linux-kernel - Re: [RFC PATCH v2 5/6] ftrace: Add ftrace_partial_regs() for converting ftrace_regs to pt

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Message-ID: <CABRcYmLFwSrfsod6y8-K1memLUZiJeb2so6pD4XaFUpwbLD9AQ@mail.gmail.com>
Date:   Wed, 9 Aug 2023 12:31:27 +0200
From:   Florent Revest <revest@...omium.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.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>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH v2 5/6] ftrace: Add ftrace_partial_regs() for
 converting ftrace_regs to pt_regs

On Mon, Aug 7, 2023 at 8:49 AM Masami Hiramatsu (Google)
<mhiramat@...nel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Add ftrace_partial_regs() which converts the ftrace_regas to pt_regs.

ftrace_regs*

> If the architecture defines its own ftrace_regs, this copies partial
> registers to pt_regs and returns it. If not, ftrace_regs is the same as
> pt_regs and ftrace_partial_regs() will return ftrace_regs::regs.
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
>  arch/arm64/include/asm/ftrace.h |   11 +++++++++++
>  include/linux/ftrace.h          |   11 +++++++++++
>  2 files changed, 22 insertions(+)
>
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index ab158196480c..b108cd6718cf 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -137,6 +137,17 @@ ftrace_override_function_with_return(struct ftrace_regs *fregs)
>         fregs->pc = fregs->lr;
>  }
>
> +static __always_inline struct pt_regs *
> +ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
> +{
> +       memcpy(regs->regs, fregs->regs, sizeof(u64) * 10);

Are you intentionally copying that tenth value (fregs.direct_tramp)
into pt_regs.regs[9] ? This seems wrong and it looks like it will bite
us back one day. Isn't it one of these cases where we can simply use
sizeof(fregs->regs) ?

> +       regs->sp = fregs->sp;
> +       regs->pc = fregs->pc;
> +       regs->x[29] = fregs->fp;
> +       regs->x[30] = fregs->lr;
> +       return regs;
> +}
> +
>  int ftrace_regs_query_register_offset(const char *name);
>
>  int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 3fb94a1a2461..7f45654441b7 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -155,6 +155,17 @@ static __always_inline struct pt_regs *ftrace_get_regs(struct ftrace_regs *fregs
>         return arch_ftrace_get_regs(fregs);
>  }
>
> +#if !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS) || \
> +       defined(CONFIG_HAVE_PT_REGS_COMPAT_FTRACE_REGS)
> +
> +static __always_inline struct pt_regs *
> +ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
> +{
> +       return arch_ftrace_get_regs((struct ftrace_regs *)fregs);
> +}

I don't think this works. Suppose you are on x86, WITH_ARGS, and with
HAVE_PT_REGS_COMPAT_FTRACE_REGS. If you register to ftrace without
FTRACE_OPS_FL_SAVE_REGS you will receive a ftrace_regs from the light
ftrace pre-trampoline that has a CS register equal to 0 and
arch_ftrace_get_regs will return NULL here, which should never happen.

Have you tested your series without registering as FTRACE_OPS_FL_SAVE_REGS ?