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    Subject[PATCH AUTOSEL 4.19 123/258] tracing: Have trace_stack nr_entries compare not be so subtle
    Date
    From: Dan Carpenter <dan.carpenter@oracle.com>

    [ Upstream commit ca16b0fbb05242f18da9d810c07d3882ffed831c ]

    Dan Carpenter reviewed the trace_stack.c code and figured he found an off by
    one bug.

    "From reviewing the code, it seems possible for
    stack_trace_max.nr_entries to be set to .max_entries and in that case we
    would be reading one element beyond the end of the stack_dump_trace[]
    array. If it's not set to .max_entries then the bug doesn't affect
    runtime."

    Although it looks to be the case, it is not. Because we have:

    static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
    { [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX };

    struct stack_trace stack_trace_max = {
    .max_entries = STACK_TRACE_ENTRIES - 1,
    .entries = &stack_dump_trace[0],
    };

    And:

    stack_trace_max.nr_entries = x;
    for (; x < i; x++)
    stack_dump_trace[x] = ULONG_MAX;

    Even if nr_entries equals max_entries, indexing with it into the
    stack_dump_trace[] array will not overflow the array. But if it is the case,
    the second part of the conditional that tests stack_dump_trace[nr_entries]
    to ULONG_MAX will always be true.

    By applying Dan's patch, it removes the subtle aspect of it and makes the if
    conditional slightly more efficient.

    Link: http://lkml.kernel.org/r/20180620110758.crunhd5bfep7zuiz@kili.mountain

    Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/trace/trace_stack.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
    index 4237eba4ef20..6e3edd745c68 100644
    --- a/kernel/trace/trace_stack.c
    +++ b/kernel/trace/trace_stack.c
    @@ -286,7 +286,7 @@ __next(struct seq_file *m, loff_t *pos)
    {
    long n = *pos - 1;

    - if (n > stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
    + if (n >= stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
    return NULL;

    m->private = (void *)n;
    --
    2.19.1
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