Re: [PATCH v17 01/16] function_graph: Pass ftrace_regs to entryfunc

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 21 Oct 2024 18:03:40 +0100
Will Deacon <will@kernel.org> wrote:

> On Wed, Oct 16, 2024 at 09:57:55AM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > Pass ftrace_regs to the fgraph_ops::entryfunc(). If ftrace_regs is not
> > available, it passes a NULL instead. User callback function can access
> > some registers (including return address) via this ftrace_regs.
> > 
> > Note that the ftrace_regs can be NULL when the arch does NOT define:
> > HAVE_DYNAMIC_FTRACE_WITH_ARGS or HAVE_DYNAMIC_FTRACE_WITH_REGS.
> > More specifically, if HAVE_DYNAMIC_FTRACE_WITH_REGS is defined but
> > not the HAVE_DYNAMIC_FTRACE_WITH_ARGS, and the ftrace ops used to
> > register the function callback does not set FTRACE_OPS_FL_SAVE_REGS.
> > In this case, ftrace_regs can be NULL in user callback.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Huacai Chen <chenhuacai@kernel.org>
> > Cc: WANG Xuerui <kernel@xen0n.name>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> > Cc: Naveen N Rao <naveen@kernel.org>
> > Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> > Cc: Paul Walmsley <paul.walmsley@sifive.com>
> > Cc: Palmer Dabbelt <palmer@dabbelt.com>
> > Cc: Albert Ou <aou@eecs.berkeley.edu>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: x86@kernel.org
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > 
> > ---
> >  Changes in v16:
> >   - Add a note when the ftrace_regs can be NULL.
> >   - Update against for the latest kernel.
> >  Changes in v11:
> >   - Update for the latest for-next branch.
> >  Changes in v8:
> >   - Just pass ftrace_regs to the handler instead of adding a new
> >     entryregfunc.
> >   - Update riscv ftrace_graph_func().
> >  Changes in v3:
> >   - Update for new multiple fgraph.
> > ---
> >  arch/arm64/kernel/ftrace.c               |   20 +++++++++++-
> >  arch/loongarch/kernel/ftrace_dyn.c       |   10 +++++-
> >  arch/powerpc/kernel/trace/ftrace.c       |    2 +
> >  arch/powerpc/kernel/trace/ftrace_64_pg.c |   10 ++++--
> >  arch/riscv/kernel/ftrace.c               |   17 ++++++++++
> >  arch/x86/kernel/ftrace.c                 |   50 +++++++++++++++++++++---------
> >  include/linux/ftrace.h                   |   17 ++++++++--
> >  kernel/trace/fgraph.c                    |   25 +++++++++------
> >  kernel/trace/ftrace.c                    |    3 +-
> >  kernel/trace/trace.h                     |    3 +-
> >  kernel/trace/trace_functions_graph.c     |    3 +-
> >  kernel/trace/trace_irqsoff.c             |    3 +-
> >  kernel/trace/trace_sched_wakeup.c        |    3 +-
> >  kernel/trace/trace_selftest.c            |    8 +++--
> >  14 files changed, 129 insertions(+), 45 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> > index b2d947175cbe..a5a285f8a7ef 100644
> > --- a/arch/arm64/kernel/ftrace.c
> > +++ b/arch/arm64/kernel/ftrace.c
> > @@ -481,7 +481,25 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
> >  void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> >  		       struct ftrace_ops *op, struct ftrace_regs *fregs)
> >  {
> > -	prepare_ftrace_return(ip, &arch_ftrace_regs(fregs)->lr, arch_ftrace_regs(fregs)->fp);
> > +	unsigned long return_hooker = (unsigned long)&return_to_handler;
> > +	unsigned long frame_pointer = arch_ftrace_regs(fregs)->fp;
> > +	unsigned long *parent = &arch_ftrace_regs(fregs)->lr;
> > +	unsigned long old;
> > +
> > +	if (unlikely(atomic_read(&current->tracing_graph_pause)))
> > +		return;
> > +
> > +	/*
> > +	 * Note:
> > +	 * No protection against faulting at *parent, which may be seen
> > +	 * on other archs. It's unlikely on AArch64.
> > +	 */
> > +	old = *parent;
> 
> Sorry to pick on this line again, but the comment is very non-committal]
> and I think this is something on which we need to be definitive.

Agreed. I think this does not happen because it is a part of
__arch_ftrace_regs, which is stored on kernel stack.

I copied it from prepare_ftrace_return(), so that also need to be
changed too.

Let me remove this comment.

Thank you,

> 
> Either the access can fault, and we should handle it, or it will never
> fault and we don't need to handle it. Saying it's unlikely means we
> need to handle it :)
> 
> Will
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>