it intermediately (not directly).
So I think your idea (storing relative offset from module) is better.

> 
> > 
> > Hmm, for avoiding the security concern, as Steve said, we may need to save
> > the module relative address, which may introduce a bit more overhead, but
> > it should be safer.
> 
> Actually, if we save the addresses of where the modules are in the
> persistent ring buffer, and expose the addresses only if they are from
> the previous boot (if it's the current boot, it just says "current"),
> then we can decipher the modules from the previous boot.

OK, but when would we save it? it is OK to do it in panic()?

> 
> > 
> > Anyway, this v1 may be able to leak the KASLR offset (or estimate it easier).
> > I think we have 2 options; (A) as Mathieu pointed, expose the offset
> > information via trace buffer. (B) as Steve pointed, fully relative offset
> > in stacktrace.
> 
> It should be fine to read the full offsets. Again, perf already does this.

Indeed. Hmm, I need to know how perf solve this limitation.

Thank you,

> 
> -- Steve


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