From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f200.google.com (mail-io0-f200.google.com [209.85.223.200]) by kanga.kvack.org (Postfix) with ESMTP id 93DF36B0260 for ; Mon, 9 Oct 2017 14:46:39 -0400 (EDT) Received: by mail-io0-f200.google.com with SMTP id 101so2793389ioj.2 for ; Mon, 09 Oct 2017 11:46:39 -0700 (PDT) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id k187sor257777iof.127.2017.10.09.11.46.38 for (Google Transport Security); Mon, 09 Oct 2017 11:46:38 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20171009183734.GA7784@leverpostej> References: <20171009150521.82775-1-glider@google.com> <20171009154610.GA22534@leverpostej> <20171009183734.GA7784@leverpostej> From: Dmitry Vyukov Date: Mon, 9 Oct 2017 20:46:18 +0200 Message-ID: Subject: Re: [PATCH v2 1/3] kcov: support comparison operands collection Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Mark Rutland Cc: Alexander Potapenko , Andrew Morton , Alexander Popov , Andrey Ryabinin , Quentin Casasnovas , andreyknvl , Kees Cook , Vegard Nossum , syzkaller , "linux-mm@kvack.org" , LKML On Mon, Oct 9, 2017 at 8:37 PM, Mark Rutland 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 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: email@kvack.org