From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: [patch 046/118] mm/swapfile.c: swap_next should increase position index Date: Thu, 30 Jan 2020 22:13:39 -0800 Message-ID: <20200131061339.vdpByFCaP%akpm@linux-foundation.org> References: <20200130221021.5f0211c56346d5485af07923@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:33242 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725907AbgAaGNk (ORCPT ); Fri, 31 Jan 2020 01:13:40 -0500 In-Reply-To: <20200130221021.5f0211c56346d5485af07923@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: akpm@linux-foundation.org, hughd@google.com, jannh@google.com, keescook@chromium.org, linux-mm@kvack.org, mm-commits@vger.kernel.org, torvalds@linux-foundation.org, viro@zeniv.linux.org.uk, vvs@virtuozzo.com From: Vasily Averin Subject: mm/swapfile.c: swap_next should increase position index If seq_file .next fuction does not change position index, read after some lseek can generate unexpected output. 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 https://bugzilla.kernel.org/show_bug.cgi?id=206283 Link: http://lkml.kernel.org/r/bd8cfd7b-ac95-9b91-f9e7-e8438bd5047d@virtuozzo.com Signed-off-by: Vasily Averin Reviewed-by: Andrew Morton Cc: Jann Horn Cc: Alexander Viro Cc: Kees Cook Cc: Hugh Dickins Signed-off-by: Andrew Morton --- mm/swapfile.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/mm/swapfile.c~swap_next-should-increase-position-index +++ a/mm/swapfile.c @@ -2737,10 +2737,10 @@ static void *swap_next(struct seq_file * else type = si->type + 1; + ++(*pos); for (; (si = swap_type_to_swap_info(type)); type++) { if (!(si->flags & SWP_USED) || !si->swap_map) continue; - ++*pos; return si; } _