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, 7 insertions, 7 deletions

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 0f12cf01070e..5669da513cd7 100644
--- a/include/linux/filter.h
+++ b/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)