Re: [PATCH 5.15.y] perf symbols: Symbol lookup with kcore can fail if multiple segments match stext

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 28, 2023 at 04:04:35PM -0700, Krister Johansen wrote:
> 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(-)
> 

Both now queued up, thanks.

greg k-h