linux-kernel - [PATCH] perf header: Set proper symbol name for vdso when build-id event found

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Date:   Fri,  1 Dec 2023 19:15:06 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] perf header: Set proper symbol name for vdso when build-id event found

From: Like Xu <likexu@...cent.com>

If using perf-record to sample a cpu-cycles:P event on a hypervisor process
when '--kcore' is not enabled, user may find some surprise in perf-report:

# perf report -i perf.data -v:
# Overhead Command   Shared Object            	Symbol
    99.71%  vcpu0    arch/x86/kvm/kvm-intel.ko  0xffffffffa10d1e30 B [k] 0x0000000000034ed0

build id event received for vmlinux: d12116149f511f7dbd0b21c45d38d3d2ec09b87f [20]
build id event received for kvm-intel.ko: a8fc0213abbafd97b10ce58ce84bec8519f9abce [20]
build id event received for [vdso]: 4d56e381df8d2c051f6bc1ef69c0118c59d5c49f [20]

# perf report:
# Overhead  Command  Shared Object     Symbol
# ........  .......  ................  .......................................
#
    99.71%  vcpu0    [kvm_intel]       [k] 0x0000000000034ed0
     0.10%  vcpu0    [kernel.vmlinux]  [k] __lock_acquire.isra.29

Users may be curious as to how 0x34ed0 was generated and wondered if this
RIP came from the guest application but perf-script-D does not point to any
samples of this address.

Based on perf/tool implementation, this is actually an offset pointing to
the vdso object (in this case it is the assembly __vmx_vcpu_run defined
in arch/x86/kvm/vmx/vmenter.S). The pattern is not reproduced on perf-tool
of some distributions, and git-bisect quickly identified the possible root
cause, which leds to this straightforward fix and after this change:

# perf report -i perf.data -v:
# Overhead Command   Shared Object            	Symbol
    99.71%  vcpu0    arch/x86/kvm/kvm-intel.ko  0x34ed0            B [k] __vmx_vcpu_run

# perf report:
# Overhead  Command  Shared Object     Symbol
# ........  .......  ................  .......................................
#
    99.71%  vcpu0    [kvm_intel]       [k] __vmx_vcpu_run

The fix also gets commit 1deec1bd96cc ("perf header: Set proper module name
when build-id event found") lit again.

Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Ian Rogers <irogers@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Fixes: b2fe96a350de ("perf tools: Fix module symbol processing")
Signed-off-by: Like Xu <likexu@...cent.com>
---
 tools/perf/util/header.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index e86b9439ffee..a33d589511ff 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -2305,8 +2305,8 @@ static int __event_process_build_id(struct perf_record_header_build_id *bev,
 
 			if (!kmod_path__parse_name(&m, filename) && m.kmod)
 				dso__set_module_info(dso, &m, machine);
-
-			dso->kernel = dso_space;
+			else
+				dso->kernel = dso_space;
 			free(m.name);
 		}
 
-- 
2.43.0