Re: [PATCH v10 05/11] arm64: Make dump_stacktrace() use arch_stack_walk()

Mark Rutland
2021-10-26T12:06:00Z

On Mon, Oct 25, 2021 at 05:49:25PM +0100, Mark Rutland wrote:
> From f3e66ca75aff3474355839f72d123276028204e1 Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland@arm.com>
> Date: Mon, 25 Oct 2021 13:23:11 +0100
> Subject: [PATCH] arm64: ftrace: use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
>
> When CONFIG_FUNCTION_GRAPH_TRACER is selected, and the function graph:
> tracer is in use, unwind_frame() may erroneously asscociate a traced
> function with an incorrect return address. This can happen when starting
> an unwind from a pt_regs, or when unwinding across an exception
> boundary.
>
> The underlying problem is that ftrace_graph_get_ret_stack() takes an
> index offset from the most recent entry added to the fgraph return
> stack. We start an unwind at offset 0, and increment the offset each
> time we encounter `return_to_handler`, which indicates a rewritten
> return address. This is broken in two cases:
>
> * Between creating a pt_regs and starting the unwind, function calls may
> place entries on the stack, leaving an abitrary offset which we can
> only determine by performing a full unwind from the caller of the
> unwind code. While this initial unwind is open-coded in
> dump_backtrace(), this is not performed for other unwinders such as
> perf_callchain_kernel().
>
> * When unwinding across an exception boundary (whether continuing an
> unwind or starting a new unwind from regs), we always consume the LR
> of the interrupted context, though this may not have been live at the
> time of the exception. Where the LR was not live but happened to
> contain `return_to_handler`, we'll recover an address from the graph
> return stack and increment the current offset, leaving subsequent
> entries off-by-one.
>
> Where the LR was not live and did not contain `return_to_handler`, we
> will still report an erroneous address, but subsequent entries will be
> unaffected.

It turns out I had this backwards, and we currently always *skip* the LR
when unwinding across regs, because:

* The entry assembly creates a synthetic frame record with the original
FP and the ELR_EL1 value (i.e. the PC at the point of the exception),
skipping the LR.

* In arch_stack_walk() we start the walk from regs->pc, and continue
with the frame record, skipping the LR.

* In the existing dump_backtrace, we skip until we hit a frame record
whose FP value matches the FP in the regs (i.e. the synthetic frame
record created by the entry assembly). That'll dump the ELR_EL1 value,
then continue to the next frame record, skipping the LR.

So case two is bogus, and only case one can happen today. This cleanup
shouldn't trigger the WARN_ON_ONCE() in unwind_frame(), and we can fix
the missing LR entry in a subsequent cleanup.

Thanks,
Mark.