utils: add a generic mlock function with a destructor

diff --git a/common/hsm_secret.c b/common/hsm_secret.c
index fd32c516..9acb044f 100644
--- a/common/hsm_secret.c
+++ b/common/hsm_secret.c
@@ -28,11 +28,6 @@
 #define PASSPHRASE_HASH_LEN 32
 #define HSM_SECRET_PLAIN_SIZE 32
 
-void destroy_secret(struct secret *secret)
-{
-	sodium_munlock(secret->data, sizeof(secret->data));
-}
-
 /* Helper function to validate a mnemonic string */
 static bool validate_mnemonic(const char *mnemonic, enum hsm_secret_error *err)
 {
@@ -70,9 +65,7 @@ struct secret *get_encryption_key(const tal_t *ctx, const char *passphrase)
 	}
 
 	/* Don't swap the encryption key ! */
-	if (sodium_mlock(secret->data, sizeof(secret->data)) != 0)
-		return tal_free(secret);
-	tal_add_destructor(secret, destroy_secret);
+	mlock_tal_memory(secret);
 
 	/* Now derive the key. */
 	if (crypto_pwhash(secret->data, sizeof(secret->data), passphrase, strlen(passphrase), salt,
@@ -254,10 +247,6 @@ static struct hsm_secret *extract_encrypted_secret(const tal_t *ctx,
 	/* Attempt decryption */
 	struct secret temp_secret;
 	decrypt_success = decrypt_hsm_secret(encryption_key, hsm_secret, &temp_secret);
-
-	/* Clear encryption key immediately after use */
-	destroy_secret(encryption_key);
-
 	if (!decrypt_success) {
 		*err = HSM_SECRET_ERR_WRONG_PASSPHRASE;
 		return tal_free(hsms);
@@ -389,11 +378,6 @@ bool encrypt_legacy_hsm_secret(const struct secret *encryption_key,
 	return true;
 }
 
-static void destroy_passphrase(char *passphrase)
-{
-	sodium_munlock(passphrase, tal_bytelen(passphrase));
-}
-
 /* Disable terminal echo if needed */
 static bool disable_echo(struct termios *saved_term)
 {
@@ -457,11 +441,7 @@ const char *read_stdin_pass(const tal_t *ctx, enum hsm_secret_error *err)
 		return NULL;
 	}
 
-	/* Memory locking is mandatory: failure means we're on an insecure system */
-	if (sodium_mlock(input, tal_bytelen(input)) != 0)
-		abort();
-
-	tal_add_destructor(input, destroy_passphrase);
+	mlock_tal_memory(input);
 
 	if (echo_disabled)
 		restore_echo(&saved_term);
diff --git a/common/hsm_secret.h b/common/hsm_secret.h
index f874f042..74a11a32 100644
--- a/common/hsm_secret.h
+++ b/common/hsm_secret.h
@@ -158,12 +158,6 @@ bool derive_seed_hash(const char *mnemonic, const char *passphrase, struct sha25
  */
 int is_legacy_hsm_secret_encrypted(const char *path);
 
-/**
- * Zero and unlock a secret's memory.
- * @secret - the secret to destroy
- */
-void destroy_secret(struct secret *secret);
-
 /**
  * Convert hsm_secret_type enum to human-readable string.
  * @type - the hsm_secret_type to convert
diff --git a/common/utils.c b/common/utils.c
index 053e09a7..aba47452 100644
--- a/common/utils.c
+++ b/common/utils.c
@@ -10,6 +10,7 @@
 #include <common/utils.h>
 #include <errno.h>
 #include <locale.h>
+#include <sodium.h>
 
 const tal_t *wally_tal_ctx = NULL;
 secp256k1_context *secp256k1_ctx;
@@ -92,6 +93,18 @@ u8 *tal_hexdata(const tal_t *ctx, const void *str, size_t len)
 	return data;
 }
 
