[PATCH v2 0/2 RESEND] cgroup: seq_file .next functions should increase position index

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v2 changes:
- improved patch description
- cgroupv2 patch includes fix for __cgroup_procs_start(), advised by Michal Koutný

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/*

This patch set fixes the problem in cgroup-related files

Vasily Averin (2):
  cgroup-v1: cgroup_pidlist_next should update position index
  cgroup: cgroup_procs_next should increase position index

 kernel/cgroup/cgroup-v1.c |  1 +
 kernel/cgroup/cgroup.c    | 10 +++++++---
 2 files changed, 8 insertions(+), 3 deletions(-)

-- 
1.8.3.1




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