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From:   Vasily Averin <vvs@virtuozzo.com>
Subject: [PATCH 0/7] seq_file .next functions should increase position index
To:     linux-kernel@vger.kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        NeilBrown <neilb@suse.com>, Waiman Long <longman@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>
Message-ID: <244674e5-760c-86bd-d08a-047042881748@virtuozzo.com>
Date:   Fri, 24 Jan 2020 10:02:36 +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/*

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

Vasily Averin (7):
  pstore_ftrace_seq_next should increase position index
  gcov_seq_next should increase position index
  t_next should increase position index
  fpid_next should increase position index
  eval_map_next should increase position index
  trigger_next should increase position index
  sysvipc_find_ipc should increase position index

 fs/pstore/inode.c                   | 2 +-
 ipc/util.c                          | 2 +-
 kernel/gcov/fs.c                    | 2 +-
 kernel/trace/ftrace.c               | 9 ++++++---
 kernel/trace/trace.c                | 4 +---
 kernel/trace/trace_events_trigger.c | 5 +++--
 6 files changed, 13 insertions(+), 11 deletions(-)

-- 
1.8.3.1