From: Dmitry Vyukov <dvyukov@google.com> To: Mark Rutland <mark.rutland@arm.com> Cc: Alexander Potapenko <glider@google.com>, Andrew Morton <akpm@linux-foundation.org>, Alexander Popov <alex.popov@linux.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Quentin Casasnovas <quentin.casasnovas@oracle.com>, andreyknvl <andreyknvl@google.com>, Kees Cook <keescook@chromium.org>, Vegard Nossum <vegard.nossum@oracle.com>, syzkaller <syzkaller@googlegroups.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org> Subject: Re: [PATCH v2 1/3] kcov: support comparison operands collection Date: Mon, 9 Oct 2017 20:46:18 +0200 [thread overview] Message-ID: <CACT4Y+apUD89-neN7GUsbdZ9a1hMgRPQk-h4dhC9iDf+_6Kh=w@mail.gmail.com> (raw) In-Reply-To: <20171009183734.GA7784@leverpostej> On Mon, Oct 9, 2017 at 8:37 PM, Mark Rutland <mark.rutland@arm.com> wrote: > On Mon, Oct 09, 2017 at 08:15:10PM +0200, 'Dmitry Vyukov' via syzkaller wrote: >> On Mon, Oct 9, 2017 at 5:46 PM, Mark Rutland <mark.rutland@arm.com> wrote: >> > On Mon, Oct 09, 2017 at 05:05:19PM +0200, Alexander Potapenko wrote: > >> > ... I note that a few places in the kernel use a 128-bit type. Are >> > 128-bit comparisons not instrumented? >> >> Yes, they are not instrumented. >> How many are there? Can you give some examples? > > From a quick scan, it doesn't looks like there are currently any > comparisons. > > It's used as a data type in a few places under arm64: > > arch/arm64/include/asm/checksum.h: __uint128_t tmp; > arch/arm64/include/asm/checksum.h: tmp = *(const __uint128_t *)iph; > arch/arm64/include/asm/fpsimd.h: __uint128_t vregs[32]; > arch/arm64/include/uapi/asm/ptrace.h: __uint128_t vregs[32]; > arch/arm64/include/uapi/asm/sigcontext.h: __uint128_t vregs[32]; > arch/arm64/kernel/signal32.c: __uint128_t raw; > arch/arm64/kvm/guest.c: __uint128_t tmp; Then I think we just continue ignoring them for now :) In the future we can extend kcov to trace 128-bits values. We will need to add a special flag and write 2 consecutive entries for them. Or something along these lines. >> >> + area = t->kcov_area; >> >> + /* The first 64-bit word is the number of subsequent PCs. */ >> >> + pos = READ_ONCE(area[0]) + 1; >> >> + if (likely(pos < t->kcov_size)) { >> >> + area[pos] = ip; >> >> + WRITE_ONCE(area[0], pos); >> > >> > Not a new problem, but if the area for one thread is mmap'd, and read by >> > another thread, these two writes could be seen out-of-order, since we >> > don't have an smp_wmb() between them. >> > >> > I guess Syzkaller doesn't read the mmap'd kcov file from another thread? >> >> >> Yes, that's the intention. If you read coverage from another thread, >> you can't know coverage from what exactly you read. So the usage >> pattern is: >> >> reset coverage; >> do something; >> read coverage; > > Ok. I guess without a use-case for reading this from another thread it doesn't > really matter. > > Thanks, > Mark.
