Re: [PATCH v2 0/4] arm64/ftrace: move to DYNAMIC_FTRACE_WITH_ARGS

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Nov 15, 2022 at 10:01:48AM -0500, Steven Rostedt wrote:
> On Thu,  3 Nov 2022 17:05:16 +0000
> Mark Rutland <mark.rutland@arm.com> wrote:
> 
> > This series replaces arm64's support for FTRACE_WITH_REGS with support
> > for FTRACE_WITH_ARGS. This removes some overhead and complexity, and
> > removes some latent issues with inconsistent presentation of struct
> > pt_regs (which can only be reliably saved/restored at exception
> > boundaries).
> > 
> > The existing FTRACE_WITH_REGS support was added for two major reasons:
> > 
> > (1) To make it possible to use the ftrace graph tracer with pointer
> >     authentication, where it's necessary to snapshot/manipulate the LR
> >     before it is signed by the instrumented function.
> > 
> > (2) To make it possible to implement LIVEPATCH in future, where we need
> >     to hook function entry before an instrumented function manipulates
> >     the stack or argument registers. Practically speaking, we need to
> >     preserve the argument/return registers, PC, LR, and SP.
> > 
> > Neither of these requires the full set of pt_regs, and only requires us
> > to save/restore a subset of registers used for passing
> > arguments/return-values and context/return information (which is the
> > minimum set we always need to save/restore today).
> > 
> > As there is no longer a need to save different sets of registers for
> > different features, we no longer need distinct `ftrace_caller` and
> > `ftrace_regs_caller` trampolines. This allows the trampoline assembly to
> > be simpler, and simplifies code which previously had to handle the two
> > trampolines.
> > 
> > I've tested this with the ftrace selftests, where there are no
> > unexpected failures.
> 
> Were there any "expected" failures?

Ah; sorry, I had meant to include the results here.

With this series applied atop v6.1-rc4 and using the ftrace selftests from that
tree, my results were the same as with baseline v6.1-rc4:

| # of passed:  104
| # of failed:  0
| # of unresolved:  7
| # of untested:  0
| # of unsupported:  2
| # of xfailed:  1
| # of undefined(test bug):  0

Where the non-passing tests were:

| [8] Test ftrace direct functions against tracers        [UNRESOLVED]
| [9] Test ftrace direct functions against kprobes        [UNRESOLVED]

... as direct functions aren't supported on arm64 (both before and after this
series).

| [16] Generic dynamic event - check if duplicate events are caught       [UNSUPPORTED]
| [74] event trigger - test inter-event histogram trigger eprobe on synthetic event       [UNSUPPORTED]

... which are due to a bug in the tests fixed by:

  https://lore.kernel.org/all/20221010074207.714077-1-svens@linux.ibm.com/

... and they both pass with that applied.

| [22] Test trace_printk from module      [UNRESOLVED]
| [31] ftrace - function trace on module  [UNRESOLVED]
| [51] Kprobe dynamic event - probing module      [UNRESOLVED]
| [61] test for the preemptirqsoff tracer [UNRESOLVED]

... which are because my test environment didn't have modules.

| [62] Meta-selftest: Checkbashisms       [UNRESOLVED]

... which is irrelevant for this series.

| [65] event trigger - test inter-event histogram trigger expected fail actions   [XFAIL]

... which is expected.

[...]

> So I ran this on top of my code through all my ftrace tests (for x86) and
> it didn't cause any regressions.
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Thanks!

Mark.

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