kallsyms: rework symbol lookup return codes

Building with W=1 in some configurations produces a false positive
warning for kallsyms:

kernel/kallsyms.c: In function '__sprint_symbol.isra':
kernel/kallsyms.c:503:17: error: 'strcpy' source argument is the same as destination [-Werror=restrict]
  503 |                 strcpy(buffer, name);
      |                 ^~~~~~~~~~~~~~~~~~~~

This originally showed up while building with -O3, but later started
happening in other configurations as well, depending on inlining
decisions. The underlying issue is that the local 'name' variable is
always initialized to the be the same as 'buffer' in the called functions
that fill the buffer, which gcc notices while inlining, though it could
see that the address check always skips the copy.

The calling conventions here are rather unusual, as all of the internal
lookup functions (bpf_address_lookup, ftrace_mod_address_lookup,
ftrace_func_address_lookup, module_address_lookup and
kallsyms_lookup_buildid) already use the provided buffer and either return
the address of that buffer to indicate success, or NULL for failure,
but the callers are written to also expect an arbitrary other buffer
to be returned.

Rework the calling conventions to return the length of the filled buffer
instead of its address, which is simpler and easier to follow as well
as avoiding the warning. Leave only the kallsyms_lookup() calling conventions
unchanged, since that is called from 16 different functions and
adapting this would be a much bigger change.

Link: https://lore.kernel.org/lkml/[email protected]/
Link: https://lore.kernel.org/lkml/[email protected]/
Tested-by: Geert Uytterhoeven <[email protected]>
Reviewed-by: Luis Chamberlain <[email protected]>
Acked-by: Steven Rostedt (Google) <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>

include/linux/filter.h

@@ -1208,18 +1208,18 @@ static inline bool bpf_jit_kallsyms_enabled(void)
 	return false;
 }
 
-const char *__bpf_address_lookup(unsigned long addr, unsigned long *size,
+int __bpf_address_lookup(unsigned long addr, unsigned long *size,
 				 unsigned long *off, char *sym);
 bool is_bpf_text_address(unsigned long addr);
 int bpf_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
 		    char *sym);
 struct bpf_prog *bpf_prog_ksym_find(unsigned long addr);
 
-static inline const char *
+static inline int
 bpf_address_lookup(unsigned long addr, unsigned long *size,
 		   unsigned long *off, char **modname, char *sym)
 {
-	const char *ret = __bpf_address_lookup(addr, size, off, sym);
+	int ret = __bpf_address_lookup(addr, size, off, sym);
 
 	if (ret && modname)
 		*modname = NULL;
@@ -1263,11 +1263,11 @@ static inline bool bpf_jit_kallsyms_enabled(void)
 	return false;
 }
 
-static inline const char *
+static inline int
 __bpf_address_lookup(unsigned long addr, unsigned long *size,
 		     unsigned long *off, char *sym)
 {
-	return NULL;
+	return 0;
 }
 
 static inline bool is_bpf_text_address(unsigned long addr)
@@ -1286,11 +1286,11 @@ static inline struct bpf_prog *bpf_prog_ksym_find(unsigned long addr)
 	return NULL;
 }
 
-static inline const char *
+static inline int
 bpf_address_lookup(unsigned long addr, unsigned long *size,
 		   unsigned long *off, char **modname, char *sym)
 {
-	return NULL;
+	return 0;
 }
 
 static inline void bpf_prog_kallsyms_add(struct bpf_prog *fp)

include/linux/ftrace.h

@@ -86,15 +86,15 @@ struct ftrace_hash;
 
 #if defined(CONFIG_FUNCTION_TRACER) && defined(CONFIG_MODULES) && \
 	defined(CONFIG_DYNAMIC_FTRACE)
-const char *
+int
 ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
 		   unsigned long *off, char **modname, char *sym);
 #else
-static inline const char *
+static inline int
 ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
 		   unsigned long *off, char **modname, char *sym)
 {
-	return NULL;
+	return 0;
 }
 #endif
 

include/linux/module.h

@@ -931,11 +931,11 @@ int module_kallsyms_on_each_symbol(const char *modname,
  * least KSYM_NAME_LEN long: a pointer to namebuf is returned if
  * found, otherwise NULL.
  */
-const char *module_address_lookup(unsigned long addr,
-				  unsigned long *symbolsize,
-				  unsigned long *offset,
-				  char **modname, const unsigned char **modbuildid,
-				  char *namebuf);
+int module_address_lookup(unsigned long addr,
+			  unsigned long *symbolsize,
+			  unsigned long *offset,
+			  char **modname, const unsigned char **modbuildid,
+			  char *namebuf);
 int lookup_module_symbol_name(unsigned long addr, char *symname);
 int lookup_module_symbol_attrs(unsigned long addr,
 			       unsigned long *size,
@@ -964,14 +964,14 @@ static inline int module_kallsyms_on_each_symbol(const char *modname,
 }
 
 /* For kallsyms to ask for address resolution.  NULL means not found. */
-static inline const char *module_address_lookup(unsigned long addr,
+static inline int module_address_lookup(unsigned long addr,
 						unsigned long *symbolsize,
 						unsigned long *offset,
 						char **modname,
 						const unsigned char **modbuildid,
 						char *namebuf)
 {
-	return NULL;
+	return 0;
 }
 
 static inline int lookup_module_symbol_name(unsigned long addr, char *symname)

kernel/bpf/core.c

@@ -736,21 +736,20 @@ static struct bpf_ksym *bpf_ksym_find(unsigned long addr)
 	return n ? container_of(n, struct bpf_ksym, tnode) : NULL;
 }
 
