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    Subject[PATCH 3.19.y-ckt 133/160] tracing: Fix setting of start_index in find_next()
    Date
    3.19.8-ckt13 -stable review patch.  If anyone has any objections, please let me know.

    ---8<------------------------------------------------------------

    From: Qiu Peiyang <peiyangx.qiu@intel.com>

    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.

    Link: http://lkml.kernel.org/r/5684B900.9000309@intel.com

    Fixes: 102c9323c35a8 "tracing: Add __tracepoint_string() to export string pointers"
    Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: Kamal Mostafa <kamal@canonical.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 c4e70b6..01443a1 100644
    --- a/kernel/trace/trace_printk.c
    +++ b/kernel/trace/trace_printk.c
    @@ -269,6 +269,7 @@ 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);
    }

    --
    1.9.1

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