Re: [PATCH v2 06/10] ftrace: selftest: remove broken trace_direct_tramp

On Thu, Mar 16, 2023 at 12:51 AM Steven Rostedt [off-list ref] wrote:

On Tue,  7 Feb 2023 19:21:31 +0100
Florent Revest [off-list ref] wrote:

quoted

From: Mark Rutland <mark.rutland@arm.com>

The ftrace selftest code has a trace_direct_tramp() function which it
uses as a direct call trampoline. This happens to work on x86, since the
direct call's return address is in the usual place, and can be returned
to via a RET, but in general the calling convention for direct calls is
different from regular function calls, and requires a trampoline written
in assembly.

On s390, regular function calls place the return address in %r14, and an
ftrace patch-site in an instrumented function places the trampoline's
return address (which is within the instrumented function) in %r0,
preserving the original %r14 value in-place. As a regular C function
will return to the address in %r14, using a C function as the trampoline
results in the trampoline returning to the caller of the instrumented
function, skipping the body of the instrumented function.

Note that the s390 issue is not detcted by the ftrace selftest code, as
the instrumented function is trivial, and returning back into the caller
happens to be equivalent.

On arm64, regular function calls place the return address in x30, and
an ftrace patch-site in an instrumented function saves this into r9
and places the trampoline's return address (within the instrumented
function) in x30. A regular C function will return to the address in
x30, but will not restore x9 into x30. Consequently, using a C function
as the trampoline results in returning to the trampoline's return
address having corrupted x30, such that when the instrumented function
returns, it will return back into itself.

To avoid future issues in this area, remove the trace_direct_tramp()
function, and require that each architecture with direct calls provides
a stub trampoline, named ftrace_stub_direct_tramp. This can be written
to handle the architecture's trampoline calling convention, and in
future could be used elsewhere (e.g. in the ftrace ops sample, to
measure the overhead of direct calls), so we may as well always build it
in.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Li Huafei <redacted>
Cc: Xu Kuohai <redacted>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Florent Revest <redacted>
Signed-off-by: Florent Revest <redacted>

Care to respin with my update requests? I can take up to this patch and
base it directly on v6.3-rc3 when it comes out. I'm expecting that to have
the fixes in other code that is breaking my tests.

Okay! :) I'll send you a subset of this series (first 6 patches with
your review addressed and rebased on v6.3-rc2 for now).

Then I'll push it out after it passes all my tests, and you can take it
and add the arm64 specific bits on top. I'm currently running these patches
as is on my tests to see if they fail (with a patched kernel for the other
code that's breaking my tests).

Does that sound OK?

Sounds good to me, yes!