queue-4.19/tracing-have-trace_stack-nr_entries-compare-not-be-s.patch

   1 From af4dc639e26f2f315eeb3bb2e9f593871d7d1f8c Mon Sep 17 00:00:00 2001
   2 From: Dan Carpenter <dan.carpenter@oracle.com>
   3 Date: Wed, 20 Jun 2018 14:08:00 +0300
   4 Subject: tracing: Have trace_stack nr_entries compare not be so subtle
   5
   6 [ Upstream commit ca16b0fbb05242f18da9d810c07d3882ffed831c ]
   7
   8 Dan Carpenter reviewed the trace_stack.c code and figured he found an off by
   9 one bug.
  10
  11  "From reviewing the code, it seems possible for
  12   stack_trace_max.nr_entries to be set to .max_entries and in that case we
  13   would be reading one element beyond the end of the stack_dump_trace[]
  14   array.  If it's not set to .max_entries then the bug doesn't affect
  15   runtime."
  16
  17 Although it looks to be the case, it is not. Because we have:
  18
  19  static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
  20          { [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX };
  21
  22  struct stack_trace stack_trace_max = {
  23         .max_entries            = STACK_TRACE_ENTRIES - 1,
  24         .entries                = &stack_dump_trace[0],
  25  };
  26
  27 And:
  28
  29         stack_trace_max.nr_entries = x;
  30         for (; x < i; x++)
  31                 stack_dump_trace[x] = ULONG_MAX;
  32
  33 Even if nr_entries equals max_entries, indexing with it into the
  34 stack_dump_trace[] array will not overflow the array. But if it is the case,
  35 the second part of the conditional that tests stack_dump_trace[nr_entries]
  36 to ULONG_MAX will always be true.
  37
  38 By applying Dan's patch, it removes the subtle aspect of it and makes the if
  39 conditional slightly more efficient.
  40
  41 Link: http://lkml.kernel.org/r/20180620110758.crunhd5bfep7zuiz@kili.mountain
  42
  43 Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
  44 Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
  45 Signed-off-by: Sasha Levin <sashal@kernel.org>
  46 ---
  47  kernel/trace/trace_stack.c | 2 +-
  48  1 file changed, 1 insertion(+), 1 deletion(-)
  49
  50 diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
  51 index 4237eba4ef20..6e3edd745c68 100644
  52 --- a/kernel/trace/trace_stack.c
  53 +++ b/kernel/trace/trace_stack.c
  54 @@ -286,7 +286,7 @@ __next(struct seq_file *m, loff_t *pos)
  55  {
  56         long n = *pos - 1;
  57
  58 -       if (n > stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
  59 +       if (n >= stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
  60                 return NULL;
  61
  62         m->private = (void *)n;
  63 --
  64 2.19.1
  65