commit 1c249565426e3a9940102c0ba9f63914f7cda73d upstream.
This problem was encountered on an arm64 system with a lot of memory. Without kernel debug symbols installed, and with both kcore and kallsyms available, perf managed to get confused and returned "unknown" for all of the kernel symbols that it tried to look up.
On this system, stext fell within the vmalloc segment. The kcore symbol matching code tries to find the first segment that contains stext and uses that to replace the segment generated from just the kallsyms information. In this case, however, there were two: a very large vmalloc segment, and the text segment. This caused perf to get confused because multiple overlapping segments were inserted into the RB tree that holds the discovered segments. However, that alone wasn't sufficient to cause the problem. Even when we could find the segment, the offsets were adjusted in such a way that the newly generated symbols didn't line up with the instruction addresses in the trace. The most obvious solution would be to consult which segment type is text from kcore, but this information is not exposed to users.
Instead, select the smallest matching segment that contains stext instead of the first matching segment. This allows us to match the text segment instead of vmalloc, if one is contained within the other.
Reviewed-by: Adrian Hunter adrian.hunter@intel.com Signed-off-by: Krister Johansen kjlx@templeofstupid.com Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: David Reaver me@davidreaver.com Cc: Ian Rogers irogers@google.com Cc: Jiri Olsa jolsa@kernel.org Cc: Mark Rutland mark.rutland@arm.com Cc: Michael Petlan mpetlan@redhat.com Cc: Namhyung Kim namhyung@kernel.org Cc: Peter Zijlstra peterz@infradead.org Link: http://lore.kernel.org/lkml/20230125183418.GD1963@templeofstupid.com Signed-off-by: Arnaldo Carvalho de Melo acme@redhat.com Signed-off-by: Krister Johansen kjlx@templeofstupid.com --- tools/perf/util/symbol.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c index a3a165ae933a..98014f937568 100644 --- a/tools/perf/util/symbol.c +++ b/tools/perf/util/symbol.c @@ -1368,10 +1368,23 @@ static int dso__load_kcore(struct dso *dso, struct map *map,
/* Find the kernel map using the '_stext' symbol */ if (!kallsyms__get_function_start(kallsyms_filename, "_stext", &stext)) { + u64 replacement_size = 0; + list_for_each_entry(new_map, &md.maps, node) { - if (stext >= new_map->start && stext < new_map->end) { + u64 new_size = new_map->end - new_map->start; + + if (!(stext >= new_map->start && stext < new_map->end)) + continue; + + /* + * On some architectures, ARM64 for example, the kernel + * text can get allocated inside of the vmalloc segment. + * Select the smallest matching segment, in case stext + * falls within more than one in the list. + */ + if (!replacement_map || new_size < replacement_size) { replacement_map = new_map; - break; + replacement_size = new_size; } } }