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From:   Lexi Shao <shaolexi@huawei.com>
To:     <linux-perf-users@vger.kernel.org>, <linux-kernel@vger.kernel.org>
CC:     <acme@kernel.org>, <mark.rutland@arm.com>, <peterz@infradead.org>,
        <mingo@redhat.com>, <alexander.shishkin@linux.intel.com>,
        <jolsa@redhat.com>, <namhyung@kernel.org>, <shaolexi@huawei.com>,
        <qiuxi1@huawei.com>, <nixiaoming@huawei.com>,
        <wangbing6@huawei.com>
Subject: [PATCH] perf symbol: ignore $a/$d symbols for ARM modules
Date:   Wed, 27 Oct 2021 17:52:35 +0800
Message-ID: <20211027095235.123358-1-shaolexi@huawei.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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@huawei.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