[RFC] perf tool: Fix memory corruption because of zero length symbols

[Ravi Bangoria]

Perf top is often crashing at very random locations on powerpc.
After investigating, I found the crash only happens when sample
is of zero length symbol. Powerpc kernel has many such symbols
which does not contain length details in vmlinux binary and thus
start and end addresses of such symbols are same.

Structure

  struct sym_hist {
        u64                   nr_samples;
        u64                   period;
        struct sym_hist_entry addr[0];
  };

has last member 'addr[]' of size zero. 'addr[]' is an array of
addresses that belongs to one symbol (function). If function
consist of 100 instructions, 'addr' points to an array of 100
'struct sym_hist_entry' elements. For zero length symbol, it
points to the *empty* array, i.e. no members in the array and
thus offset 0 is also invalid for such array.

  static int __symbol__inc_addr_samples(...)
  {
        ...
        offset = addr - sym->start;
        h = annotation__histogram(notes, evidx);
        h->nr_samples++;
        h->addr[offset].nr_samples++;
        h->period += sample->period;
        h->addr[offset].period += sample->period;
        ...
  }

Here, when 'addr' is same as 'sym->start', 'offset' becomes 0,
which is valid for normal symbols but *invalid* for zero length
symbols and thus updating h->addr[offset] causes memory corruption.

Fix this by adding one dummy element for zero length symbols.

Fixes: edee44be5919 ("perf annotate: Don't throw error for zero length symbols")
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
---
 tools/perf/util/annotate.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/annotate.c b/tools/perf/util/annotate.c
index 4397a8b..15db525 100644
--- a/tools/perf/util/annotate.c
+++ b/tools/perf/util/annotate.c
@@ -606,9 +606,19 @@ static struct arch *arch__find(const char *name)
 int symbol__alloc_hist(struct symbol *sym)
 {
 	struct annotation *notes = symbol__annotation(sym);
-	const size_t size = symbol__size(sym);
+	size_t size = symbol__size(sym);
 	size_t sizeof_sym_hist;
 
+	/*
+	 * Add buffer of one element for zero length symbol.
+	 * When sample is taken from first instruction of
+	 * zero length symbol, perf still resolves it and
+	 * shows symbol name in perf report and allows to
+	 * annotate it.
+	 */
+	if (size == 0)
+		size = 1;
+
 	/* Check for overflow when calculating sizeof_sym_hist */
 	if (size > (SIZE_MAX - sizeof(struct sym_hist)) / sizeof(struct sym_hist_entry))
 		return -1;
-- 
1.8.3.1