On 11/25/21 8:56 AM, Mark Brown wrote:

On Tue, Nov 23, 2021 at 01:37:22PM -0600, madvenka@linux.microsoft.com wrote:

quoted

Introduce arch_stack_walk_reliable() for ARM64. This works like
arch_stack_walk() except that it returns -EINVAL if the stack trace is not
reliable.

quoted

Until all the reliability checks are in place, arch_stack_walk_reliable()
may not be used by livepatch. But it may be used by debug and test code.

Probably also worth noting that this doesn't select
HAVE_RELIABLE_STACKTRACE which is what any actual users are going to use
to identify if the architecture has the feature.  I would have been
tempted to add arch_stack_walk() as a separate patch but equally having
the user code there (even if it itself can't yet be used...) helps with
reviewing the actual unwinder so I don't mind.

I did not select HAVE_RELIABLE_STACKTRACE just in case we think that some
more reliability checks need to be added. But if reviewers agree
that this patch series contains all the reliability checks we need, I
will add a patch to select HAVE_RELIABLE_STACKTRACE to the series.

quoted

+static void unwind_check_reliability(struct task_struct *task,
+				     struct stackframe *frame)
+{
+	if (frame->fp == (unsigned long)task_pt_regs(task)->stackframe) {
+		/* Final frame; no more unwind, no need to check reliability */
+		return;
+	}

If the unwinder carries on for some reason (the code for that is
elsewhere and may be updated separately...) then this will start
checking again.  I'm not sure if this is a *problem* as such but the
thing about this being the final frame coupled with not actually
explicitly stopping the unwind here makes me think this should at least
be clearer, the comment begs the question about what happens if
something decides it is not in fact the final frame.

I can address this by adding an explicit comment to that effect.
For example, define a separate function to check for the final frame:

/*
 * Check if this is the final frame. Unwind must stop at the final
 * frame.
 */
static inline bool unwind_is_final_frame(struct task_struct *task,
                                         struct stackframe *frame)
{
	return frame->fp == (unsigned long)task_pt_regs(task)->stackframe;
}

Then, use this function in unwind_check_reliability() and unwind_continue().

Is this acceptable?

Madhavan