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Date: Mon, 11 Feb 2019 15:16:36 +0100 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Dan Carpenter <dan.carpenter@...cle.com>, "Steven Rostedt (VMware)" <rostedt@...dmis.org>, Sasha Levin <sashal@...nel.org> Subject: [PATCH 4.19 116/313] tracing: Have trace_stack nr_entries compare not be so subtle 4.19-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@...cle.com> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org> Signed-off-by: Sasha Levin <sashal@...nel.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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