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    Subject[094/152] tracing: Fix panic when lseek() called on "trace" opened for writing
    2.6.36-stable review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Slava Pestov <slavapestov@google.com>

    commit 364829b1263b44aa60383824e4c1289d83d78ca7 upstream.

    The file_ops struct for the "trace" special file defined llseek as seq_lseek().
    However, if the file was opened for writing only, seq_open() was not called,
    and the seek would dereference a null pointer, file->private_data.

    This patch introduces a new wrapper for seq_lseek() which checks if the file
    descriptor is opened for reading first. If not, it does nothing.

    Signed-off-by: Slava Pestov <slavapestov@google.com>
    LKML-Reference: <1290640396-24179-1-git-send-email-slavapestov@google.com>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    kernel/trace/trace.c | 10 +++++++++-
    1 file changed, 9 insertions(+), 1 deletion(-)

    --- a/kernel/trace/trace.c
    +++ b/kernel/trace/trace.c
    @@ -2320,11 +2320,19 @@ tracing_write_stub(struct file *filp, co
    return count;
    }

    +static loff_t tracing_seek(struct file *file, loff_t offset, int origin)
    +{
    + if (file->f_mode & FMODE_READ)
    + return seq_lseek(file, offset, origin);
    + else
    + return 0;
    +}
    +
    static const struct file_operations tracing_fops = {
    .open = tracing_open,
    .read = seq_read,
    .write = tracing_write_stub,
    - .llseek = seq_lseek,
    + .llseek = tracing_seek,
    .release = tracing_release,
    };




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