[PATCH] perf: fix symbols__fixup_end heuristic for corner cases

[Daniel Borkmann]

The current symbols__fixup_end() heuristic for the last entry in the
rb tree is suboptimal as it leads to not being able to recognize the
symbol in the call graph in a couple of corner cases, for example:

 i) If the symbol has a start address (f.e. exposed via kallsyms)
    that is at a page boundary, then the roundup(curr->start, 4096)
    for the last entry will result in curr->start == curr->end with
    a symbol length of zero.

ii) If the symbol has a start address that is shortly before a page
    boundary, then also here, curr->end - curr->start will just be
    very few bytes, where it's unrealistic that we could perform a
    match against.

Instead, change the heuristic to roundup(curr->start, 4096) + 4096,
so that we can catch such corner cases and have a better chance to
find that specific symbol. It's still just best effort as the real
end of the symbol is unknown to us (and could even be at a larger
offset than the current range), but better than the current situation.

Alexei reported that he recently run into case i) with a JITed eBPF
program (these are all page aligned) as the last symbol which wasn't
properly shown in the call graph (while other eBPF program symbols
in the rb tree were displayed correctly). Since this is a generic
issue, lets try to improve the heuristic a bit.

Fixes: 2e538c4a1847 ("perf tools: Improve kernel/modules symbol lookup")
Reported-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
 tools/perf/util/symbol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 70e389b..9b4d8ba 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -202,7 +202,7 @@ void symbols__fixup_end(struct rb_root *symbols)
 
 	/* Last entry */
 	if (curr->end == curr->start)
-		curr->end = roundup(curr->start, 4096);
+		curr->end = roundup(curr->start, 4096) + 4096;
 }
 
 void __map_groups__fixup_end(struct map_groups *mg, enum map_type type)
-- 
1.9.3