linux-kernel - Re: module: save load_info for livepatch modules

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Date:	Thu, 12 Nov 2015 11:24:04 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	Miroslav Benes <mbenes@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: module: save load_info for livepatch modules

On Thu 2015-11-12 00:33:12, Jessica Yu wrote:
> +++ Miroslav Benes [11/11/15 15:17 +0100]:
> >On Mon, 9 Nov 2015, Jessica Yu wrote:
> >
> >>diff --git a/include/linux/module.h b/include/linux/module.h
> >>index 3a19c79..c8680b1 100644
> >>--- a/include/linux/module.h
> >>+++ b/include/linux/module.h
> >
> >[...]
> >
> >>+#ifdef CONFIG_LIVEPATCH
> >>+extern void klp_prepare_patch_module(struct module *mod,
> >>+				     struct load_info *info);
> >>+extern int
> >>+apply_relocate_add(Elf64_Shdr *sechdrs, const char *strtab,
> >>+		   unsigned int symindex, unsigned int relsec,
> >>+		   struct module *me);
> >>+#endif
> >>+
> >> #else /* !CONFIG_MODULES... */
> >
> >apply_relocate_add() is already in include/linux/moduleloader.h (guarded
> >by CONFIG_MODULES_USE_ELF_RELA), so maybe we can just include that where
> >we need it. As for the klp_prepare_patch_module() wouldn't it be better to
> >have it in our livepatch.h and include that in kernel/module.c?
> 
> Yeah, Petr pointed this out as well :-) I will just include
> moduleloader.h for the apply_relocate_add() declaration.
> 
> It also looks like we have some disagreement over where to put
> klp_prepare_patch_module(), either in livepatch/core.c (and add the
> function declaration in livepatch.h, and have module.c include
> livepatch.h) or in kernel/module.c, keeping the
> klp_prepare_patch_module() declaration in module.h. Maybe Rusty can
> provide some input.
> 
> >> /* Given an address, look for it in the exception tables. */
> >>diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> >>index 6e53441..087a8c7 100644
> >>--- a/kernel/livepatch/core.c
> >>+++ b/kernel/livepatch/core.c
> >>@@ -1001,6 +1001,23 @@ static struct notifier_block klp_module_nb = {
> >> 	.priority = INT_MIN+1, /* called late but before ftrace notifier */
> >> };
> >>
> >>+/*
> >>+ * Save necessary information from info in order to be able to
> >>+ * patch modules that might be loaded later
> >>+ */
> >>+void klp_prepare_patch_module(struct module *mod, struct load_info *info)
> >>+{
> >>+	Elf_Shdr *symsect;
> >>+
> >>+	symsect = info->sechdrs + info->index.sym;
> >>+	/* update sh_addr to point to symtab */
> >>+	symsect->sh_addr = (unsigned long)info->hdr + symsect->sh_offset;
> >>+
> >>+	mod->info = kzalloc(sizeof(*info), GFP_KERNEL);
> >>+	memcpy(mod->info, info, sizeof(*info));
> >>+
> >>+}
> >
> >What about arch-specific 'struct mod_arch_specific'? We need to preserve
> >it somewhere as well for s390x and other non-x86 architectures.
> 
> Ah! Thank you for catching this, I overlooked this important detail.
> Yes, we do need to save the arch-specific struct. We would be in
> trouble for s390 relocs if we didn't. I am trying to think of a way to
> save this information for s390, since s390's module_finalize() frees
> mod->arch.syminfo, which we definitely need in order for the call to
> apply_relocate_add() to work. Maybe we can add an extra call right
> before module_finalize() that will do some livepatch-specific
> processing and copy this information (this would be in
> post_relocation() in kernel/module.c). Perhaps this patchset cannot be
> entirely free of arch-specific code after all :-( Still thinking.

I think about adding a flag somewhere, e.g. mod->preserve_relocs.
It might be set in simplify_symbols() when SHN_LIVEPATCH is found.
It might be checked when freeing the needed structures in both
the generic and arch-specific code.

> >>+#ifdef CONFIG_LIVEPATCH
> >>+	/*
> >>+	 * Save sechdrs, indices, and other data from info
> >>+	 * in order to patch to-be-loaded modules.
> >>+	 * Do not call free_copy() for livepatch modules.
> >>+	 */
> >>+	if (get_modinfo((struct load_info *)info, "livepatch"))
> >>+		klp_prepare_patch_module(mod, info);
> >>+	else
> >>+		free_copy(info);
> >>+#else
> >> 	/* Get rid of temporary copy. */
> >> 	free_copy(info);
> >>+#endif
> >
> >Maybe I am missing something but isn't it necessary to call vfree() on
> >info somewhere in the end?
> 
> So free_copy() will call vfree(info->hdr), except in livepatch modules
> we want to keep all the elf section information stored there, so we
> avoid calling free_copy(), As for the info struct itself, if you look
> at the init_module and finit_module syscall definitions in
> kernel/module.c, you will see that info is actually a local function
> variable, simply passed in to the call to load_module(), and will be
> automatically deallocated when the syscall returns. :-) No need to
> explicitly free info.

We still have to free the copied or preserved structures when
the module is unloaded.

Thank you,
Petr
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