Messages in this thread | | | Date | Wed, 25 Jan 2023 09:29:31 +0200 | Subject | Re: [PATCH] perf/util: Symbol lookup can fail if multiple segmets match stext | From | Adrian Hunter <> |
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On 25/01/23 00:35, Krister Johansen wrote: > 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. > > Signed-off-by: Krister Johansen <kjlx@templeofstupid.com> > --- > tools/perf/util/symbol.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c > index a3a165ae933a..14ac4189eaff 100644 > --- a/tools/perf/util/symbol.c > +++ b/tools/perf/util/symbol.c > @@ -1368,10 +1368,16 @@ 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;
We'd usually put a blank line here
> 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; > +
Really needs a comment, and please be specific e.g.
ARM64 vmalloc segment overlaps the kernel text segment, so choosing the smaller segment will get the kernel text.
> + if (!replacement_map || new_size < replacement_size) { > replacement_map = new_map; > - break; > + replacement_size = new_size; > } > } > }
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