module: fix bne2 "gave up waiting for init of module libcrc32c"

Problem: it's hard to avoid an init routine stumbling over a
request_module these days.  And it's not clear it's always a bad idea:
for example, a module like kvm with dynamic dependencies on kvm-intel
or kvm-amd would be neater if it could simply request_module the right
one.

In this particular case, it's libcrc32c:

	libcrc32c_mod_init
	 crypto_alloc_shash
	  crypto_alloc_tfm
	   crypto_find_alg
	    crypto_alg_mod_lookup
	     crypto_larval_lookup
	      request_module

If another module is waiting inside resolve_symbol() for libcrc32c to
finish initializing (ie. bne2 depends on libcrc32c) then it does so
holding the module lock, and our request_module() can't make progress
until that is released.

Waiting inside resolve_symbol() without the lock isn't all that hard:
we just need to pass the -EBUSY up the call chain so we can sleep
where we don't hold the lock.  Error reporting is a bit trickier: we
need to copy the name of the unfinished module before releasing the
lock.

Other notes:
1) This also fixes a theoretical issue where a weak dependency would allow
   symbol version mismatches to be ignored.
2) We rename use_module to ref_module to make life easier for the only
   external user (the out-of-tree ksplice patches).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Tim Abbot <tabbott@ksplice.com>
Tested-by: Brandon Philips <bphilips@suse.de>

diff --git a/kernel/module.c b/kernel/module.c
index f99558e..8c6b428 100644 (file)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -582,33 +582,26 @@ static int add_module_usage(struct module *a, struct module *b)
 }
 
 /* Module a uses b: caller needs module_mutex() */
-int use_module(struct module *a, struct module *b)
+int ref_module(struct module *a, struct module *b)
 {
        int err;
 
-       if (b == NULL || already_uses(a, b)) return 1;
-
-       /* If we're interrupted or time out, we fail. */
-       if (wait_event_interruptible_timeout(
-                   module_wq, (err = strong_try_module_get(b)) != -EBUSY,
-                   30 * HZ) <= 0) {
-               printk("%s: gave up waiting for init of module %s.\n",
-                      a->name, b->name);
+       if (b == NULL || already_uses(a, b))
                return 0;
-       }
 
-       /* If strong_try_module_get() returned a different error, we fail. */
+       /* If module isn't available, we fail. */
+       err = strong_try_module_get(b);
        if (err)
-               return 0;
+               return err;
 
        err = add_module_usage(a, b);
        if (err) {
                module_put(b);
-               return 0;
+               return err;
        }
-       return 1;
+       return 0;
 }
-EXPORT_SYMBOL_GPL(use_module);
+EXPORT_SYMBOL_GPL(ref_module);
 
 /* Clear the unload stuff of the module. */
 static void module_unload_free(struct module *mod)
@@ -893,11 +886,11 @@ static inline void module_unload_free(struct module *mod)
 {
 }
 
-int use_module(struct module *a, struct module *b)
+int ref_module(struct module *a, struct module *b)
 {
-       return strong_try_module_get(b) == 0;
+       return strong_try_module_get(b);
 }
-EXPORT_SYMBOL_GPL(use_module);
+EXPORT_SYMBOL_GPL(ref_module);
 
 static inline void module_unload_init(struct module *mod)
 {
@@ -1062,26 +1055,58 @@ static inline int same_magic(const char *amagic, const char *bmagic,
 static const struct kernel_symbol *resolve_symbol(Elf_Shdr *sechdrs,
                                                  unsigned int versindex,
                                                  const char *name,
-                                                 struct module *mod)
+                                                 struct module *mod,
+                                                 char ownername[])
 {
        struct module *owner;
        const struct kernel_symbol *sym;
        const unsigned long *crc;
+       int err;
 
        mutex_lock(&module_mutex);
        sym = find_symbol(name, &owner, &crc,
                          !(mod->taints & (1 << TAINT_PROPRIETARY_MODULE)), true);
-       /* use_module can fail due to OOM,
-          or module initialization or unloading */
-       if (sym) {
-               if (!check_version(sechdrs, versindex, name, mod, crc, owner)
-                   || !use_module(mod, owner))
-                       sym = NULL;
+       if (!sym)
+               goto unlock;
+
+       if (!check_version(sechdrs, versindex, name, mod, crc, owner)) {
+               sym = ERR_PTR(-EINVAL);
+               goto getname;
        }
+
+       err = ref_module(mod, owner);
+       if (err) {
+               sym = ERR_PTR(err);
+               goto getname;
+       }
+
+getname:
+       /* We must make copy under the lock if we failed to get ref. */
+       strncpy(ownername, module_name(owner), MODULE_NAME_LEN);
+unlock:
        mutex_unlock(&module_mutex);
        return sym;
 }
 
+static const struct kernel_symbol *resolve_symbol_wait(Elf_Shdr *sechdrs,
+                                                      unsigned int versindex,
+                                                      const char *name,
+                                                      struct module *mod)
+{
+       const struct kernel_symbol *ksym;
+       char ownername[MODULE_NAME_LEN];
+
+       if (wait_event_interruptible_timeout(module_wq,
+                       !IS_ERR(ksym = resolve_symbol(sechdrs, versindex, name,
+                                                     mod, ownername)) ||
+                       PTR_ERR(ksym) != -EBUSY,
+                                            30 * HZ) <= 0) {
+               printk(KERN_WARNING "%s: gave up waiting for init of module %s.\n",
+                      mod->name, ownername);
+       }
+       return ksym;
+}
+
 /*
  * /sys/module/foo/sections stuff
  * J. Corbet <corbet@lwn.net>
@@ -1638,21 +1663,23 @@ static int simplify_symbols(Elf_Shdr *sechdrs,
                        break;
 
                case SHN_UNDEF:
-                       ksym = resolve_symbol(sechdrs, versindex,
-                                             strtab + sym[i].st_name, mod);
+                       ksym = resolve_symbol_wait(sechdrs, versindex,
+                                                  strtab + sym[i].st_name,
+                                                  mod);
                        /* Ok if resolved.  */
-                       if (ksym) {
+                       if (ksym && !IS_ERR(ksym)) {
                                sym[i].st_value = ksym->value;
                                break;
                        }
 
                        /* Ok if weak.  */
-                       if (ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
+                       if (!ksym && ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
                                break;
 
-                       printk(KERN_WARNING "%s: Unknown symbol %s\n",
-                              mod->name, strtab + sym[i].st_name);
-                       ret = -ENOENT;
+                       printk(KERN_WARNING "%s: Unknown symbol %s (err %li)\n",
+                              mod->name, strtab + sym[i].st_name,
+                              PTR_ERR(ksym));
+                       ret = PTR_ERR(ksym) ?: -ENOENT;
                        break;
 
                default: