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Subject[PATCH 4.20 138/352] tracing: Have trace_stack nr_entries compare not be so subtle
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4.20-stable review patch.  If anyone has any objections, please let me know.

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

[ 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;
--
2.19.1


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