| Message ID | 20240722202502.70301-1-andrey.konovalov@linux.dev (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | x86, kcov: ignore stack trace coverage | expand |
On Mon, 22 Jul 2024 at 22:25, <andrey.konovalov@linux.dev> wrote: > > From: Andrey Konovalov <andreyknvl@gmail.com> > > When a multitude of kernel debugging options are enabled, they often > collect and save the current stack trace. The coverage produced by the > related routines is not relevant for the KCOV's intended use case > (guiding the fuzzing process). > > Thus, disable instrumentation of the x86 stack trace collection code. > > KCOV instrumentaion of the generic kernel/stacktrace.c was already > disabled in commit 43e76af85fa7 ("kcov: ignore fault-inject and > stacktrace"). This patch is an x86-specific addition. > > In addition to freeing up the KCOV buffer capacity for holding more > relevant coverage, this patch also speeds up the kernel boot time with > the config from the syzbot USB fuzzing instance by ~25%. > > Fixes: 43e76af85fa7 ("kcov: ignore fault-inject and stacktrace") > Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com> > > --- > > I'm not sure whether it makes sense to backport this patch to stable > kernels, but I do think that it makes sense to take it into mainline > as a fix: currently, the USB fuzzing instance is choking on the amount > of coverage produced by KCOV and thus doesn't perform well. > > For reference, without this patch, for the following program: > > r0 = syz_usb_connect_ath9k(0x3, 0x5a, &(0x7f0000000080)={{0x12, 0x1, > 0x200, 0xff, 0xff, 0xff, 0x40, 0xcf3, 0x9271, 0x108, 0x1, 0x2, 0x3, 0x1, > [{{0x9, 0x2, 0x48, 0x1, 0x1, 0x0, 0x80, 0xfa, {{0x9, 0x4, 0x0, 0x0, 0x6, > 0xff, 0x0, 0x0, 0x0, "", {{0x9, 0x5, 0x1, 0x2, 0x200, 0x0, 0x0, 0x0, ""}, > {0x9, 0x5, 0x82, 0x2, 0x200, 0x0, 0x0, 0x0, ""}, {0x9, 0x5, 0x83, 0x3, > 0x40, 0x1, 0x0, 0x0, ""}, {0x9, 0x5, 0x4, 0x3, 0x40, 0x1, 0x0, 0x0, ""}, > {0x9, 0x5, 0x5, 0x2, 0x200, 0x0, 0x0, 0x0, ""}, {0x9, 0x5, 0x6, 0x2, > 0x200, 0x0, 0x0, 0x0, ""}}}}}}]}}, 0x0) > > KCOV produces ~500k coverage entries. > > Here are the top ones sorted by the number of occurrences: > > 23027 /home/user/src/arch/x86/kernel/unwind_orc.c:99 > 17335 /home/user/src/arch/x86/kernel/unwind_orc.c:100 > 16460 /home/user/src/arch/x86/include/asm/stacktrace.h:60 (discriminator 3) > 16460 /home/user/src/arch/x86/include/asm/stacktrace.h:60 > 16191 /home/user/src/security/tomoyo/domain.c:183 (discriminator 1) > 16128 /home/user/src/security/tomoyo/domain.c:184 (discriminator 8) > 11384 /home/user/src/arch/x86/kernel/unwind_orc.c:109 > 11155 /home/user/src/arch/x86/include/asm/stacktrace.h:59 > 10997 /home/user/src/arch/x86/kernel/unwind_orc.c:665 > 10768 /home/user/src/include/asm-generic/rwonce.h:67 > 9994 /home/user/src/arch/x86/kernel/unwind_orc.c:390 > 9994 /home/user/src/arch/x86/kernel/unwind_orc.c:389 > ... > > With this patch, the number of entries drops to ~140k. > > (For reference, here are the top entries with this patch applied: > > 16191 /home/user/src/security/tomoyo/domain.c:183 (discriminator 1) > 16128 /home/user/src/security/tomoyo/domain.c:184 (discriminator 8) > 3528 /home/user/src/security/tomoyo/domain.c:173 (discriminator 2) > 3528 /home/user/src/security/tomoyo/domain.c:173 > 3528 /home/user/src/security/tomoyo/domain.c:171 (discriminator 5) > 2877 /home/user/src/lib/vsprintf.c:646 > 2672 /home/user/src/lib/vsprintf.c:651 > 2672 /home/user/src/lib/vsprintf.c:649 > 2230 /home/user/src/lib/vsprintf.c:2559 > ... > > I'm not sure why tomoyo produces such a large number of entries, but > that will require a separate fix anyway if it's unintended.) > --- > arch/x86/kernel/Makefile | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile > index 20a0dd51700a..241e21723fa5 100644 > --- a/arch/x86/kernel/Makefile > +++ b/arch/x86/kernel/Makefile > @@ -40,6 +40,14 @@ KMSAN_SANITIZE_sev.o := n > KCOV_INSTRUMENT_head$(BITS).o := n > KCOV_INSTRUMENT_sev.o := n > > +# These produce large amounts of uninteresting coverage. > +KCOV_INSTRUMENT_dumpstack.o := n > +KCOV_INSTRUMENT_dumpstack_$(BITS).o := n > +KCOV_INSTRUMENT_stacktrace.o := n > +KCOV_INSTRUMENT_unwind_orc.o := n > +KCOV_INSTRUMENT_unwind_frame.o := n > +KCOV_INSTRUMENT_unwind_guess.o := n I've sent something similar recently, I think it should be in tip/x86 queue now: https://lore.kernel.org/all/eaf54b8634970b73552dcd38bf9be6ef55238c10.1718092070.git.dvyukov@google.com/ > CFLAGS_irq.o := -I $(src)/../include/asm/trace > > obj-y += head_$(BITS).o > -- > 2.25.1 >
On Mon, Jul 22, 2024 at 10:36 PM Dmitry Vyukov <dvyukov@google.com> wrote: > > > +# These produce large amounts of uninteresting coverage. > > +KCOV_INSTRUMENT_dumpstack.o := n > > +KCOV_INSTRUMENT_dumpstack_$(BITS).o := n > > +KCOV_INSTRUMENT_stacktrace.o := n > > +KCOV_INSTRUMENT_unwind_orc.o := n > > +KCOV_INSTRUMENT_unwind_frame.o := n > > +KCOV_INSTRUMENT_unwind_guess.o := n Ah, I even reviewed it, completely forgot :( That's great then, thank you! This patch can be ignored.
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile index 20a0dd51700a..241e21723fa5 100644 --- a/arch/x86/kernel/Makefile +++ b/arch/x86/kernel/Makefile @@ -40,6 +40,14 @@ KMSAN_SANITIZE_sev.o := n KCOV_INSTRUMENT_head$(BITS).o := n KCOV_INSTRUMENT_sev.o := n +# These produce large amounts of uninteresting coverage. +KCOV_INSTRUMENT_dumpstack.o := n +KCOV_INSTRUMENT_dumpstack_$(BITS).o := n +KCOV_INSTRUMENT_stacktrace.o := n +KCOV_INSTRUMENT_unwind_orc.o := n +KCOV_INSTRUMENT_unwind_frame.o := n +KCOV_INSTRUMENT_unwind_guess.o := n + CFLAGS_irq.o := -I $(src)/../include/asm/trace obj-y += head_$(BITS).o