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Mon, 19 Jan 2026 02:09:16 -0800 (PST) From: James Clark Date: Mon, 19 Jan 2026 10:08:16 +0000 Subject: [PATCH 1/2] perf cs-etm: Fix decoding for sparse CPU maps Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260119-james-perf-coresight-cpu-map-segfault-v1-1-a9cfd67653ae@linaro.org> References: <20260119-james-perf-coresight-cpu-map-segfault-v1-0-a9cfd67653ae@linaro.org> In-Reply-To: <20260119-james-perf-coresight-cpu-map-segfault-v1-0-a9cfd67653ae@linaro.org> To: Suzuki K Poulose , Mike Leach , John Garry , Will Deacon , Leo Yan , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Thomas Falcon Cc: coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, James Clark X-Mailer: b4 0.14.0 The ETM decoder incorrectly assumed that auxtrace queue indices were equivalent to CPU number. This assumption is used for inserting records into the queue, and for fetching queues when given a CPU number. This assumption held when Perf always opened a dummy event on every CPU, even if the user provided a subset of CPUs on the commandline, resulting in the indices aligning. For example: # event : name =3D cs_etm//u, , id =3D { 2451, 2452 }, type =3D 11 (cs_et= m), size =3D 136, config =3D 0x4010, { sample_period, samp> # event : name =3D dummy:u, , id =3D { 2453, 2454, 2455, 2456 }, type =3D= 1 (PERF_TYPE_SOFTWARE), size =3D 136, config =3D 0x9 (PER> 0 0 0x200 [0xd0]: PERF_RECORD_ID_INDEX nr: 6 ... id: 2451 idx: 2 cpu: 2 tid: -1 ... id: 2452 idx: 3 cpu: 3 tid: -1 ... id: 2453 idx: 0 cpu: 0 tid: -1 ... id: 2454 idx: 1 cpu: 1 tid: -1 ... id: 2455 idx: 2 cpu: 2 tid: -1 ... id: 2456 idx: 3 cpu: 3 tid: -1 Since commit 811082e4b668 ("perf parse-events: Support user CPUs mixed with threads/processes") the dummy event no longer behaves in this way, making the ETM event indices start from 0 on the first CPU recorded regardless of its ID: # event : name =3D cs_etm//u, , id =3D { 771, 772 }, type =3D 11 (cs_etm)= , size =3D 144, config =3D 0x4010, { sample_period, sample> # event : name =3D dummy:u, , id =3D { 773, 774 }, type =3D 1 (PERF_TYPE_= SOFTWARE), size =3D 144, config =3D 0x9 (PERF_COUNT_SW_DUM> 0 0 0x200 [0x90]: PERF_RECORD_ID_INDEX nr: 4 ... id: 771 idx: 0 cpu: 2 tid: -1 ... id: 772 idx: 1 cpu: 3 tid: -1 ... id: 773 idx: 0 cpu: 2 tid: -1 ... id: 774 idx: 1 cpu: 3 tid: -1 This causes the following segfault when decoding: $ perf record -e cs_etm//u -C 2,3 -- true $ perf report perf: Segmentation fault -------- backtrace -------- #0 0xaaaabf9fd020 in ui__signal_backtrace setup.c:110 #1 0xffffab5c7930 in __kernel_rt_sigreturn [vdso][930] #2 0xaaaabfb68d30 in cs_etm_decoder__reset cs-etm-decoder.c:85 #3 0xaaaabfb65930 in cs_etm__get_data_block cs-etm.c:2032 #4 0xaaaabfb666fc in cs_etm__run_per_cpu_timeless_decoder cs-etm.c:2551 #5 0xaaaabfb6692c in (cs_etm__process_timeless_queues cs-etm.c:2612 #6 0xaaaabfb63390 in cs_etm__flush_events cs-etm.c:921 #7 0xaaaabfb324c0 in auxtrace__flush_events auxtrace.c:2915 #8 0xaaaabfaac378 in __perf_session__process_events session.c:2285 #9 0xaaaabfaacc9c in perf_session__process_events session.c:2442 #10 0xaaaabf8d3d90 in __cmd_report builtin-report.c:1085 #11 0xaaaabf8d6944 in cmd_report builtin-report.c:1866 #12 0xaaaabf95ebfc in run_builtin perf.c:351 #13 0xaaaabf95eeb0 in handle_internal_command perf.c:404 #14 0xaaaabf95f068 in run_argv perf.c:451 #15 0xaaaabf95f390 in main perf.c:558 #16 0xffffaab97400 in __libc_start_call_main libc_start_call_main.h:74 #17 0xffffaab974d8 in __libc_start_main@@GLIBC_2.34 libc-start.c:128 #18 0xaaaabf8aa8f0 in _start perf[7a8f0] Fix it by inserting into the queues based on CPU number, rather than using the index. Fixes: 811082e4b668 ("perf parse-events: Support user CPUs mixed with threa= ds/processes") Signed-off-by: James Clark --- tools/perf/util/cs-etm.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c index 25d56e0f1c07..12b55c2bc2ca 100644 --- a/tools/perf/util/cs-etm.c +++ b/tools/perf/util/cs-etm.c @@ -3086,7 +3086,7 @@ static int cs_etm__queue_aux_fragment(struct perf_ses= sion *session, off_t file_o =20 if (aux_offset >=3D auxtrace_event->offset && aux_offset + aux_size <=3D auxtrace_event->offset + auxtrace_event->s= ize) { - struct cs_etm_queue *etmq =3D etm->queues.queue_array[auxtrace_event->id= x].priv; + struct cs_etm_queue *etmq =3D cs_etm__get_queue(etm, auxtrace_event->cpu= ); =20 /* * If this AUX event was inside this buffer somewhere, create a new auxt= race event @@ -3095,6 +3095,7 @@ static int cs_etm__queue_aux_fragment(struct perf_ses= sion *session, off_t file_o auxtrace_fragment.auxtrace =3D *auxtrace_event; auxtrace_fragment.auxtrace.size =3D aux_size; auxtrace_fragment.auxtrace.offset =3D aux_offset; + auxtrace_fragment.auxtrace.idx =3D etmq->queue_nr; file_offset +=3D aux_offset - auxtrace_event->offset + auxtrace_event->h= eader.size; =20 pr_debug3("CS ETM: Queue buffer size: %#"PRI_lx64" offset: %#"PRI_lx64 --=20 2.34.1