linux-kernel - Re: [PATCH] kallsyms: store type information in its own array

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Date:   Thu, 7 Mar 2019 15:37:20 -0800
From:   Eugene Loh <eugene.loh@...cle.com>
To:     jeyu@...nel.org, linux-kernel@...r.kernel.org,
        vincent.whitchurch@...s.com
Subject: Re: [PATCH] kallsyms: store type information in its own array

It's been 1.5 weeks.  Could I get some feedback on this patch? Thanks.


On 02/25/2019 11:59 AM, eugene.loh@...cle.com wrote:
> From: Eugene Loh <eugene.loh@...cle.com>
>
> When a module is loaded, its symbols' Elf_Sym information is stored
> in a symtab.  Further, type information is also captured.  Since
> Elf_Sym has no type field, historically the st_info field has been
> hijacked for storing type:  st_info was overwritten.
>
> commit 5439c985c5a83a8419f762115afdf560ab72a452 ("module: Overwrite
> st_size instead of st_info") changes that practice, as its one-liner
> indicates.  Unfortunately, this change overwrites symbol size,
> information that a tool like DTrace expects to find.
>
> Allocate a typetab array to store type information so that no Elf_Sym
> field needs to be overwritten.
>
> Fixes: 5439c985c5a8 ("module: Overwrite st_size instead of st_info")
> Signed-off-by: Eugene Loh <eugene.loh@...cle.com>
> Reviewed-by: Nick Alcock <nick.alcock@...cle.com>
> ---
>   include/linux/module.h   |  1 +
>   kernel/module-internal.h |  2 +-
>   kernel/module.c          | 21 ++++++++++++++-------
>   3 files changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index f5bc4c0..9c1bc21 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -315,6 +315,7 @@ struct mod_kallsyms {
>   	Elf_Sym *symtab;
>   	unsigned int num_symtab;
>   	char *strtab;
> +	char *typetab;
>   };
>   
>   #ifdef CONFIG_LIVEPATCH
> diff --git a/kernel/module-internal.h b/kernel/module-internal.h
> index 79c9be2..67828af 100644
> --- a/kernel/module-internal.h
> +++ b/kernel/module-internal.h
> @@ -20,7 +20,7 @@ struct load_info {
>   	unsigned long len;
>   	Elf_Shdr *sechdrs;
>   	char *secstrings, *strtab;
> -	unsigned long symoffs, stroffs;
> +	unsigned long symoffs, stroffs, init_typeoff, core_typeoff;
>   	struct _ddebug *debug;
>   	unsigned int num_debug;
>   	bool sig_ok;
> diff --git a/kernel/module.c b/kernel/module.c
> index 2ad1b52..c7c1bcd 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2647,6 +2647,8 @@ static void layout_symtab(struct module *mod, struct load_info *info)
>   	info->symoffs = ALIGN(mod->core_layout.size, symsect->sh_addralign ?: 1);
>   	info->stroffs = mod->core_layout.size = info->symoffs + ndst * sizeof(Elf_Sym);
>   	mod->core_layout.size += strtab_size;
> +	info->core_typeoff = mod->core_layout.size;
> +	mod->core_layout.size += ndst * sizeof(char);
>   	mod->core_layout.size = debug_align(mod->core_layout.size);
>   
>   	/* Put string table section at end of init part of module. */
> @@ -2660,6 +2662,8 @@ static void layout_symtab(struct module *mod, struct load_info *info)
>   				      __alignof__(struct mod_kallsyms));
>   	info->mod_kallsyms_init_off = mod->init_layout.size;
>   	mod->init_layout.size += sizeof(struct mod_kallsyms);
> +	info->init_typeoff = mod->init_layout.size;
> +	mod->init_layout.size += nsrc * sizeof(char);
>   	mod->init_layout.size = debug_align(mod->init_layout.size);
>   }
>   
> @@ -2683,20 +2687,23 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
>   	mod->kallsyms->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
>   	/* Make sure we get permanent strtab: don't use info->strtab. */
>   	mod->kallsyms->strtab = (void *)info->sechdrs[info->index.str].sh_addr;
> +	mod->kallsyms->typetab = mod->init_layout.base + info->init_typeoff;
>   
> -	/* Set types up while we still have access to sections. */
> -	for (i = 0; i < mod->kallsyms->num_symtab; i++)
> -		mod->kallsyms->symtab[i].st_size
> -			= elf_type(&mod->kallsyms->symtab[i], info);
> -
> -	/* Now populate the cut down core kallsyms for after init. */
> +	/*
> +	 * Now populate the cut down core kallsyms for after init
> +	 * and set types up while we still have access to sections.
> +	 */
>   	mod->core_kallsyms.symtab = dst = mod->core_layout.base + info->symoffs;
>   	mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs;
> +	mod->core_kallsyms.typetab = mod->core_layout.base + info->core_typeoff;
>   	src = mod->kallsyms->symtab;
>   	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)) {
> +			mod->core_kallsyms.typetab[ndst] =
> +			    mod->kallsyms->typetab[i];
>   			dst[ndst] = src[i];
>   			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
>   			s += strlcpy(s, &mod->kallsyms->strtab[src[i].st_name],
> @@ -4084,7 +4091,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
>   			const Elf_Sym *sym = &kallsyms->symtab[symnum];
>   
>   			*value = kallsyms_symbol_value(sym);
> -			*type = sym->st_size;
> +			*type = kallsyms->typetab[symnum];
>   			strlcpy(name, kallsyms_symbol_name(kallsyms, symnum), KSYM_NAME_LEN);
>   			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
>   			*exported = is_exported(name, *value, mod);