Re: livepatch: reuse module loader code to write relocations

Jessica Yu
2016-02-10T01:21:49Z

+++ Petr Mladek [09/02/16 15:01 +0100]:
>On Wed 2016-02-03 20:11:09, Jessica Yu wrote:
>> Reuse module loader code to write relocations, thereby eliminating the need
>> for architecture specific relocation code in livepatch. Specifically, reuse
>> the apply_relocate_add() function in the module loader to write relocations
>> instead of duplicating functionality in livepatch's arch-dependent
>> klp_write_module_reloc() function.
>>
>> In order to accomplish this, livepatch modules manage their own relocation
>> sections (marked with the SHF_RELA_LIVEPATCH section flag) and
>> livepatch-specific symbols (marked with SHN_LIVEPATCH symbol section
>> index). To apply livepatch relocation sections, livepatch symbols
>> referenced by relocs are resolved and then apply_relocate_add() is called
>> to apply those relocations.
>>
>> In addition, remove x86 livepatch relocation code and the s390
>> klp_write_module_reloc() function stub. They are no longer needed since
>> relocation work has been offloaded to module loader.
>>
>> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
>> index 7aa975d..c1fe57c 100644
>> --- a/kernel/livepatch/core.c
>> +++ b/kernel/livepatch/core.c
>> @@ -28,6 +28,9 @@
>> #include <linux/list.h>
>> #include <linux/kallsyms.h>
>> #include <linux/livepatch.h>
>> +#include <linux/elf.h>
>> +#include <linux/string.h>
>> +#include <linux/moduleloader.h>
>> #include <asm/cacheflush.h>
>>
>> /**
>> @@ -87,6 +90,166 @@ static bool klp_is_object_loaded(struct klp_object *obj)
>> return !obj->name || obj->mod;
>> }
>>
>> +/*
>> + * Check if a livepatch symbol is formatted properly.
>> + *
>> + * See Documentation/livepatch/module-elf-format.txt for a
>> + * detailed outline of requirements.
>> + */
>> +static int klp_check_symbol_format(struct module *pmod, Elf_Sym *sym)
>> +{
>> + size_t len;
>> + char *s, *objname, *symname;
>> +
>> + if (sym->st_shndx != SHN_LIVEPATCH)
>> + return -EINVAL;
>> +
>> + /*
>> + * Livepatch symbol names must follow this format:
>> + * .klp.sym.objname.symbol_name,sympos
>> + */
>> + s = pmod->strtab + sym->st_name;
>> + /* [.klp.sym.]objname.symbol_name,sympos */
>> + if (!s || strncmp(s, KLP_SYM_PREFIX, KLP_SYM_PREFIX_LEN))
>> + return -EINVAL;
>> +
>> + /* .klp.sym.[objname].symbol_name,sympos */
>> + objname = s + KLP_SYM_PREFIX_LEN;
>> + len = strcspn(objname, ".");
>> + if (!(len > 0))
>> + return -EINVAL;
>> +
>> + /* .klp.sym.objname.symbol_name,[sympos] */
>> + if (!strchr(s, ','))
>> + return -EINVAL;
>> +
>> + /* .klp.sym.objname.[symbol_name],sympos */
>> + symname = objname + len + 1;
>> + len = strcspn(symname, ",");
>> + if (!(len > 0))
>> + return -EINVAL;
>> +
>> + return 0;
>> +}
>> +
>> +/*
>> + * Check if a livepatch relocation section is formatted properly.
>> + *
>> + * See Documentation/livepatch/module-elf-format.txt for a
>> + * detailed outline of requirements.
>> + */
>> +static int klp_check_relasec_format(struct module *pmod, Elf_Shdr *relasec)
>> +{
>> + char *secname;
>> + size_t len;
>> +
>> + secname = pmod->klp_info->secstrings + relasec->sh_name;
>> + /* [.klp.rela.]objname.section_name */
>> + if (!secname || strncmp(secname, KLP_RELASEC_PREFIX,
>> + KLP_RELASEC_PREFIX_LEN))
>> + return -EINVAL;
>> +
>> + /* .klp.rela.[objname].section_name */
>> + len = strcspn(secname + KLP_RELASEC_PREFIX_LEN, ".");
>> + if (!(len > 0))
>> + return -EINVAL;
>> +
>> + return 0;
>> +}
>> +
>> +/*
>> + * Check if obj->name matches the objname encoded in the rela
>> + * section name (.klp.rela.[objname].section_name)
>> + *
>> + * Must pass klp_check_relasec_format() before calling this.
>> + */
>> +static bool klp_relasec_matches_object(struct module *pmod, Elf_Shdr *relasec,
>> + struct klp_object *obj)
>> +{
>> + size_t len;
>> + const char *obj_objname, *sec_objname, *secname;
>> +
>> + secname = pmod->klp_info->secstrings + relasec->sh_name;
>> + /* .klp.rela.[objname].section_name */
>> + sec_objname = secname + KLP_RELASEC_PREFIX_LEN;
>> + obj_objname = klp_is_module(obj) ? obj->name : "vmlinux";
>> +
>> + /* Get length of the objname encoded in the section name */
>> + len = strcspn(sec_objname, ".");
>> +
>> + if (strlen(obj_objname) != len)
>> + return false;
>> +
>> + return strncmp(sec_objname, obj_objname, len) ? false : true;
>> +}
>> +
>> +/*
>> + * klp_get_* helper functions
>> + *
>> + * klp_get_* functions extract different components of the name
>> + * of a livepatch symbol. The full symbol name from the strtab
>> + * is passed in as parameter @s, and @result is filled in with
>> + * the extracted component.
>> + *
>> + * These functions assume a correctly formatted symbol and the
>> + * klp_check_symbol_format() test *must* pass before calling any
>> + * of these functions.
>> + */
>> +
>> +/* .klp.sym.[objname].symbol_name,sympos */
>> +static int klp_get_sym_objname(char *s, char **result)
>> +{
>> + size_t len;
>> + char *objname, *objname_start;
>> +
>> + /* .klp.sym.[objname].symbol_name,sympos */
>> + objname_start = s + KLP_SYM_PREFIX_LEN;
>> + len = strcspn(objname_start, ".");
>> + objname = kstrndup(objname_start, len, GFP_KERNEL);
>> + if (objname == NULL)
>> + return -ENOMEM;
>> +
>> + /* klp_find_object_symbol() treats NULL as vmlinux */
>> + if (!strcmp(objname, "vmlinux")) {
>> + *result = NULL;
>> + kfree(objname);
>> + } else
>> + *result = objname;
>> +
>> + return 0;
>> +}
>> +
>> +/* .klp.sym.objname.[symbol_name],sympos */
>> +static int klp_get_symbol_name(char *s, char **result)
>> +{
>> + size_t len;
>> + char *objname, *symname;
>> +
>> + /* .klp.sym.[objname].symbol_name,sympos */
>> + objname = s + KLP_SYM_PREFIX_LEN;
>> + len = strcspn(objname, ".");
>> +
>> + /* .klp.sym.objname.[symbol_name],sympos */
>> + symname = objname + len + 1;
>> + len = strcspn(symname, ",");
>> +
>> + *result = kstrndup(symname, len, GFP_KERNEL);
>> + if (*result == NULL)
>> + return -ENOMEM;
>> +
>> + return 0;
>> +}
>> +
>> +/* .klp.sym.objname.symbol_name,[sympos] */
>> +static int klp_get_sympos(char *s, unsigned long *result)
>> +{
>> + char *sympos;
>> +
>> + /* .klp.sym.symbol_name,[sympos] */
>> + sympos = strchr(s, ',') + 1;
>> + return kstrtol(sympos, 10, result);
>> +}
>
>The usage of the helper function is nicely strightforward.
>Also I like a lot all the comments with the [] brackets
>that highlight what each check or search is for.
>
>But I think that there is a lot of duplicated code in
>the check_ and in the get_ functions. Also there is
>a lot of strdup/free games.

