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

Eugene Loh
2019-03-07T23:37:32Z

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

On 02/25/2019 11:59 AM, eugene.loh@oracle.com wrote:
> From: Eugene Loh <eugene.loh@oracle.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@oracle.com>
> Reviewed-by: Nick Alcock <nick.alcock@oracle.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);