linux-kernel - [PATCH v2 1/2] perf symbol: ignore $a/$d symbols for ARM modules

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Date:   Fri, 29 Oct 2021 14:50:37 +0800
From:   Lexi Shao <shaolexi@...wei.com>
To:     <linux-kernel@...r.kernel.org>, <linux-perf-users@...r.kernel.org>
CC:     <james.clark@....com>, <acme@...nel.org>,
        <alexander.shishkin@...ux.intel.com>, <jolsa@...hat.com>,
        <mark.rutland@....com>, <mingo@...hat.com>, <namhyung@...nel.org>,
        <nixiaoming@...wei.com>, <peterz@...radead.org>,
        <qiuxi1@...wei.com>, <shaolexi@...wei.com>, <wangbing6@...wei.com>,
        <jeyu@...nel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <andrii@...nel.org>, <kafai@...com>, <songliubraving@...com>,
        <yhs@...com>, <john.fastabend@...il.com>, <kpsingh@...nel.org>,
        <natechancellor@...il.com>, <ndesaulniers@...gle.com>,
        <bpf@...r.kernel.org>, <clang-built-linux@...glegroups.com>
Subject: [PATCH v2 1/2] perf symbol: ignore $a/$d symbols for ARM modules

On ARM machine, kernel symbols from modules can be resolved to $a
instead of printing the actual symbol name. Ignore symbols starting with
"$" when building kallsyms rbtree.

A sample stacktrace is shown as follows:

c0f2e39c schedule_hrtimeout+0x14 ([kernel.kallsyms])
bf4a66d8 $a+0x78 ([test_module])
c0a4f5f4 kthread+0x15c ([kernel.kallsyms])
c0a001f8 ret_from_fork+0x14 ([kernel.kallsyms])

On ARM machine, $a/$d symbols are used by the compiler to mark the
beginning of code/data part in code section. These symbols are filtered
out when linking vmlinux(see scripts/kallsyms.c ignored_prefixes), but
are left on modules. So there are $a symbols in /proc/kallsyms which
share the same addresses with the actual module symbols and confuses perf
when resolving symbols.

After this patch, the module symbol name is printed:

c0f2e39c schedule_hrtimeout+0x14 ([kernel.kallsyms])
bf4a66d8 test_func+0x78 ([test_module])
c0a4f5f4 kthread+0x15c ([kernel.kallsyms])
c0a001f8 ret_from_fork+0x14 ([kernel.kallsyms])

Signed-off-by: Lexi Shao <shaolexi@...wei.com>
Reviewed-by: James Clark <james.clark@....com>
---
 tools/perf/util/symbol.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 0fc9a5410739..35116aed74eb 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -702,6 +702,10 @@ static int map__process_kallsym_symbol(void *arg, const char *name,
 	if (!symbol_type__filter(type))
 		return 0;
 
+	/* Ignore local symbols for ARM modules */
+	if (name[0] == '$')
+		return 0;
+
 	/*
 	 * module symbols are not sorted so we add all
 	 * symbols, setting length to 0, and rely on
-- 
2.12.3