Patch "tracing: Fix setting of start_index in find_next()" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

tracing: Fix setting of start_index in find_next()

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tracing-fix-setting-of-start_index-in-find_next.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Qiu Peiyang <[email protected]>
Date: Thu, 31 Dec 2015 13:11:28 +0800
Subject: tracing: Fix setting of start_index in find_next()

From: Qiu Peiyang <[email protected]>

commit f36d1be2930ede0a1947686e1126ffda5d5ee1bb upstream.

When we do cat /sys/kernel/debug/tracing/printk_formats, we hit kernel panic at t_show.

general protection fault: 0000 [#1] PREEMPT SMP
CPU: 0 PID: 2957 Comm: sh Tainted: G W  O 3.14.55-x86_64-01062-gd4acdc7 #2
RIP: 0010:[<ffffffff811375b2>]
 [<ffffffff811375b2>] t_show+0x22/0xe0
RSP: 0000:ffff88002b4ebe80  EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000004
RDX: 0000000000000004 RSI: ffffffff81fd26a6 RDI: ffff880032f9f7b1
RBP: ffff88002b4ebe98 R08: 0000000000001000 R09: 000000000000ffec
R10: 0000000000000000 R11: 000000000000000f R12: ffff880004d9b6c0
R13: 7365725f6d706400 R14: ffff880004d9b6c0 R15: ffffffff82020570
FS:  0000000000000000(0000) GS:ffff88003aa00000(0063) knlGS:00000000f776bc40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00000000f6c02ff0 CR3: 000000002c2b3000 CR4: 00000000001007f0
Call Trace:
 [<ffffffff811dc076>] seq_read+0x2f6/0x3e0
 [<ffffffff811b749b>] vfs_read+0x9b/0x160
 [<ffffffff811b7f69>] SyS_read+0x49/0xb0
 [<ffffffff81a3a4b9>] ia32_do_call+0x13/0x13
 ---[ end trace 5bd9eb630614861e ]---
Kernel panic - not syncing: Fatal exception

When the first time find_next calls find_next_mod_format, it should iterate the trace_bprintk_fmt_list to find the first print format of the module. However in current code, start_index is smaller than *pos at first, and code will not iterate the list. Latter container_of will get the wrong address with former v, which will cause mod_fmt be a meaningless object and so is the returned mod_fmt->fmt.

This patch will fix it by correcting the start_index. After fixed, when the first time calls find_next_mod_format, start_index will be equal to *pos, and code will iterate the trace_bprintk_fmt_list to get the right module printk format, so is the returned mod_fmt->fmt.

---
 kernel/trace/trace_printk.c |    1 +
 1 file changed, 1 insertion(+)

@@ -267,6 +267,7 @@ static const char **find_next(void *v, l
 	if (*pos < last_index + start_index)
 		return __start___tracepoint_str + (*pos - last_index);
 
+	start_index += last_index;
 	return find_next_mod_format(start_index, v, fmt, pos);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/tracing-fix-setting-of-start_index-in-find_next.patch