linux-kernel - Re: [RFC PATCH v2 1/3] livepatch: Clear relocation targets on a module removal

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Date:   Wed, 2 Oct 2019 15:22:52 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jikos@...nel.org, jpoimboe@...hat.com, joe.lawrence@...hat.com,
        nstange@...e.de, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 1/3] livepatch: Clear relocation targets on a
 module removal

On Thu 2019-09-05 14:45:12, Miroslav Benes wrote:
> We thus decided to reverse the relocation patching (clear all relocation
> targets on x86_64, or return back nops on powerpc). The solution is not
> universal and is too much arch-specific, but it may prove to be simpler
> in the end.
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index a93b10c48000..e461d456e447 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -741,6 +741,51 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
>  	return 0;
>  }
>  
> +#ifdef CONFIG_LIVEPATCH
> +void clear_relocate_add(Elf64_Shdr *sechdrs,
> +		       const char *strtab,
> +		       unsigned int symindex,
> +		       unsigned int relsec,
> +		       struct module *me)
> +{
> +	unsigned int i;
> +	Elf64_Rela *rela = (void *)sechdrs[relsec].sh_addr;
> +	Elf64_Sym *sym;
> +	unsigned long *location;
> +	const char *symname;
> +	u32 *instruction;
> +
> +	pr_debug("Applying ADD relocate section %u to %u\n", relsec,

s/Applying/Clearing/

> +	       sechdrs[relsec].sh_info);
> +
> +	for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rela); i++) {
> +		location = (void *)sechdrs[sechdrs[relsec].sh_info].sh_addr
> +			+ rela[i].r_offset;
> +		sym = (Elf64_Sym *)sechdrs[symindex].sh_addr
> +			+ ELF64_R_SYM(rela[i].r_info);
> +		symname = me->core_kallsyms.strtab
> +			+ sym->st_name;
> +
> +		if (ELF64_R_TYPE(rela[i].r_info) != R_PPC_REL24)
> +			continue;

I expected that the code below would reverse the operations
in apply_relocate_add() for case R_PPC_REL24. But it is not
obvious for me.

It might be because I am not familiar with the code. Or would
it deserve some comments?

> +
> +		if (sym->st_shndx != SHN_UNDEF &&
> +		    sym->st_shndx != SHN_LIVEPATCH)
> +			continue;
> +
> +		instruction = (u32 *)location;
> +		if (is_mprofile_mcount_callsite(symname, instruction))
> +			continue;
> +
> +		if (!instr_is_relative_link_branch(*instruction))
> +			continue;
> +
> +		instruction += 1;
> +		*instruction = PPC_INST_NOP;
> +	}
> +}
> +#endif
> +
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index ab4a4606d19b..f0b380d2a17a 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -295,6 +295,45 @@ static int klp_write_object_relocations(struct module *pmod,
>  	return ret;
>  }
>  
> +static void klp_clear_object_relocations(struct module *pmod,
> +					struct klp_object *obj)
> +{
> +	int i, cnt;
> +	const char *objname, *secname;
> +	char sec_objname[MODULE_NAME_LEN];
> +	Elf_Shdr *sec;
> +
> +	objname = klp_is_module(obj) ? obj->name : "vmlinux";
> +
> +	/* For each klp relocation section */
> +	for (i = 1; i < pmod->klp_info->hdr.e_shnum; i++) {
> +		sec = pmod->klp_info->sechdrs + i;
> +		secname = pmod->klp_info->secstrings + sec->sh_name;
> +		if (!(sec->sh_flags & SHF_RELA_LIVEPATCH))
> +			continue;
> +
> +		/*
> +		 * Format: .klp.rela.sec_objname.section_name
> +		 * See comment in klp_resolve_symbols() for an explanation
> +		 * of the selected field width value.
> +		 */
> +		secname = pmod->klp_info->secstrings + sec->sh_name;
> +		cnt = sscanf(secname, ".klp.rela.%55[^.]", sec_objname);
> +		if (cnt != 1) {
> +			pr_err("section %s has an incorrectly formatted name\n",
> +			       secname);
> +			continue;
> +		}
> +
> +		if (strcmp(objname, sec_objname))
> +			continue;
> +

It would make the review easier when the order of 1st and 2nd
patch was swaped. I mean that I would not need to check twice
that the two functions actually share the same code.

> +		clear_relocate_add(pmod->klp_info->sechdrs,
> +				   pmod->core_kallsyms.strtab,
> +				   pmod->klp_info->symndx, i, pmod);
> +	}
> +}
> +
>  /*
>   * Sysfs Interface
>   *

I was not able to check correctness of the ppc and s390 parts.
Otherwise, it looks good to me.

Best Regards,
Petr