Follow the way modprobe handles this by making this a Kconfig option which
defaults to the current /sbin/request-key.

Also changed "char const" to "const char" as checkpatch complains
otherwise.

Link: https://github.com/NixOS/nixpkgs/blob/6b316287bae2ee04c9b93c8c858d930fd07d7338/pkgs/os-specific/linux/kernel/request-key-helper.patch
Signed-off-by: Gary Guo <gary@garyguo.net>
---
I did not update mentions of /sbin/request-key in documentation and
elsewhere, as "/sbin/request-key" is concise while "request-key UMH" is
more mouthful and less clear.

Number of distros that doesn't have /sbin is limited so I think it wouldn't
create much confusion. Similarly, there are a lot of places where
/sbin/modprobe is mentioned despite it is technically configurable.
---
 security/keys/Kconfig       | 9 +++++++++
 security/keys/request_key.c | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index f4510d8cb485..ee3c3d85fc03 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig

@@ -40,6 +40,15 @@ config KEYS_REQUEST_CACHE
 	  key.  Pathwalk will call multiple methods for each dentry traversed
 	  (permission, d_revalidate, lookup, getxattr, getacl, ...).
 
+config REQUEST_KEY_PATH
+	string "Path to the request-key binary"
+	default "/sbin/request-key"
+	help
+	  Path of the request-key usermode helper binary.
+
+	  This program is invoked by the kernel when the kernel is asked for
+	  a key that it doesn't have immediately available.
+
 config PERSISTENT_KEYRINGS
 	bool "Enable register of persistent per-UID keyrings"
 	help

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index a7673ad86d18..ac8f9d1a87ad 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c

@@ -117,7 +117,7 @@ static int call_usermodehelper_keys(const char *path, char **argv, char **envp,
  */
 static int call_sbin_request_key(struct key *authkey, void *aux)
 {
-	static char const request_key[] = "/sbin/request-key";
+	static const char request_key[] = CONFIG_REQUEST_KEY_PATH;
 	struct request_key_auth *rka = get_request_key_auth(authkey);
 	const struct cred *cred = current_cred();
 	key_serial_t prkey, sskey;

base-commit: 6e845bcb78c95af935094040bd4edc3c2b6dd784
-- 
2.54.0