Message-ID: <20151112100548.GK4431@pathway.suse.cz>
Date:	Thu, 12 Nov 2015 11:05:48 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	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>,
	Miroslav Benes <mbenes@...e.cz>, 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 Wed 2015-11-11 23:44:08, Jessica Yu wrote:
> +++ Petr Mladek [11/11/15 15:31 +0100]:
> >On Mon 2015-11-09 23:45:52, Jessica Yu wrote:
> >>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;
> >
> >Is livepatch the only user of this value? By other words, is this safe?
> 
> I think it is safe to say yes. klp_prepare_patch_module() is only
> called at the very end of load_module(), right before
> do_init_module(). Normally, at that point, info->hdr will have already
> been freed by free_copy() along with the elf section information
> associated with it. But if we have a livepatch module, we don't free.
> So we should be the very last user, and there should be nobody
> utilizing the memory associated with the load_info struct anymore at
> that point.

I see. It looks safe at this point. But still I wonder if it would be
possible to calculate this later in the livepatch code. It will allow
to potentially use the info structure also by other subsystem.

BTW: Where is "sh_addr" value used, please? I see it used only
in the third patch as info->sechdrs[relindex].sh_addr. But it is
an array. I am not sure if it is the same variable.


> >>+	mod->info = kzalloc(sizeof(*info), GFP_KERNEL);
> >>+	memcpy(mod->info, info, sizeof(*info));
> >>+
> >>+}
> >
> >It is strange that this funtion is defined in livepatch/core.c
> >but declared in module.h. I would move the definition to
> >module.c.
> 
> Right, I was trying to keep all the livepatch-related functions
> together in livepatch/core.c. but I can move it to module.c if it
> makes more sense/Rusty doesn't object to it :-)

Sure. I think that we could use some generic name, e.g. copy_module_info().

> >> static int __init klp_init(void)
> >> {
> >> 	int ret;
> >>diff --git a/kernel/module.c b/kernel/module.c
> >>index 8f051a1..8ae3ca5 100644
> >>--- a/kernel/module.c
> >>+++ b/kernel/module.c
> >>@@ -2137,6 +2123,11 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
> >> 			       (long)sym[i].st_value);
> >> 			break;
> >>
> >>+#ifdef CONFIG_LIVEPATCH
> >>+		case SHN_LIVEPATCH:
> >>+			break;
> >>+#endif
> >
> >IMHO, even a kernel compiled without CONFIG_LIVEPATCH should handle livepatch
> >modules with grace. It means to reject loading.
> 
> I think even right now, without considering this patchset, we don't
> reject modules "gracefully" when we load a livepatch module without
> CONFIG_LIVEPATCH. The module loader will complain and reject the
> livepatch module, saying something like "Unknown symbol
> klp_register_patch." This behavior is the same with or without
> this patch series applied. If we want to add a bit more logic to
> gracefully reject patch modules, perhaps that should be a different
> patch altogether, as I think it is unrelated to the goal of this one :-)

Yup, the module load would fail anyway because of the missing symbol.
But I think that we should fail on the first error occurence.

In each case, IMHO, we should not do the "default:" action for this
section even when complied without CONFIG_LIVEPATCH.


> >> 		case SHN_UNDEF:
> >> 			ksym = resolve_symbol_wait(mod, info, name);
> >> 			/* Ok if resolved.  */
> >>@@ -2185,6 +2176,11 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
> >> 		if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC))
> >> 			continue;
> >>
> >>+#ifdef CONFIG_LIVEPATCH
> >>+		if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
> >>+			continue;
> >>+#endif

I guess that if we do not trigger the error above, and do
not have the check here, we will try to call apply_relocate() below.
I guess that it will fail. If we are lucky it will print "Unknown
relocation". I think that we could do better.

> >>+
> >> 		if (info->sechdrs[i].sh_type == SHT_REL)
> >> 			err = apply_relocate(info->sechdrs, info->strtab,
> >> 					     info->index.sym, i, mod);
> >>@@ -3530,8 +3526,20 @@ static int load_module(struct load_info *info, const char __user *uargs,
> >> 	if (err < 0)
> >> 		goto bug_cleanup;
> >>
> >>+#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
> >
> >I would move this #else one line above and get rid of the
> >double free_copy(info); But it is a matter of taste.
> 
> Maybe I'm missing something, but I think we do need the double
> free_copy(), because in the CONFIG_LIVEPATCH case, we still want to
> call free_copy() for non-livepatch modules. And we want to avoid
> calling free_copy() for livepatch modules (hence the extra else).

Ah, this was just a cosmetic change. I meant to use something like:

#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
#endif
		/* Get rid of temporary copy. */
		free_copy(info);


It is a matter of taste. Maybe, your variant was better in the end.

Thank you,
Petr
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