-const char *__bpf_address_lookup(unsigned long addr, unsigned long *size,
+int __bpf_address_lookup(unsigned long addr, unsigned long *size,
 				 unsigned long *off, char *sym)
 {
 	struct bpf_ksym *ksym;
-	char *ret = NULL;
+	int ret = 0;
 
 	rcu_read_lock();
 	ksym = bpf_ksym_find(addr);
 	if (ksym) {
 		unsigned long symbol_start = ksym->start;
 		unsigned long symbol_end = ksym->end;
 
-		strscpy(sym, ksym->name, KSYM_NAME_LEN);
+		ret = strscpy(sym, ksym->name, KSYM_NAME_LEN);
 
-		ret = sym;
 		if (size)
 			*size = symbol_end - symbol_start;
 		if (off)

kernel/kallsyms.c

@@ -388,12 +388,12 @@ int kallsyms_lookup_size_offset(unsigned long addr, unsigned long *symbolsize,
 	       !!__bpf_address_lookup(addr, symbolsize, offset, namebuf);
 }
 
-static const char *kallsyms_lookup_buildid(unsigned long addr,
+static int kallsyms_lookup_buildid(unsigned long addr,
 			unsigned long *symbolsize,
 			unsigned long *offset, char **modname,
 			const unsigned char **modbuildid, char *namebuf)
 {
-	const char *ret;
+	int ret;
 
 	namebuf[KSYM_NAME_LEN - 1] = 0;
 	namebuf[0] = 0;
@@ -410,7 +410,7 @@ static const char *kallsyms_lookup_buildid(unsigned long addr,
 		if (modbuildid)
 			*modbuildid = NULL;
 
-		ret = namebuf;
+		ret = strlen(namebuf);
 		goto found;
 	}
 
@@ -442,8 +442,13 @@ const char *kallsyms_lookup(unsigned long addr,
 			    unsigned long *offset,
 			    char **modname, char *namebuf)
 {
-	return kallsyms_lookup_buildid(addr, symbolsize, offset, modname,
-				       NULL, namebuf);
+	int ret = kallsyms_lookup_buildid(addr, symbolsize, offset, modname,
+					  NULL, namebuf);
+
+	if (!ret)
+		return NULL;
+
+	return namebuf;
 }
 
 int lookup_symbol_name(unsigned long addr, char *symname)
@@ -478,19 +483,15 @@ static int __sprint_symbol(char *buffer, unsigned long address,
 {
 	char *modname;
 	const unsigned char *buildid;
-	const char *name;
 	unsigned long offset, size;
 	int len;
 
 	address += symbol_offset;
-	name = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
+	len = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
 				       buffer);
-	if (!name)
+	if (!len)
 		return sprintf(buffer, "0x%lx", address - symbol_offset);
 
-	if (name != buffer)
-		strcpy(buffer, name);
-	len = strlen(buffer);
 	offset -= symbol_offset;
 
 	if (add_offset)

kernel/module/kallsyms.c

@@ -321,14 +321,15 @@ void * __weak dereference_module_function_descriptor(struct module *mod,
  * For kallsyms to ask for address resolution.  NULL means not found.  Careful
  * not to lock to avoid deadlock on oopses, simply disable preemption.
  */
-const char *module_address_lookup(unsigned long addr,
-				  unsigned long *size,
-			    unsigned long *offset,
-			    char **modname,
-			    const unsigned char **modbuildid,
-			    char *namebuf)
+int module_address_lookup(unsigned long addr,
+			  unsigned long *size,
+			  unsigned long *offset,
+			  char **modname,
+			  const unsigned char **modbuildid,
+			  char *namebuf)
 {
-	const char *ret = NULL;
+	const char *sym;
+	int ret = 0;
 	struct module *mod;
 
 	preempt_disable();
@@ -344,12 +345,10 @@ const char *module_address_lookup(unsigned long addr,
 #endif
 		}
 
-		ret = find_kallsyms_symbol(mod, addr, size, offset);
-	}
-	/* Make a copy in here where it's safe */
-	if (ret) {
-		strscpy(namebuf, ret, KSYM_NAME_LEN);
-		ret = namebuf;
+		sym = find_kallsyms_symbol(mod, addr, size, offset);
+
+		if (sym)
+			ret = strscpy(namebuf, sym, KSYM_NAME_LEN);
 	}
 	preempt_enable();
 

kernel/trace/ftrace.c

@@ -6969,7 +6969,7 @@ allocate_ftrace_mod_map(struct module *mod,
 	return mod_map;
 }
 
-static const char *
+static int
 ftrace_func_address_lookup(struct ftrace_mod_map *mod_map,
 			   unsigned long addr, unsigned long *size,
 			   unsigned long *off, char *sym)
@@ -6990,21 +6990,18 @@ ftrace_func_address_lookup(struct ftrace_mod_map *mod_map,
 			*size = found_func->size;
 		if (off)
 			*off = addr - found_func->ip;
-		if (sym)
-			strscpy(sym, found_func->name, KSYM_NAME_LEN);
-
-		return found_func->name;
+		return strscpy(sym, found_func->name, KSYM_NAME_LEN);
 	}
 
-	return NULL;
+	return 0;
 }
 
-const char *
+int
 ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
 		   unsigned long *off, char **modname, char *sym)
 {
 	struct ftrace_mod_map *mod_map;
-	const char *ret = NULL;
+	int ret = 0;
 
 	/* mod_map is freed via call_rcu() */
 	preempt_disable();