perf symbols: Ignore $a/$d symbols for ARM modules

On anARM 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 an 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])

Reviewed-by: James Clark <james.clark@arm.com>
Signed-off-by: Lexi Shao <shaolexi@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: QiuXi <qiuxi1@huawei.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Wangbing <wangbing6@huawei.com>
Cc: Xiaoming Ni <nixiaoming@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Cc: clang-built-linux@googlegroups.com
Link: https://lore.kernel.org/r/20211029065038.39449-2-shaolexi@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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