Hi, On Wed, Aug 30, 2017 at 06:23:29PM +0200, Dmitry Vyukov wrote: > From: Victor Chibotaru <tchibo@xxxxxxxxxx> > > Enables kcov to collect comparison operands from instrumented code. > This is done by using Clang's -fsanitize=trace-cmp instrumentation > (currently not available for GCC). What's needed to build the kernel with Clang these days? I was under the impression that it still wasn't possible to build arm64 with clang due to a number of missing features (e.g. the %a assembler output template). > The comparison operands help a lot in fuzz testing. E.g. they are > used in Syzkaller to cover the interiors of conditional statements > with way less attempts and thus make previously unreachable code > reachable. > > To allow separate collection of coverage and comparison operands two > different work modes are implemented. Mode selection is now done via > a KCOV_ENABLE ioctl call with corresponding argument value. > > Signed-off-by: Victor Chibotaru <tchibo@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Mark Rutland <mark.rutland@xxxxxxx> > Cc: Alexander Popov <alex.popov@xxxxxxxxx> > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx> > Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx> > Cc: syzkaller@xxxxxxxxxxxxxxxx > Cc: linux-mm@xxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx > --- > Clang instrumentation: > https://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow How stable is this? The comment at the end says "This interface is a subject to change." [...] > diff --git a/kernel/kcov.c b/kernel/kcov.c > index cd771993f96f..2abce5dfa2df 100644 > --- a/kernel/kcov.c > +++ b/kernel/kcov.c > @@ -21,13 +21,21 @@ > #include <linux/kcov.h> > #include <asm/setup.h> > > +/* Number of words written per one comparison. */ > +#define KCOV_WORDS_PER_CMP 3 Could you please expand the comment to cover what a "word" is? Generally, "word" is an ambiguous term, and it's used inconsitently in this file as of this patch. For comparison coverage, a "word" is assumed to always be 64-bit, (which makes sxense given 64-bit comparisons), but for branch coverage a "word" refers to an unsigned long, which would be 32-bit on a 32-bit platform. [...] > +static bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t) Perhaps kcov_mode_is_active()? That would better describe what is being checked. > +{ > + enum kcov_mode mode; > + > + /* > + * We are interested in code coverage as a function of a syscall inputs, > + * so we ignore code executed in interrupts. > + */ > + if (!t || !in_task()) > + return false; > + mode = READ_ONCE(t->kcov_mode); > + /* > + * There is some code that runs in interrupts but for which > + * in_interrupt() returns false (e.g. preempt_schedule_irq()). > + * READ_ONCE()/barrier() effectively provides load-acquire wrt > + * interrupts, there are paired barrier()/WRITE_ONCE() in > + * kcov_ioctl_locked(). > + */ > + barrier(); > + if (mode != needed_mode) > + return false; > + return true; This would be simlper as: barrier(); return mode == needed_mode; [...] > +#ifdef CONFIG_KCOV_ENABLE_COMPARISONS > +static void write_comp_data(u64 type, u64 arg1, u64 arg2) > +{ > + struct task_struct *t; > + u64 *area; > + u64 count, start_index, end_pos, max_pos; > + > + t = current; > + if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t)) > + return; > + > + /* > + * We write all comparison arguments and types as u64. > + * The buffer was allocated for t->kcov_size unsigned longs. > + */ > + area = (u64 *)t->kcov_area; > + max_pos = t->kcov_size * sizeof(unsigned long); Perhaps it would make more sense for k->kcov_size to be in bytes, if different options will have differing record sizes? > + > + count = READ_ONCE(area[0]); > + > + /* Every record is KCOV_WORDS_PER_CMP words. */ As above, please be explicit about what a "word" is, or avoid using "word" terminology. Thanks, Mark. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
