Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Apr 09, 2021 at 01:09:09PM +0100, Mark Rutland wrote:
> On Mon, Apr 05, 2021 at 03:43:09PM -0500, madvenka@linux.microsoft.com wrote:
> > From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
> > 
> > There are a number of places in kernel code where the stack trace is not
> > reliable. Enhance the unwinder to check for those cases and mark the
> > stack trace as unreliable. Once all of the checks are in place, the unwinder
> > can provide a reliable stack trace. But before this can be used for livepatch,
> > some other entity needs to guarantee that the frame pointers are all set up
> > correctly in kernel functions. objtool is currently being worked on to
> > fill that gap.
> > 
> > Except for the return address check, all the other checks involve checking
> > the return PC of every frame against certain kernel functions. To do this,
> > implement some infrastructure code:
> > 
> > 	- Define a special_functions[] array and populate the array with
> > 	  the special functions
> 
> I'm not too keen on having to manually collate this within the unwinder,
> as it's very painful from a maintenance perspective.

Agreed.

> I'd much rather we could associate this information with the
> implementations of these functions, so that they're more likely to
> stay in sync.
> 
> Further, I believe all the special cases are assembly functions, and
> most of those are already in special sections to begin with. I reckon
> it'd be simpler and more robust to reject unwinding based on the
> section. If we need to unwind across specific functions in those
> sections, we could opt-in with some metadata. So e.g. we could reject
> all functions in ".entry.text", special casing the EL0 entry functions
> if necessary.

Couldn't this also end up being somewhat fragile?  Saying "certain
sections are deemed unreliable" isn't necessarily obvious to somebody
who doesn't already know about it, and it could be overlooked or
forgotten over time.  And there's no way to enforce it stays that way.

FWIW, over the years we've had zero issues with encoding the frame
pointer on x86.  After you save pt_regs, you encode the frame pointer to
point to it.  Ideally in the same macro so it's hard to overlook.

If you're concerned about debuggers getting confused by the encoding -
which debuggers specifically?  In my experience, if vmlinux has
debuginfo, gdb and most other debuggers will use DWARF (which is already
broken in asm code) and completely ignore frame pointers.

> I think there's a lot more code that we cannot unwind, e.g. KVM
> exception code, or almost anything marked with SYM_CODE_END().

Just a reminder that livepatch only unwinds blocked tasks (plus the
'current' task which calls into livepatch).  So practically speaking, it
doesn't matter whether the 'unreliable' detection has full coverage.
The only exceptions which really matter are those which end up calling
schedule(), e.g. preemption or page faults.

Being able to consistently detect *all* possible unreliable paths would
be nice in theory, but it's unnecessary and may not be worth the extra
complexity.

-- 
Josh