Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220503150410.2d9e88aa@rorschach.local.home>
In-Reply-To: <20220503150410.2d9e88aa@rorschach.local.home>
From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 6 May 2022 15:14:43 -0700
Message-ID: <CAEf4BzYJan2c0oy-eww++VC57ak=+QOt6a9SWUT1M__AKF8VSA@mail.gmail.com>
Subject: Re: : [PATCH] ftrace/x86: Add FTRACE_MCOUNT_MAX_OFFSET to avoid
 adding weak functions
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, May 3, 2022 at 12:04 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
>
> If an unused weak function was traced, it's call to fentry will still
> exist, which gets added into the __mcount_loc table. Ftrace will use
> kallsyms to retrieve the name for each location in __mcount_loc to display
> it in the available_filter_functions and used to enable functions via the
> name matching in set_ftrace_filter/notrace. Enabling these functions do
> nothing but enable an unused call to ftrace_caller. If a traced weak
> function is overridden, the symbol of the function would be used for it,
> which will either created duplicate names, or if the previous function was
> not traced, it would be incorrectly listed in available_filter_functions
> as a function that can be traced.
>
> This became an issue with BPF[1] as there are tooling that enables the
> direct callers via ftrace but then checks to see if the functions were
> actually enabled. The case of one function that was marked notrace, but
> was followed by an unused weak function that was traced. The unused
> function's call to fentry was added to the __mcount_loc section, and
> kallsyms retrieved the untraced function's symbol as the weak function was
> overridden. Since the untraced function would not get traced, the BPF
> check would detect this and fail.
>
> The real fix would be to fix kallsyms to not show address of weak
> functions as the function before it. But that would require adding code in
> the build to add function size to kallsyms so that it can know when the
> function ends instead of just using the start of the next known symbol.
>
> In the mean time, this is a work around. Add a FTRACE_MCOUNT_MAX_OFFSET
> macro that if defined, ftrace will ignore any function that has its call
> to fentry/mcount that has an offset from the symbol that is greater than
> FTRACE_MCOUNT_MAX_OFFSET.
>
> If CONFIG_HAVE_FENTRY is defined for x86, define FTRACE_MCOUNT_MAX_OFFSET
> to zero, which will have ftrace ignore all locations that are not at the
> start of the function.
>
> [1] https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@kernel.org/
>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  arch/x86/include/asm/ftrace.h |  5 ++++
>  kernel/trace/ftrace.c         | 50 +++++++++++++++++++++++++++++++++--
>  2 files changed, 53 insertions(+), 2 deletions(-)
>

Thanks for investigating and fixing this! I guess we'll need ENDBR
handling, but otherwise it looks good to me!