Re: : [PATCH] ftrace/x86: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak functions

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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!