On Wed, May 25, 2022 at 06:05:53PM -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. > > 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. ^^^ that paragraph is obsolete by your own changes thing below :-) > [1] https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@xxxxxxxxxx/ > > Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx> > --- > Changes since v1: https://lore.kernel.org/all/20220503150410.2d9e88aa@xxxxxxxxxxxxxxxxxxxx/ > > - Changed MAX_OFFSET to 4 on x86 if KERNEL_IBT is enabled > (Reminded by Peter Zijlstra) > > arch/x86/include/asm/ftrace.h | 10 +++++++ > kernel/trace/ftrace.c | 50 +++++++++++++++++++++++++++++++++-- > 2 files changed, 58 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h > index 024d9797646e..53675fe2d847 100644 > --- a/arch/x86/include/asm/ftrace.h > +++ b/arch/x86/include/asm/ftrace.h > @@ -9,6 +9,16 @@ > # define MCOUNT_ADDR ((unsigned long)(__fentry__)) > #define MCOUNT_INSN_SIZE 5 /* sizeof mcount call */ > > +/* Ignore unused weak functions which will have non zero offsets */ > +#ifdef CONFIG_HAVE_FENTRY > +# ifdef CONFIG_X86_KERNEL_IBT > +/* endbr64 is 4 bytes in front of the fentry */ > +# define FTRACE_MCOUNT_MAX_OFFSET 4 > +# else > +# define FTRACE_MCOUNT_MAX_OFFSET 0 > +# endif > +#endif #define FTRACE_MCOUNT_MAX_OFFSET ENDBR_INSN_SIZE Should do the same I think, less lines etc..
