Re: module: save load_info for livepatch modules

[Jessica Yu]

+++ Petr Mladek [12/11/15 11:05 +0100]:
> 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.

We can technically reassign sh_addr later in livepatch somewhere, yes,
I'd have to think more about where it'd make the most sense to do
this. Maybe in patch_init? It just seemed at the time a bit clearer to
do it in klp_prepare_patch_module() (soon to be called
copy_module_info() probably).

> 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.

I will add a more informative comment in the code, see my reply to
Miroslav.

> > >>+	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.

See comment below -- 

> > >> 		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.

For the loading of livepatch modules in !CONFIG_LIVEPATCH kernels, we
should probably gracefully reject it in the beginning of load_module()
(so that MODULE_INFO flag might come in handy here after all). If it's
a livepatch module && !CONFIG_LIVEPATCH, reject it. Then we wouldn't
even call apply_relocations() here, we wouldn't run into the
possibility of this check falling through, nor would
simplify_symbols() be even called.

> > >>+
> > >> 		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);

Oh OK, so that's what you meant. :-)

Thanks,
Jessica
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