linux-kernel - [PATCH v2 2/2] kallsyms: ignore arm mapping symbols when loading module

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Date:   Fri, 29 Oct 2021 14:50:38 +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 2/2] kallsyms: ignore arm mapping symbols when loading module

Arm modules contains mapping symbols(e.g. $a $d) which are ignored in
module_kallsyms_on_each_symbol. However, these symbols are still
displayed when catting /proc/kallsyms. This confuses tools(e.g. perf)
that resolves kernel symbols with address using information from
/proc/kallsyms. See discussion in Link:
https://lore.kernel.org/all/c7dfbd17-85fd-b914-b90f-082abc64c9d1@arm.com/

Being left out in vmlinux(see scripts/kallsyms.c is_ignored_symbol) and
kernelspace API implies that these symbols are not used in any cases.
So we can ignore them in the first place by not adding them to module
kallsyms.

Signed-off-by: Lexi Shao <shaolexi@...wei.com>
---
 kernel/module.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 5c26a76e800b..b30cbbe144c7 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2662,16 +2662,22 @@ static char elf_type(const Elf_Sym *sym, const struct load_info *info)
 	return '?';
 }
 
-static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
-			unsigned int shnum, unsigned int pcpundx)
+static inline int is_arm_mapping_symbol(const char *str);
+static bool is_core_symbol(const Elf_Sym *src, const struct load_info *info)
 {
 	const Elf_Shdr *sec;
+	const Elf_Shdr *sechdrs = info->sechdrs;
+	unsigned int shnum = info->hdr->e_shnum;
+	unsigned int pcpundx = info->index.pcpu;
 
 	if (src->st_shndx == SHN_UNDEF
 	    || src->st_shndx >= shnum
 	    || !src->st_name)
 		return false;
 
+	if (is_arm_mapping_symbol(&info->strtab[src->st_name]))
+		return false;
+
 #ifdef CONFIG_KALLSYMS_ALL
 	if (src->st_shndx == pcpundx)
 		return true;
@@ -2714,8 +2720,7 @@ static void layout_symtab(struct module *mod, struct load_info *info)
 	/* Compute total space required for the core symbols' strtab. */
 	for (ndst = i = 0; i < nsrc; i++) {
 		if (i == 0 || is_livepatch_module(mod) ||
-		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
-				   info->index.pcpu)) {
+		    is_core_symbol(src+i, info)) {
 			strtab_size += strlen(&info->strtab[src[i].st_name])+1;
 			ndst++;
 		}
@@ -2778,8 +2783,7 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
 	for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) {
 		mod->kallsyms->typetab[i] = elf_type(src + i, info);
 		if (i == 0 || is_livepatch_module(mod) ||
-		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
-				   info->index.pcpu)) {
+		    is_core_symbol(src+i, info)) {
 			mod->core_kallsyms.typetab[ndst] =
 			    mod->kallsyms->typetab[i];
 			dst[ndst] = src[i];
@@ -4246,8 +4250,7 @@ static const char *find_kallsyms_symbol(struct module *mod,
 		 * We ignore unnamed symbols: they're uninformative
 		 * and inserted at a whim.
 		 */
-		if (*kallsyms_symbol_name(kallsyms, i) == '\0'
-		    || is_arm_mapping_symbol(kallsyms_symbol_name(kallsyms, i)))
+		if (*kallsyms_symbol_name(kallsyms, i) == '\0')
 			continue;
 
 		if (thisval <= addr && thisval > bestval) {
-- 
2.12.3