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SubjectRe: [PATCH 4.20 138/352] tracing: Have trace_stack nr_entries compare not be so subtle
On Mon, 11 Feb 2019 15:16:05 +0100
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> 4.20-stable review patch. If anyone has any objections, please let me know.
>

This is not a bug fix and was not meant for stable. Please do not apply.

-- Steve

> ------------------
>
> [ 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 2b0d1ee3241c..e2a153fc1afc 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;

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