linux-kernel - Re: [PATCH v5 2/6] module: preserve Elf information for livepatch modules

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Date:	Mon, 21 Mar 2016 09:06:31 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Miroslav Benes <mbenes@...e.cz>, 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: [PATCH v5 2/6] module: preserve Elf information for livepatch
 modules

On Wed, Mar 16, 2016 at 03:47:04PM -0400, Jessica Yu wrote:
> For livepatch modules, copy Elf section, symbol, and string information
> from the load_info struct in the module loader. Persist copies of the
> original symbol table and string table.
> 
> Livepatch manages its own relocation sections in order to reuse module
> loader code to write relocations. Livepatch modules must preserve Elf
> information such as section indices in order to apply livepatch relocation
> sections using the module loader's apply_relocate_add() function.
> 
> In order to apply livepatch relocation sections, livepatch modules must
> keep a complete copy of their original symbol table in memory. Normally, a
> stripped down copy of a module's symbol table (containing only "core"
> symbols) is made available through module->core_symtab. But for livepatch
> modules, the symbol table copied into memory on module load must be exactly
> the same as the symbol table produced when the patch module was compiled.
> This is because the relocations in each livepatch relocation section refer
> to their respective symbols with their symbol indices, and the original
> symbol indices (and thus the symtab ordering) must be preserved in order
> for apply_relocate_add() to find the right symbol.
> 
> Signed-off-by: Jessica Yu <jeyu@...hat.com>
> ---
>  include/linux/module.h |  25 ++++++++++
>  kernel/module.c        | 123 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 146 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 2bb0c30..3daf2b3 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -330,6 +330,15 @@ struct mod_kallsyms {
>  	char *strtab;
>  };
>  
> +#ifdef CONFIG_LIVEPATCH
> +struct klp_modinfo {
> +	Elf_Ehdr hdr;
> +	Elf_Shdr *sechdrs;
> +	char *secstrings;
> +	unsigned int symndx;
> +};
> +#endif
> +
>  struct module {
>  	enum module_state state;
>  
> @@ -456,7 +465,11 @@ struct module {
>  #endif
>  
>  #ifdef CONFIG_LIVEPATCH
> +	bool klp; /* Is this a livepatch module? */
>  	bool klp_alive;
> +
> +	/* Elf information */
> +	struct klp_modinfo *klp_info;
>  #endif
>  
>  #ifdef CONFIG_MODULE_UNLOAD
> @@ -630,6 +643,18 @@ static inline bool module_requested_async_probing(struct module *module)
>  	return module && module->async_probe_requested;
>  }
>  
> +#ifdef CONFIG_LIVEPATCH
> +static inline bool is_livepatch_module(struct module *mod)
> +{
> +	return mod->klp;
> +}
> +#else /* !CONFIG_LIVEPATCH */
> +static inline bool is_livepatch_module(struct module *mod)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_LIVEPATCH */
> +
>  #else /* !CONFIG_MODULES... */
>  
>  /* Given an address, look for it in the exception tables. */
> diff --git a/kernel/module.c b/kernel/module.c
> index 87cfeb2..80b7fd9 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1971,6 +1971,82 @@ static void module_enable_nx(const struct module *mod) { }
>  static void module_disable_nx(const struct module *mod) { }
>  #endif
>  
> +#ifdef CONFIG_LIVEPATCH
> +/*
> + * Persist Elf information about a module. Copy the Elf header,
> + * section header table, section string table, and symtab section
> + * index from info to mod->klp_info.
> + */
> +static int copy_module_elf(struct module *mod, struct load_info *info)
> +{
> +	unsigned int size, symndx;
> +	int ret;
> +
> +	size = sizeof(*mod->klp_info);
> +	mod->klp_info = kmalloc(size, GFP_KERNEL);
> +	if (mod->klp_info == NULL)
> +		return -ENOMEM;
> +
> +	/* Elf header */
> +	size = sizeof(Elf_Ehdr);
> +	memcpy(&mod->klp_info->hdr, info->hdr, size);
> +
> +	/* Elf section header table */
> +	size = sizeof(Elf_Shdr) * info->hdr->e_shnum;
> +	mod->klp_info->sechdrs = kmalloc(size, GFP_KERNEL);
> +	if (mod->klp_info->sechdrs == NULL) {
> +		ret = -ENOMEM;
> +		goto free_info;
> +	}
> +	memcpy(mod->klp_info->sechdrs, info->sechdrs, size);
> +
> +	/* Elf section name string table */
> +	size = info->sechdrs[info->hdr->e_shstrndx].sh_size;
> +	mod->klp_info->secstrings = kmalloc(size, GFP_KERNEL);
> +	if (mod->klp_info->secstrings == NULL) {
> +		ret = -ENOMEM;
> +		goto free_sechdrs;
> +	}
> +	memcpy(mod->klp_info->secstrings, info->secstrings, size);
> +
> +	/* Elf symbol section index */
> +	symndx = info->index.sym;
> +	mod->klp_info->symndx = symndx;

nit: The 'symndx' local variable is superfluous.

> +
> +	/*
> +	 * For livepatch modules, core_symtab is a complete copy

s/core_symtab/core_kallsyms.symtab/ ?

> +	 * of the original symbol table. Adjust sh_addr to point
> +	 * to core_symtab since the copy of the symtab in module
> +	 * init memory is freed at the end of do_init_module().
> +	 */
> +	mod->klp_info->sechdrs[symndx].sh_addr = (unsigned long) mod->core_kallsyms.symtab;

-- 
Josh