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

[Peter Zijlstra]

On Tue, May 03, 2022 at 03:04:10PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> 
> 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.

So for x86-64... objtool knows about these holes and *could* squash
these entries if you want (at the cost of requiring link time objtool
run).

Also see commit:

  4adb23686795 ("objtool: Ignore extra-symbol code")

But yeah, ensuring fentry is close ought to work.

> 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.

You forgot about IBT again? __fentry__ no longer lives at +0 on x86
anymore.