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SubjectRe: [RFC][PATCH 0/3 v3] ftrace: Add access to function arguments for all callbacks
On Wed, 28 Oct 2020 20:08:16 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> This is something I wanted to implement a long time ago, but held off until
> there was a good reason to do so. Now it appears that having access to the
> arguments of the function by default is very useful. As a bonus, because
> arguments must be saved regardless before calling a callback, because they
> need to be restored before returning back to the start of the traced
> function, there's not much work to do to have them always be there for
> normal function callbacks.
>
> The basic idea is that if CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS is set, then
> all callbacks registered to ftrace can use the regs parameter for the stack
> and arguments (kernel_stack_pointer(regs), regs_get_kernel_argument(regs, n)),
> without the need to set REGS that causes overhead by saving all registers as
> REGS simulates a breakpoint.
>
> This could be extended to move the REGS portion to kprobes itself, and
> remove the SAVE_REGS flag completely, but for now, kprobes still uses the
> full SAVE_REGS support.

Yeah, I would like to keep using the SAVE_REGS support until SAVE_ARGS
implemented on all arch. But in parallel, I will prepare generic
kprobe-on-ftrace handler.

> The last patch extends the WITH_ARGS to allow default function tracing to
> modify the instruction pointer, where livepatching for x86 no longer needs
> to save all registers.
>
> The idea of this approach is to give enough information to a callback that
> it could retrieve all arguments, which includes the stack pointer and
> instruction pointer.
>
> This can also be extended to modify the function graph tracer to use the
> function tracer instead of having a separate trampoline.

Anyway, this series (3 patches) looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>

Thank you!

>
> Changes since v2:
>
> - Change all callback args to use struct ftrace_regs, and if something
> requires the full regs, it must use the ftrace_get_regs() helper macro,
> which will return NULL if the regs isn't full (FL_SAVE_REGS set).
> This addresses a concern by both Peter Zijlsta and Thomas Gleixner that
> a partially filled pt_regs may be used inappropriately.
>
> - No test patch in this version.
>
> - Use case using livepatching is added. This passes the test_livepatch.sh
> selftest.
>
> Steven Rostedt (VMware) (3):
> ftrace: Have the callbacks receive a struct ftrace_regs instead of pt_regs
> ftrace/x86: Allow for arguments to be passed in to ftrace_regs by default
> livepatching: Use the default ftrace_ops instead of REGS when ARGS is available
>
> ----
> arch/powerpc/include/asm/livepatch.h | 4 +++-
> arch/s390/include/asm/livepatch.h | 5 ++++-
> arch/x86/Kconfig | 1 +
> arch/x86/include/asm/ftrace.h | 18 ++++++++++++++++++
> arch/x86/include/asm/livepatch.h | 4 ++--
> arch/x86/kernel/ftrace_64.S | 15 +++++++++++++--
> arch/x86/kernel/kprobes/ftrace.c | 3 ++-
> include/linux/ftrace.h | 28 ++++++++++++++++++++++++++--
> include/linux/kprobes.h | 2 +-
> kernel/livepatch/patch.c | 10 ++++++----
> kernel/trace/Kconfig | 9 +++++++++
> kernel/trace/ftrace.c | 27 +++++++++++++++------------
> kernel/trace/trace_event_perf.c | 2 +-
> kernel/trace/trace_functions.c | 9 ++++-----
> kernel/trace/trace_irqsoff.c | 2 +-
> kernel/trace/trace_sched_wakeup.c | 2 +-
> kernel/trace/trace_selftest.c | 20 +++++++++++---------
> kernel/trace/trace_stack.c | 2 +-
> 18 files changed, 119 insertions(+), 44 deletions(-)


--
Masami Hiramatsu <mhiramat@kernel.org>

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