Fix race between cat /proc/*/wchan and rmmod et al

kallsyms_lookup() can go iterating over modules list unprotected which is OK
for emergency situations (oops), but not OK for regular stuff like
/proc/*/wchan.

Introduce lookup_symbol_name()/lookup_module_symbol_name() which copy symbol
name into caller-supplied buffer or return -ERANGE.  All copying is done with
module_mutex held, so...

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kallsyms.c

@@ -269,6 +269,23 @@ const char *kallsyms_lookup(unsigned long addr,
 	return NULL;
 }
 
+int lookup_symbol_name(unsigned long addr, char *symname)
+{
+	symname[0] = '\0';
+	symname[KSYM_NAME_LEN] = '\0';
+
+	if (is_ksym_addr(addr)) {
+		unsigned long pos;
+
+		pos = get_symbol_pos(addr, NULL, NULL);
+		/* Grab name */
+		kallsyms_expand_symbol(get_symbol_offset(pos), symname);
+		return 0;
+	}
+	/* see if it's in a module */
+	return lookup_module_symbol_name(addr, symname);
+}
+
 /* Look up a kernel symbol and return it in a text buffer. */
 int sprint_symbol(char *buffer, unsigned long address)
 {

kernel/module.c

@@ -2126,6 +2126,29 @@ const char *module_address_lookup(unsigned long addr,
 	return NULL;
 }
 
+int lookup_module_symbol_name(unsigned long addr, char *symname)
+{
+	struct module *mod;
+
+	mutex_lock(&module_mutex);
+	list_for_each_entry(mod, &modules, list) {
+		if (within(addr, mod->module_init, mod->init_size) ||
+		    within(addr, mod->module_core, mod->core_size)) {
+			const char *sym;
+
+			sym = get_ksymbol(mod, addr, NULL, NULL);
+			if (!sym)
+				goto out;
+			strlcpy(symname, sym, KSYM_NAME_LEN + 1);
+			mutex_unlock(&module_mutex);
+			return 0;
+		}
+	}
+out:
+	mutex_unlock(&module_mutex);
+	return -ERANGE;
+}
+
 int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
 			char *name, char *module_name, int *exported)
 {

kernel/time/timer_list.c

@@ -38,15 +38,12 @@ DECLARE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases);
 
 static void print_name_offset(struct seq_file *m, void *sym)
 {
-	unsigned long addr = (unsigned long)sym;
-	char namebuf[KSYM_NAME_LEN+1];
-	const char *sym_name;
+	char symname[KSYM_NAME_LEN+1];
 
-	sym_name = kallsyms_lookup(addr, NULL, NULL, NULL, namebuf);
-	if (sym_name)
-		SEQ_printf(m, "%s", sym_name);
-	else
+	if (lookup_symbol_name((unsigned long)sym, symname) < 0)
 		SEQ_printf(m, "<%p>", sym);
+	else
+		SEQ_printf(m, "%s", symname);
 }
 
 static void

kernel/time/timer_stats.c

@@ -257,14 +257,12 @@ void timer_stats_update_stats(void *timer, pid_t pid, void *startf,
 
 static void print_name_offset(struct seq_file *m, unsigned long addr)
 {
-	char namebuf[KSYM_NAME_LEN+1];
-	const char *sym_name;
+	char symname[KSYM_NAME_LEN+1];
 
-	sym_name = kallsyms_lookup(addr, NULL, NULL, NULL, namebuf);
-	if (sym_name)
-		seq_printf(m, "%s", sym_name);
-	else
+	if (lookup_symbol_name(addr, symname) < 0)
 		seq_printf(m, "<%p>", (void *)addr);
+	else
+		seq_printf(m, "%s", symname);
 }
 
 static int tstats_show(struct seq_file *m, void *v)