Re: [PATCH] trace: correct start_index in find_next

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On 2015/12/31 13:11, Qiu, PeiyangX wrote:
> From: Qiu Peiyang <peiyangx.qiu@intel.com>
>
> 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.
>
> Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>
> ---
>  kernel/trace/trace_printk.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
> index 1c2b285..64f0a26 100644
> --- a/kernel/trace/trace_printk.c
> +++ b/kernel/trace/trace_printk.c
> @@ -273,6 +273,7 @@ static const char **find_next(void *v, loff_t *pos)
>  	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);
>  }

Rusty, Steven,

How is this patch? It fixes a kernel panic.

Yanmin