From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: [patch 33/35] fs/seq_file.c: seq_read(): add info message about buggy .next functions Date: Fri, 10 Apr 2020 14:34:06 -0700 Message-ID: <20200410213406.YKfS5tNA-%akpm@linux-foundation.org> References: <20200410143047.bf34a933ce1affdc042c7c80@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:48338 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726582AbgDJVeH (ORCPT ); Fri, 10 Apr 2020 17:34:07 -0400 In-Reply-To: <20200410143047.bf34a933ce1affdc042c7c80@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: akpm@linux-foundation.org, dave@stgolabs.net, linux-mm@kvack.org, longman@redhat.com, manfred@colorfullife.com, mingo@redhat.com, mm-commits@vger.kernel.org, neilb@suse.com, oberpar@linux.ibm.com, rostedt@goodmis.org, torvalds@linux-foundation.org, viro@zeniv.linux.org.uk, vvs@virtuozzo.com From: Vasily Averin Subject: fs/seq_file.c: seq_read(): add info message about buggy .next functions Patch series "seq_file .next functions should increase position index". In Aug 2018 NeilBrown noticed commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") "Some ->next functions do not increment *pos when they return NULL... Note that such ->next functions are buggy and should be fixed. A simple demonstration is dd if=/proc/swaps bs=1000 skip=1 Choose any block size larger than the size of /proc/swaps. This will always show the whole last line of /proc/swaps" Described problem is still actual. If you make lseek into middle of last output line following read will output end of last line and whole last line once again. $ dd if=/proc/swaps bs=1 # usual output Filename Type Size Used Priority /dev/dm-0 partition 4194812 97536 -2 104+0 records in 104+0 records out 104 bytes copied $ dd if=/proc/swaps bs=40 skip=1 # last line was generated twice dd: /proc/swaps: cannot skip to specified offset v/dm-0 partition 4194812 97536 -2 /dev/dm-0 partition 4194812 97536 -2 3+1 records in 3+1 records out 131 bytes copied There are lot of other affected files, I've found 30+ including /proc/net/ip_tables_matches and /proc/sysvipc/* I've sent patches into maillists of affected subsystems already, this patch-set fixes the problem in files related to pstore, tracing, gcov, sysvipc and other subsystems processed via linux-kernel@ mailing list directly https://bugzilla.kernel.org/show_bug.cgi?id=206283 This patch (of 4): Add debug code to seq_read() to detect missed or out-of-tree incorrect .next seq_file functions. [akpm@linux-foundation.org: s/pr_info/pr_info_ratelimited/, per Qian Cai] https://bugzilla.kernel.org/show_bug.cgi?id=206283 Link: http://lkml.kernel.org/r/244674e5-760c-86bd-d08a-047042881748@virtuozzo.com Link: http://lkml.kernel.org/r/7c24087c-e280-e580-5b0c-0cdaeb14cd18@virtuozzo.com Signed-off-by: Vasily Averin Cc: NeilBrown Cc: Al Viro Cc: Steven Rostedt Cc: Davidlohr Bueso Cc: Ingo Molnar Cc: Manfred Spraul Cc: Peter Oberparleiter Cc: Waiman Long Signed-off-by: Andrew Morton --- fs/seq_file.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) --- a/fs/seq_file.c~seq_read-info-message-about-buggy-next-functions +++ a/fs/seq_file.c @@ -232,9 +232,12 @@ Fill: loff_t pos = m->index; p = m->op->next(m, p, &m->index); - if (pos == m->index) - /* Buggy ->next function */ + if (pos == m->index) { + pr_info_ratelimited("buggy seq_file .next function %ps " + "did not updated position index\n", + m->op->next); m->index++; + } if (!p || IS_ERR(p)) { err = PTR_ERR(p); break; _