Re: module: preserve Elf information for livepatch modules

+++ Rusty Russell [11/01/16 11:55 +1030]:

Hi Jessica,

       Nice patch series.  Minor issues below, but they're really
just nits which can be patched afterwards if you're getting sick of
rebasing :)

Thanks Rusty!

quoted

+#ifdef CONFIG_LIVEPATCH
+/*
+ * copy_module_elf - preserve Elf information about a module
+ *
+ * Copy relevant Elf information from the load_info struct.
+ * Note: not all fields from the original load_info are
+ * copied into mod->info.

This makes me nervous, to have a struct which is half-initialized.

Better would be to have a separate type for this, eg:

struct livepatch_modinfo {
       Elf_Ehdr hdr;
       Elf_Shdr *sechdrs;
       char *secstrings;
       /* FIXME: Which of these do you need? */
       struct {
       	unsigned int sym, str, mod, vers, info, pcpu;
       } index;
};

This also avoids an extra allocation as hdr is no longer a ptr.

Sure, sounds good.

quoted

+	/*
+	 * Update symtab's sh_addr to point to a valid
+	 * symbol table, as the temporary symtab in module
+	 * init memory will be freed
+	 */
+	mod->info->sechdrs[mod->info->index.sym].sh_addr = (unsigned long)mod->core_symtab;

This comment is a bit misleading: it's actually pointing into the
temporary module copy, which will be discarded.  The init section is
slightly different...

Ah, perhaps I'm misunderstanding something..

Since copy_module_elf() is called after move_module(), my
understanding is that all the section sh_addr's should be pointing
to either module core memory or module init memory (instead of the
initial temporary copy of the module in info->hdr). Since the symbol
table section is marked with INIT_OFFSET_MASK, it will reside in
module init memory (and freed near the end of do_init_module()),
which is why I am updating the sh_addr here to point to core_symtab
instead. For livepatch modules, the core_symtab will be a complete
copy of the symbol table instead of the slimmed down version. Please
correct me if my understanding is incorrect.

Thanks for the patch review,
Jessica