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 <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Tim Abbot <[e-mail]>
Tested-by: Brandon Philips <[e-mail]>

kernel/module.c

···
 }
 
 /* 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)
···
 {
 }
 
-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)
 {
···
 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;
 }
 
 /*
···
 			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: