Const-correct SSL_get_srtp_profiles.

This is part of a very deep dependency chain. I'm sniffing at making all
the add_clienthello callbacks const. Between HelloVerifyRequest,
HelloRetryRequest, and soon ECH, we're creating lots of ClientHellos per
connection. That's probably easiest to manage if constructing a
ClientHello had no side effects.

Update-Note: The change to the return type isn't quite compatible, but I
only found one caller of this function, which has since been fixed. (If
we need to return a non-const value for compatibility, we can do that
and document that the caller should not mutate the output.)

Change-Id: I21f18f7438920a5b03d874fa548f054af3a42c4a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/47664
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/ssl.h

@@ -3054,8 +3054,8 @@ OPENSSL_EXPORT int SSL_CTX_set_srtp_profiles(SSL_CTX *ctx,
 OPENSSL_EXPORT int SSL_set_srtp_profiles(SSL *ssl, const char *profiles);
 
 // SSL_get_srtp_profiles returns the SRTP profiles supported by |ssl|.
-OPENSSL_EXPORT STACK_OF(SRTP_PROTECTION_PROFILE) *SSL_get_srtp_profiles(
-    SSL *ssl);
+OPENSSL_EXPORT const STACK_OF(SRTP_PROTECTION_PROFILE) *SSL_get_srtp_profiles(
+    const SSL *ssl);
 
 // SSL_get_selected_srtp_profile returns the selected SRTP profile, or NULL if
 // SRTP was not negotiated.

ssl/d1_srtp.cc

@@ -202,7 +202,7 @@ int SSL_set_srtp_profiles(SSL *ssl, const char *profiles) {
          ssl_ctx_make_profiles(profiles, &ssl->config->srtp_profiles);
 }
 
-STACK_OF(SRTP_PROTECTION_PROFILE) *SSL_get_srtp_profiles(SSL *ssl) {
+const STACK_OF(SRTP_PROTECTION_PROFILE) *SSL_get_srtp_profiles(const SSL *ssl) {
   if (ssl == nullptr) {
     return nullptr;
   }

ssl/t1_lib.cc

@@ -1720,7 +1720,8 @@ static void ext_srtp_init(SSL_HANDSHAKE *hs) {
 
 static bool ext_srtp_add_clienthello(SSL_HANDSHAKE *hs, CBB *out) {
   SSL *const ssl = hs->ssl;
-  STACK_OF(SRTP_PROTECTION_PROFILE) *profiles = SSL_get_srtp_profiles(ssl);
+  const STACK_OF(SRTP_PROTECTION_PROFILE) *profiles =
+      SSL_get_srtp_profiles(ssl);
   if (profiles == NULL ||
       sk_SRTP_PROTECTION_PROFILE_num(profiles) == 0) {
     return true;
@@ -1776,11 +1777,8 @@ static bool ext_srtp_parse_serverhello(SSL_HANDSHAKE *hs, uint8_t *out_alert,
     return false;
   }
 
-  STACK_OF(SRTP_PROTECTION_PROFILE) *profiles = SSL_get_srtp_profiles(ssl);
-
-  // Check to see if the server gave us something we support (and presumably
-  // offered).
-  for (const SRTP_PROTECTION_PROFILE *profile : profiles) {
+  // Check to see if the server gave us something we support and offered.
+  for (const SRTP_PROTECTION_PROFILE *profile : SSL_get_srtp_profiles(ssl)) {
     if (profile->id == profile_id) {
       ssl->s3->srtp_profile = profile;
       return true;