linux-kernel - Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability checks in the unwinder

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Message-ID: <20210521200806.nk3m7aldelmi3l2r@treble>
Date:   Fri, 21 May 2021 15:08:06 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     Mark Brown <broonie@...nel.org>, mark.rutland@....com,
        ardb@...nel.org, jthierry@...hat.com, catalin.marinas@....com,
        will@...nel.org, jmorris@...ei.org, pasha.tatashin@...een.com,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability
 checks in the unwinder

On Fri, May 21, 2021 at 02:41:56PM -0500, Madhavan T. Venkataraman wrote:
> > Or is frame->reliable supposed to be checked after all?  Looking at the
> > code again, I'm not sure.
> > 
> > Either way it would be good to document the interface more clearly in a
> > comment above the function.
> > 
> 
> So, arch_stack_walk_reliable() would do this:
> 
> 	start_backtrace(frame);
> 
> 	while (...) {
> 		if (!frame->reliable)
> 			return error;
> 
> 		consume_entry(...);
> 
> 		ret = unwind_frame(...);
> 
> 		if (ret)
> 			break;
> 	}
> 
> 	if (ret == -ENOENT)
> 		return success;
> 	return error;
> 
> 
> Something like that.

I see.  So basically there are six possible combinations of return
states:

  1) No error		frame->reliable
  2) No error		!frame->reliable
  3) -ENOENT		frame->reliable
  5) -ENOENT		!frame->reliable (doesn't happen in practice)
  4) Other error	frame->reliable  (doesn't happen in practice)
  6) Other error	!frame->reliable


On x86 we have fewer combinations:

  1) No error		state->error
  2) No error		!state->error
  3) Error		state->error
  4) Error		!state->error (doesn't happen in practice)


I think the x86 interface seems more robust, because it's more narrow
and has fewer edge cases.  Also it doesn't have to distinguish between
error enums, which can get hairy if a downstream callee happens to
return -ENOENT for a different reason.

-- 
Josh