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

[Lexi Shao]

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...@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 <shao...@huawei.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

[James Clark]

On 29/10/2021 07:50, Lexi Shao wrote:
> 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...@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 <shao...@huawei.com>

I tested this and it has removed the $ symbols from kallsyms where I saw
them before.

Reviewed-by: James Clark <james...@arm.com>