Re: module: save load_info for livepatch modules

From: Jessica Yu
Date: Wed Nov 11 2015 - 23:44:18 EST

+++ Petr Mladek [11/11/15 15:31 +0100]:
On Mon 2015-11-09 23:45:52, Jessica Yu wrote:
In livepatch modules, preserve section, symbol, string information from
the load_info struct in the module loader. This information is used to
patch modules that are not loaded in memory yet; specifically it is used
to resolve remaining symbols and write relocations when the target
module loads.

Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
include/linux/module.h | 25 +++++++++++++++++++++++++
kernel/livepatch/core.c | 17 +++++++++++++++++
kernel/module.c | 36 ++++++++++++++++++++++--------------
3 files changed, 64 insertions(+), 14 deletions(-)

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
@@ -635,6 +651,15 @@ static inline bool module_requested_async_probing(struct module *module)
return module && module->async_probe_requested;

+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);

This function is already declared in moduleloader.h.
It is implemted only when CONFIG_MODULES_USE_ELF_RELA is defined.

I guess that we want to include moduleloader.h in livepatch.

#else /* !CONFIG_MODULES... */

/* 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;

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.

+ 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

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 :-)

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
@@ -318,20 +318,6 @@ int unregister_module_notifier(struct notifier_block *nb)

-struct load_info {
- Elf_Ehdr *hdr;
- unsigned long len;
- Elf_Shdr *sechdrs;
- char *secstrings, *strtab;
- unsigned long symoffs, stroffs;
- struct _ddebug *debug;
- unsigned int num_debug;
- bool sig_ok;
- struct {
- unsigned int sym, str, mod, vers, info, pcpu;
- } index;
/* We require a truly strong try_module_get(): 0 means failure due to
ongoing or failed initialization etc. */
static inline int strong_try_module_get(struct module *mod)
@@ -2137,6 +2123,11 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)

+ break;

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 :-)

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))

+ if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
+ continue;
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;

+ /*
+ * 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);

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).

/* Get rid of temporary copy. */

Thanks for the comments,
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