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

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Date:   Tue, 2 Nov 2021 09:43:00 +0000
From:   James Clark <james.clark@....com>
To:     Lexi Shao <shaolexi@...wei.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Cc:     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, 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: Re: [PATCH v2 2/2] kallsyms: ignore arm mapping symbols when loading
 module



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-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>


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

Reviewed-by: James Clark <james.clark@....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) {
>