+++ 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().
From 9cfd14675206adf55a85e5f5322b36ea89a523e4 Mon Sep 17 00:00:00 2001
From: Jessica Yu <jeyu@xxxxxxxxxx>
Date: Mon, 4 Jun 2018 14:35:56 +0200
Subject: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api
---
kernel/module.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/module.c b/kernel/module.c
index c9bea7f2b43e..dfa61490b37d 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -4070,7 +4070,7 @@ static unsigned long mod_find_symname(struct module *mod, const char *name)
for (i = 0; i < kallsyms->num_symtab; i++)
if (strcmp(name, symname(kallsyms, i)) == 0 &&
- kallsyms->symtab[i].st_info != 'U')
+ kallsyms->symtab[i].st_shndx != SHN_UNDEF)
return kallsyms->symtab[i].st_value;
return 0;
}
@@ -4116,6 +4116,10 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
if (mod->state == MODULE_STATE_UNFORMED)
continue;
for (i = 0; i < kallsyms->num_symtab; i++) {
+
+ if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
+ continue;
+
ret = fn(data, symname(kallsyms, i),
mod, kallsyms->symtab[i].st_value);
if (ret != 0)
--
2.12.3
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