linux-kernel - Re: livepatch: reuse module loader code to write relocations

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Date:	Mon, 21 Mar 2016 15:18:32 -0400
From:	Jessica Yu <jeyu@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
	Jonathan Corbet <corbet@....net>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: livepatch: reuse module loader code to write relocations

+++ Miroslav Benes [21/03/16 14:55 +0100]:
>On Wed, 16 Mar 2016, Jessica Yu wrote:
>
>[...]
>
>> +struct klp_buf {
>> +	char symname[KSYM_SYMBOL_LEN];
>
>I think it is better to make this KSYM_NAME_LEN. KSYM_SYMBOL_LEN looks
>like something different and KSYM_NAME_LEN is 128 which you reference
>below.
>

Ack, I did mean to use KSYM_NAME_LEN, thanks.

>> +	char objname[MODULE_NAME_LEN];
>> +};
>
>[...]
>
>> +static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod)
>> +{
>> +	int i, cnt, vmlinux, ret;
>> +	struct klp_buf bufs = {0};
>> +	Elf_Rela *relas;
>> +	Elf_Sym *sym;
>> +	char *symname;
>> +	unsigned long sympos;
>> +
>> +	relas = (Elf_Rela *) relasec->sh_addr;
>> +	/* For each rela in this klp relocation section */
>> +	for (i = 0; i < relasec->sh_size / sizeof(Elf_Rela); i++) {
>> +		sym = pmod->core_kallsyms.symtab + ELF_R_SYM(relas[i].r_info);
>> +		if (sym->st_shndx != SHN_LIVEPATCH)
>> +			return -EINVAL;
>> +
>> +		klp_clear_buf(&bufs);
>> +
>> +		/* Format: .klp.sym.objname.symbol_name,sympos */
>> +		symname = pmod->core_kallsyms.strtab + sym->st_name;
>> +		cnt = sscanf(symname, ".klp.sym.%64[^.].%128[^,],%lu",
>> +			     bufs.objname, bufs.symname, &sympos);
>
>It would be really nice to change actual values for their macro
>definitions, but this would be a mess which is not worth it. Anyway
>shouldn't those width modifiers be %63 and %127 to make a room for \0?
>

Yes, this is a concern and I'm not sure what the best way to fix it
is. If both MODULE_NAME_LEN and KSYM_NAME_LEN were straight up
constants, then I think Josh's stringify approach would have worked
perfectly. However since MODULE_NAME_LEN translates to an expression
(64 - sizeof(unsigned long)), which the preprocessor cannot evaluate,
we will need another approach. Building the format strings at run time
might be messier than we'd like. Alternatively we could just go the
simple route and simply be a bit more aggressive on the upper bound
for the format width; though the size of long varies on different
architectures, afaik the max size it could ever be on any arch is 8
bytes, so perhaps 64 - 8 = 56 (then - 1 to make room for \0) might be
an appropriate field width. This would deserve a comment as well.

>> +		if (cnt != 3)
>> +			return -EINVAL;
>> +
>> +		/* klp_find_object_symbol() treats a NULL objname as vmlinux */
>> +		vmlinux = !strcmp(bufs.objname, "vmlinux");
>> +		ret = klp_find_object_symbol(vmlinux ? NULL : bufs.objname,
>> +					     bufs.symname, sympos,
>> +					     (unsigned long *) &sym->st_value);
>> +		if (ret)
>> +			return ret;
>>  	}
>> -	preempt_enable();
>>
>> -	/*
>> -	 * Check if it's in another .o within the patch module. This also
>> -	 * checks that the external symbol is unique.
>> -	 */
>> -	return klp_find_object_symbol(pmod->name, name, 0, addr);
>> +	return 0;
>>  }
>>
>>  static int klp_write_object_relocations(struct module *pmod,
>>  					struct klp_object *obj)
>>  {
>> -	int ret = 0;
>> -	unsigned long val;
>> -	struct klp_reloc *reloc;
>> +	int i, cnt, ret = 0;
>> +	const char *objname, *secname;
>> +	struct klp_buf bufs = {0};
>> +	Elf_Shdr *sec;
>>
>>  	if (WARN_ON(!klp_is_object_loaded(obj)))
>>  		return -EINVAL;
>>
>> -	if (WARN_ON(!obj->relocs))
>> -		return -EINVAL;
>> +	objname = klp_is_module(obj) ? obj->name : "vmlinux";
>>
>>  	module_disable_ro(pmod);
>> +	/* For each klp relocation section */
>> +	for (i = 1; i < pmod->klp_info->hdr.e_shnum; i++) {
>> +		sec = pmod->klp_info->sechdrs + i;
>> +		if (!(sec->sh_flags & SHF_RELA_LIVEPATCH))
>> +			continue;
>>
>> -	for (reloc = obj->relocs; reloc->name; reloc++) {
>> -		/* discover the address of the referenced symbol */
>> -		if (reloc->external) {
>> -			if (reloc->sympos > 0) {
>> -				pr_err("non-zero sympos for external reloc symbol '%s' is not supported\n",
>> -				       reloc->name);
>> -				ret = -EINVAL;
>> -				goto out;
>> -			}
>> -			ret = klp_find_external_symbol(pmod, reloc->name, &val);
>> -		} else
>> -			ret = klp_find_object_symbol(obj->name,
>> -						     reloc->name,
>> -						     reloc->sympos,
>> -						     &val);
>> -		if (ret)
>> -			goto out;
>> +		klp_clear_buf(&bufs);
>>
>> -		ret = klp_write_module_reloc(pmod, reloc->type, reloc->loc,
>> -					     val + reloc->addend);
>> -		if (ret) {
>> -			pr_err("relocation failed for symbol '%s' at 0x%016lx (%d)\n",
>> -			       reloc->name, val, ret);
>> -			goto out;
>> +		/* Check if this klp relocation section belongs to obj */
>> +		secname = pmod->klp_info->secstrings + sec->sh_name;
>> +		cnt = sscanf(secname, ".klp.rela.%64[^.]", bufs.objname);
>
>Same here.
>
>Otherwise it looks really good (which applies for the whole series), so
>after fixing these nits you can add my
>
>Reviewed-by: Miroslav Benes <mbenes@...e.cz>
>
>Cheers,
>Miroslav