WARNING: multiple messages have this Message-ID (diff)
From: Dmitry Vyukov <dvyukov@google.com> To: Mark Rutland <mark.rutland@arm.com> Cc: Alexander Potapenko <glider@google.com>, Andrew Morton <akpm@linux-foundation.org>, Alexander Popov <alex.popov@linux.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Quentin Casasnovas <quentin.casasnovas@oracle.com>, andreyknvl <andreyknvl@google.com>, Kees Cook <keescook@chromium.org>, Vegard Nossum <vegard.nossum@oracle.com>, syzkaller <syzkaller@googlegroups.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org> Subject: Re: [PATCH v2 1/3] kcov: support comparison operands collection Date: Mon, 9 Oct 2017 20:46:18 +0200 [thread overview] Message-ID: <CACT4Y+apUD89-neN7GUsbdZ9a1hMgRPQk-h4dhC9iDf+_6Kh=w@mail.gmail.com> (raw) In-Reply-To: <20171009183734.GA7784@leverpostej> On Mon, Oct 9, 2017 at 8:37 PM, Mark Rutland <mark.rutland@arm.com> wrote: > On Mon, Oct 09, 2017 at 08:15:10PM +0200, 'Dmitry Vyukov' via syzkaller wrote: >> On Mon, Oct 9, 2017 at 5:46 PM, Mark Rutland <mark.rutland@arm.com> wrote: >> > On Mon, Oct 09, 2017 at 05:05:19PM +0200, Alexander Potapenko wrote: > >> > ... I note that a few places in the kernel use a 128-bit type. Are >> > 128-bit comparisons not instrumented? >> >> Yes, they are not instrumented. >> How many are there? Can you give some examples? > > From a quick scan, it doesn't looks like there are currently any > comparisons. > > It's used as a data type in a few places under arm64: > > arch/arm64/include/asm/checksum.h: __uint128_t tmp; > arch/arm64/include/asm/checksum.h: tmp = *(const __uint128_t *)iph; > arch/arm64/include/asm/fpsimd.h: __uint128_t vregs[32]; > arch/arm64/include/uapi/asm/ptrace.h: __uint128_t vregs[32]; > arch/arm64/include/uapi/asm/sigcontext.h: __uint128_t vregs[32]; > arch/arm64/kernel/signal32.c: __uint128_t raw; > arch/arm64/kvm/guest.c: __uint128_t tmp; Then I think we just continue ignoring them for now :) In the future we can extend kcov to trace 128-bits values. We will need to add a special flag and write 2 consecutive entries for them. Or something along these lines. >> >> + area = t->kcov_area; >> >> + /* The first 64-bit word is the number of subsequent PCs. */ >> >> + pos = READ_ONCE(area[0]) + 1; >> >> + if (likely(pos < t->kcov_size)) { >> >> + area[pos] = ip; >> >> + WRITE_ONCE(area[0], pos); >> > >> > Not a new problem, but if the area for one thread is mmap'd, and read by >> > another thread, these two writes could be seen out-of-order, since we >> > don't have an smp_wmb() between them. >> > >> > I guess Syzkaller doesn't read the mmap'd kcov file from another thread? >> >> >> Yes, that's the intention. If you read coverage from another thread, >> you can't know coverage from what exactly you read. So the usage >> pattern is: >> >> reset coverage; >> do something; >> read coverage; > > Ok. I guess without a use-case for reading this from another thread it doesn't > really matter. > > Thanks, > Mark. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-10-09 18:46 UTC|newest] Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-10-09 15:05 [PATCH v2 1/3] kcov: support comparison operands collection Alexander Potapenko 2017-10-09 15:05 ` Alexander Potapenko 2017-10-09 15:05 ` [PATCH v2 2/3] Makefile: support flag -fsanitizer-coverage=trace-cmp Alexander Potapenko 2017-10-09 15:05 ` Alexander Potapenko 2017-10-09 15:53 ` Andrey Ryabinin 2017-10-09 15:53 ` Andrey Ryabinin 2017-10-10 15:28 ` Alexander Potapenko 2017-10-10 15:28 ` Alexander Potapenko 2017-10-10 21:53 ` kbuild test robot 2017-10-09 15:05 ` [PATCH v2 3/3] kcov: update documentation Alexander Potapenko 2017-10-09 15:05 ` Alexander Potapenko 2017-10-09 15:46 ` [PATCH v2 1/3] kcov: support comparison operands collection Mark Rutland 2017-10-09 15:46 ` Mark Rutland 2017-10-09 18:15 ` Dmitry Vyukov 2017-10-09 18:15 ` Dmitry Vyukov 2017-10-09 18:37 ` Mark Rutland 2017-10-09 18:37 ` Mark Rutland 2017-10-09 18:46 ` Dmitry Vyukov [this message] 2017-10-09 18:46 ` Dmitry Vyukov 2017-10-10 9:56 ` Mark Rutland 2017-10-10 9:56 ` Mark Rutland 2017-10-10 15:28 ` Alexander Potapenko 2017-10-10 15:28 ` Alexander Potapenko 2017-10-10 15:34 ` Dmitry Vyukov 2017-10-10 15:34 ` Dmitry Vyukov 2017-10-11 9:56 ` Alexander Potapenko 2017-10-11 9:56 ` Alexander Potapenko
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