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Subject[PATCH 0/7] seq_file .next functions should increase position index
Date
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

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