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[209.85.220.65]) by mx.google.com with SMTPS id v62sor4726503jaa.10.2019.03.20.03.38.32 for (Google Transport Security); Wed, 20 Mar 2019 03:38:32 -0700 (PDT) Received-SPF: pass (google.com: domain of dvyukov@google.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=QJ0Hixfu; spf=pass (google.com: domain of dvyukov@google.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=dvyukov@google.com; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=jmTWTsjVde8Buo2u+HYWbLOnOnpUNQfeTfb58oufkzI=; b=QJ0Hixfu5hcP+OFD1pi83mFWhFYFQQSuLAY5azSzK1Z0hCoctBQFEUwJ3f+Y/VGYEs 3PrcLIjfbZB1UJki9W2oG8Yvj7/3C8jMjCHX8G7+CvIhafbtBJUq/RVj7VtbdIQlLxyv IyabyWBUzqVe8JITAbfCLJ8ln9FqUkgcwKaXhMwHvHpGMR0AUoCE0zClrsX7pJPc2LOP Y7bIuhc4F9jTAxR/SGC2rNDbUNXrPorOtIJoaoQgmOaIs+INa7CDUpL7zCU/HQOuCrmt 9UYNEZvjD7PjjoK7qcv0p5ONx5+qNWVQSKus6VlZjbyYmKVSreGt8Gpn5cVtS0uiOJBe sbIA== X-Google-Smtp-Source: APXvYqy33umIL3MTXjGhjlwdTvUuLGhoU88o0cygKpqVUozi+RPmVzBEzCyLcYDMDt639xeDV//bSwRJbj7diqKadhU= X-Received: by 2002:a05:6638:211:: with SMTP id e17mr3466378jaq.35.1553078311470; Wed, 20 Mar 2019 03:38:31 -0700 (PDT) MIME-Version: 1.0 References: <000000000000db3d130584506672@google.com> <426293c3-bf63-88ad-06fb-83927ab0d7c0@I-love.SAKURA.ne.jp> In-Reply-To: <426293c3-bf63-88ad-06fb-83927ab0d7c0@I-love.SAKURA.ne.jp> From: Dmitry Vyukov Date: Wed, 20 Mar 2019 11:38:20 +0100 Message-ID: Subject: Re: kernel panic: corrupted stack end in wb_workfn To: Tetsuo Handa Cc: Andrey Ryabinin , syzbot , Andrew Morton , Qian Cai , David Miller , guro@fb.com, Johannes Weiner , Josef Bacik , Kirill Tkhai , LKML , Linux-MM , linux-sctp@vger.kernel.org, Mel Gorman , Michal Hocko , netdev , Neil Horman , Shakeel Butt , syzkaller-bugs , Al Viro , Vladislav Yasevich , Matthew Wilcox , Xin Long Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Mar 20, 2019 at 11:24 AM Tetsuo Handa wrote: > > On 2019/03/20 18:59, Dmitry Vyukov wrote: > >> From bisection log: > >> > >> testing release v4.17 > >> testing commit 29dcea88779c856c7dc92040a0c01233263101d4 with gcc (GCC) 8.1.0 > >> run #0: crashed: kernel panic: corrupted stack end in wb_workfn > >> run #1: crashed: kernel panic: corrupted stack end in worker_thread > >> run #2: crashed: kernel panic: Out of memory and no killable processes... > >> run #3: crashed: kernel panic: corrupted stack end in wb_workfn > >> run #4: crashed: kernel panic: corrupted stack end in wb_workfn > >> run #5: crashed: kernel panic: corrupted stack end in wb_workfn > >> run #6: crashed: kernel panic: corrupted stack end in wb_workfn > >> run #7: crashed: kernel panic: corrupted stack end in wb_workfn > >> run #8: crashed: kernel panic: Out of memory and no killable processes... > >> run #9: crashed: kernel panic: corrupted stack end in wb_workfn > >> testing release v4.16 > >> testing commit 0adb32858b0bddf4ada5f364a84ed60b196dbcda with gcc (GCC) 8.1.0 > >> run #0: OK > >> run #1: OK > >> run #2: OK > >> run #3: OK > >> run #4: OK > >> run #5: crashed: kernel panic: Out of memory and no killable processes... > >> run #6: OK > >> run #7: crashed: kernel panic: Out of memory and no killable processes... > >> run #8: OK > >> run #9: OK > >> testing release v4.15 > >> testing commit d8a5b80568a9cb66810e75b182018e9edb68e8ff with gcc (GCC) 8.1.0 > >> all runs: OK > >> # git bisect start v4.16 v4.15 > >> > >> Why bisect started between 4.16 4.15 instead of 4.17 4.16? > > > > Because 4.16 was still crashing and 4.15 was not crashing. 4.15..4.16 > > looks like the right range, no? > > No, syzbot should bisect between 4.16 and 4.17 regarding this bug, for > "Stack corruption" can't manifest as "Out of memory and no killable processes". > > "kernel panic: Out of memory and no killable processes..." is completely > unrelated to "kernel panic: corrupted stack end in wb_workfn". Do you think this predicate is possible to code? Looking at the examples we have, distinguishing different bugs does not look feasible to me. If the predicate is not accurate, you just trade one set of false positives to another set of false positives and then you at the beginning of an infinite slippery slope refining it. Also, if we see a different bug (assuming we can distinguish them), does it mean that the original bug is not present? Or it's also present, but we just hit the other one first? This also does not look feasible to answer. And if you give a wrong answer, bisection goes the wrong way and we are where we started. Just with more complex code and things being even harder to explain to other people. I mean, yes, I agree, kernel bug bisection won't be perfect. But do you see anything actionable here?