I agree, I do not really like the repetition and all
the strdup/free's myself.

>The length of the elemets is limited by definition.
>The functions are called under klp_mutex.
>
>Therefore it might be easier to maintain a two parse
>function that would fill static buffers and return
>error in case of wrong value.
>
>I mean something like:
>
>static char mod_name[MODULE_NAME_LEN + 1];
>static char symbol_name[KSYM_SYMBOL_LEN + 1];
>
>static int
>klp_parse_symbol_format(struct module *pmod, Elf_Sym *sym,
> char *mod_name, char *symbol_name,
> int *sympos)
>{
> char *substr;
>
>
> /*
> * Livepatch symbol names must follow this format:
> * .klp.sym.objname.symbol_name,sympos
> */
> s = pmod->strtab + sym->st_name;
> /* [.klp.sym.]objname.symbol_name,sympos */
> if (!s || strncmp(s, KLP_SYM_PREFIX, KLP_SYM_PREFIX_LEN))
> return -EINVAL;
>
> /* .klp.sym.[objname].symbol_name,sympos */
> substr = s + KLP_SYM_PREFIX_LEN;
> len = strcspn(substr, ".");
> if (!len || len > MODULE_NAME_LEN)
> return -EINVAL;
> strncpy(mod_name, substr, len);
> mod_name[len] = '\0';
>
> /* .klp.sym.objname.[symbol_name],sympos */
> substr = substr + len + 1;
> len = strcspn(substr, ",");
> if (!len || len > KSYM_SYMBOL_LEN)
> return -EINVAL;
> strncpy(symbol_name, substr, len);
> mod_name[len] = '\0';
>
> /* .klp.sym.objname.symbol_name,[sympos] */
> substr = substr + len + 1;
> len = strlen(substr);
> if (!len)
> return -EINVAL;
>
> return kstrtol(substr, 10, sympos);
>}
>
>How does that sound, please?

I like this idea, it is a lot cleaner and more concise
than using all those helper functions. :-)

Thanks,
Jessica