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 be 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 addresses 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 (unless IBT is enabled), which will have ftrace ignore all locations
that are not at the start of the function (or one after the ENDBR
instruction).

A worker thread is added at boot up to scan all the ftrace record entries,
and will mark any that fail the FTRACE_MCOUNT_MAX_OFFSET test as disabled.
They will still appear in the available_filter_functions file as:

  __ftrace_invalid_address___<invalid-offset>

(showing the offset that caused it to be invalid).

This is required for tools that use libtracefs (like trace-cmd does) that
scan the available_filter_functions and enable set_ftrace_filter and
set_ftrace_notrace using indexes of the function listed in the file (this
is a speedup, as enabling thousands of files via names is an O(n^2)
operation and can take minutes to complete, where the indexing takes less
than a second).

The invalid functions cannot be removed from available_filter_functions as
the names there correspond to the ftrace records in the array that manages
them (and the indexing depends on this).

[1] https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@kernel.org/

Link: https://lkml.kernel.org/r/20220526141912.794c2786@gandalf.local.homeSigned-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>