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/20200107214042.855757-1-arnd@arndb.de/
Link: https://lore.kernel.org/lkml/20240326130647.7bfb1d92@gandalf.local.home/
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

1 files changed, 12 insertions, 11 deletions

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 22ea19a36e6e..98b9622d372e 100644
--- a/kernel/kallsyms.c
+++ b/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)