+static void destroy_munlock(const tal_t *ptr)
+{
+	sodium_munlock((void *)ptr, tal_bytelen(ptr));
+}
+
+void mlock_tal_memory(const tal_t *ptr)
+{
+	if (sodium_mlock((void *)ptr, tal_bytelen(ptr)) != 0)
+		abort();
+	tal_add_destructor(ptr, destroy_munlock);
+}
+
 bool tal_arr_eq_(const void *a, const void *b, size_t unused)
 {
 	return memeq(a, tal_bytelen(a), b, tal_bytelen(b));
diff --git a/common/utils.h b/common/utils.h
index 550748b2..d9b441e4 100644
--- a/common/utils.h
+++ b/common/utils.h
@@ -53,6 +53,16 @@ char *tal_hex(const tal_t *ctx, const tal_t *data);
 /* Allocate and fill a buffer with the data of this hex string. */
 u8 *tal_hexdata(const tal_t *ctx, const void *str, size_t len);
 
+/**
+ * mlock_tal_memory - lock a tal-allocated memory with sodium_mlock.
+ * @ptr - the tal-allocated memory to lock
+ *
+ * This is a generic function to replace the pattern of sodium_mlock + tal_add_destructor.
+ *
+ * Aborts on failure (memory locking is mandatory for security).
+ */
+void mlock_tal_memory(const tal_t *ptr);
+
 /* Note: p is never a complex expression, otherwise this multi-evaluates! */
 #define tal_arr_expand(p, s)						\
 	do {								\
diff --git a/hsmd/hsmd.c b/hsmd/hsmd.c
index 11bcfe27..78750908 100644
--- a/hsmd/hsmd.c
+++ b/hsmd/hsmd.c
@@ -455,7 +455,7 @@ static void load_hsm(const char *passphrase)
 	hsm_secret->mnemonic = tal_steal(hsm_secret, hsms->mnemonic);
 
 	/*~ Don't swap this secret data to disk for security. */
-	sodium_mlock(hsm_secret->secret_data, tal_bytelen(hsm_secret->secret_data));
+	mlock_tal_memory(hsm_secret->secret_data);
 }
 
 /*~ We have a pre-init call in developer mode, to set dev flags */
diff --git a/hsmd/libhsmd.c b/hsmd/libhsmd.c
index bff142b2..b362c812 100644
--- a/hsmd/libhsmd.c
+++ b/hsmd/libhsmd.c
@@ -2477,7 +2477,7 @@ u8 *hsmd_init(const u8 *secret_data, size_t secret_len, const u64 hsmd_version,
 
 	/*~ Store the secret (32 or 64 bytes) - use NULL context for persistence */
 	secretstuff.bip32_seed = notleak(tal_dup_arr(NULL, u8, secret_data, secret_len, 0));
-	sodium_mlock(secretstuff.bip32_seed, tal_bytelen(secretstuff.bip32_seed));
+	mlock_tal_memory(secretstuff.bip32_seed);
 
 	assert(bip32_key_version.bip32_pubkey_version == BIP32_VER_MAIN_PUBLIC
 			|| bip32_key_version.bip32_pubkey_version == BIP32_VER_TEST_PUBLIC);
diff --git a/tools/hsmtool.c b/tools/hsmtool.c
index 2ec8934e..5d93b740 100644
--- a/tools/hsmtool.c
+++ b/tools/hsmtool.c
@@ -223,9 +223,6 @@ static void encrypt_hsm(const char *hsm_secret_path)
 	if (!encrypt_legacy_hsm_secret(encryption_key, &legacy_secret, encrypted_hsm_secret))
 		errx(ERROR_LIBSODIUM, "Could not encrypt the hsm_secret seed.");
 
-	/* Securely discard the encryption key */
-	destroy_secret(encryption_key);
-
 	/* Create a backup file, "just in case". */
 	rename(hsm_secret_path, backup);
 	fd = open(hsm_secret_path, O_CREAT|O_EXCL|O_WRONLY, 0400);