bpf: Add config to allow loading modules with BTF mismatches

BTF mismatch can occur for a separately-built module even when the ABI is
otherwise compatible and nothing else would prevent successfully loading.

Add a new Kconfig to control how mismatches are handled. By default, preserve
the current behavior of refusing to load the module. If MODULE_ALLOW_BTF_MISMATCH
is enabled, load the module but ignore its BTF information.
Suggested-by: Yonghong Song <yhs@fb.com>
Suggested-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/CAADnVQJ+OVPnBz8z3vNu8gKXX42jCUqfuvhWAyCQDu8N_yqqwQ@mail.gmail.com
Link: https://lore.kernel.org/bpf/20220223012814.1898677-1-connoro@google.com

kernel/bpf/btf.c

@@ -6398,7 +6398,8 @@ static int btf_module_notify(struct notifier_block *nb, unsigned long op,
 			pr_warn("failed to validate module [%s] BTF: %ld\n",
 				mod->name, PTR_ERR(btf));
 			kfree(btf_mod);
-			err = PTR_ERR(btf);
+			if (!IS_ENABLED(CONFIG_MODULE_ALLOW_BTF_MISMATCH))
+				err = PTR_ERR(btf);
 			goto out;
 		}
 		err = btf_alloc_id(btf);

lib/Kconfig.debug

@@ -339,6 +339,16 @@ config DEBUG_INFO_BTF_MODULES
 	help
 	  Generate compact split BTF type information for kernel modules.
 
+config MODULE_ALLOW_BTF_MISMATCH
+	bool "Allow loading modules with non-matching BTF type info"
+	depends on DEBUG_INFO_BTF_MODULES
+	help
+	  For modules whose split BTF does not match vmlinux, load without
+	  BTF rather than refusing to load. The default behavior with
+	  module BTF enabled is to reject modules with such mismatches;
+	  this option will still load module BTF where possible but ignore
+	  it when a mismatch is found.
+
 config GDB_SCRIPTS
 	bool "Provide GDB scripts for kernel debugging"
 	help