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Message-Id: <20241023075507.ebeadc70579fc8a642213a55@kernel.org>
Date: Wed, 23 Oct 2024 07:55:07 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Steven Rostedt
<rostedt@...dmis.org>, Florent Revest <revest@...omium.org>,
linux-trace-kernel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>, bpf <bpf@...r.kernel.org>, Alexei
Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Alan Maguire
<alan.maguire@...cle.com>, Mark Rutland <mark.rutland@....com>,
linux-arch@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>,
Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
Christophe Leroy <christophe.leroy@...roup.eu>, Naveen N Rao
<naveen@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, Paul
Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
<dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin"
<hpa@...or.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v17 01/16] function_graph: Pass ftrace_regs to entryfunc
On Mon, 21 Oct 2024 18:03:40 +0100
Will Deacon <will@...nel.org> wrote:
> On Wed, Oct 16, 2024 at 09:57:55AM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.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@...nel.org>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Huacai Chen <chenhuacai@...nel.org>
> > Cc: WANG Xuerui <kernel@...0n.name>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Cc: Nicholas Piggin <npiggin@...il.com>
> > Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> > Cc: Naveen N Rao <naveen@...nel.org>
> > Cc: Madhavan Srinivasan <maddy@...ux.ibm.com>
> > Cc: Paul Walmsley <paul.walmsley@...ive.com>
> > Cc: Palmer Dabbelt <palmer@...belt.com>
> > Cc: Albert Ou <aou@...s.berkeley.edu>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: x86@...nel.org
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.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(¤t->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@...nel.org>
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