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From:   Vasily Averin <vvs@virtuozzo.com>
Subject: [PATCH 0/1] jbd2: seq_file .next functions should increase position
 index
To:     linux-ext4@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>
Cc:     Jan Kara <jack@suse.com>
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Date:   Thu, 23 Jan 2020 12:05:05 +0300
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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/*

Following patch fixes ext4-related file, script below generates endless output
 $ q=;while read -r r;do echo "$((++q)) $r";done </proc/fs/jbd2/DEV/info

https://bugzilla.kernel.org/show_bug.cgi?id=206283

Vasily Averin (1):
  jbd2_seq_info_next should increase position index

 fs/jbd2/journal.c | 1 +
 1 file changed, 1 insertion(+)

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1.8.3.1