linux-kernel - Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api

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Date:   Mon, 4 Jun 2018 08:16:48 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api

On Mon, Jun 04, 2018 at 03:01:31PM +0200, Jessica Yu wrote:
> +++ Jessica Yu [04/06/18 11:54 +0200]:
> > +++ Jessica Yu [04/06/18 10:05 +0200]:
> > > +++ Josh Poimboeuf [02/06/18 12:32 -0500]:
> > > > Hi Jessica,
> > > > 
> > > > I found a bug:
> > > > 
> > > > [root@f25 ~]# modprobe livepatch-sample
> > > > [root@f25 ~]# grep ' u ' /proc/kallsyms
> > > > ffffffff81161080 u klp_enable_patch	[livepatch_sample]
> > > > ffffffff81a01800 u __fentry__	[livepatch_sample]
> > > > ffffffff81161250 u klp_unregister_patch	[livepatch_sample]
> > > > ffffffff81161870 u klp_register_patch	[livepatch_sample]
> > > > ffffffff8131f0b0 u seq_printf	[livepatch_sample]
> > > > 
> > > > Notice that livepatch modules' undefined symbols are showing up in
> > > > /proc/kallsyms.  This can confuse klp_find_object_symbol() which can
> > > > cause subtle bugs in livepatch.
> > > > 
> > > > I stared at the module kallsyms code for a bit, but I don't see the bug.
> > > > Maybe it has something to do with how we save the symbol table in
> > > > copy_module_elf().  Any ideas?
> > > 
> > > Hi Josh!
> > > 
> > > This is because we preserve the entire symbol table for livepatch
> > > modules, including the SHN_UNDEF symbols. IIRC, this is so that we can
> > > still apply relocations properly with apply_relocate_add() after a
> > > to-be-patched object is loaded. Normally we don't save these SHN_UNDEF
> > > symbols for modules so they do not appear in /proc/kallsyms.
> > 
> > Hm, if having the full symtab in kallsyms is causing trouble, one
> > possibility would be to just have the module kallsyms code simply
> > skip/ignore undef symbols. That's what we technically do for normal
> > modules anyway (we normally cut undef syms out of the symtab). Haven't
> > tested this idea but does that sound like it'd help?
> 
> See if the following patch (untested) helps. It does not fix the
> /proc/kallsyms lookup, that requires a separate patch. But it should
> exclude the undef symbols from module_kallsyms_on_each_symbol() and
> thus also from klp_find_object_symbol().

That seems like it would work.  But wouldn't it be more robust if we
don't store the SHN_UNDEF symbols to start with?  Really it's only the
SHN_LIVEPATCH symbols that we need to keep, right?

What do you think about the following (untested)?

diff --git a/kernel/module.c b/kernel/module.c
index c9bea7f2b43e..78ec9de856e3 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2586,6 +2586,9 @@ static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
 {
 	const Elf_Shdr *sec;
 
+	if (src->st_shndx == SHN_LIVEPATCH)
+		return true;
+
 	if (src->st_shndx == SHN_UNDEF
 	    || src->st_shndx >= shnum
 	    || !src->st_name)
@@ -2632,9 +2635,9 @@ static void layout_symtab(struct module *mod, struct load_info *info)
 
 	/* Compute total space required for the core symbols' strtab. */
 	for (ndst = i = 0; i < nsrc; i++) {
-		if (i == 0 || is_livepatch_module(mod) ||
-		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
-				   info->index.pcpu)) {
+		if (i == 0 || is_core_symbol(src+i, info->sechdrs,
+					     info->hdr->e_shnum,
+					     info->index.pcpu)) {
 			strtab_size += strlen(&info->strtab[src[i].st_name])+1;
 			ndst++;
 		}
@@ -2691,9 +2694,9 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
 	mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs;
 	src = mod->kallsyms->symtab;
 	for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) {
-		if (i == 0 || is_livepatch_module(mod) ||
-		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
-				   info->index.pcpu)) {
+		if (i == 0 || is_core_symbol(src+i, info->sechdrs,
+					     info->hdr->e_shnum,
+					     info->index.pcpu)) {
 			dst[ndst] = src[i];
 			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
 			s += strlcpy(s, &mod->kallsyms->strtab[src[i].st_name],