Re: module: save load_info for livepatch modules

+++ Miroslav Benes [11/11/15 15:17 +0100]:

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On Mon, 9 Nov 2015, Jessica Yu wrote:

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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

[...]

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+#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.

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/* 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.