Remove in-place TLS record assembly for now.

Decrypting is very easy to do in-place, but encrypting in-place is a hassle.
The rules actually were wrong due to record-splitting. The aliasing prefix and
the alignment prefix actually differ by 1. Take it out for now in preparation
for tightening the aliasing rules.

If we decide to do in-place encrypt later, probably it'd be more useful to
return header + in-place ciphertext + trailer. (That, in turn, needs a
scatter/gather thing on the AEAD thanks to TLS 1.3's padding and record type
construction.) We may also wish to rethink how record-splitting works here.

Change-Id: I0187d39c541e76ef933b7c2c193323164fd8a156
Reviewed-on: https://boringssl-review.googlesource.com/8230
Reviewed-by: Adam Langley <agl@google.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/crypto/internal.h b/crypto/internal.h
index 433072ce..54e78d52 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -152,6 +152,19 @@ typedef __uint128_t uint128_t;
 #endif
 
 
+/* buffers_alias returns one if |a| and |b| alias and zero otherwise. */
+static inline int buffers_alias(const uint8_t *a, size_t a_len,
+                                const uint8_t *b, size_t b_len) {
+  /* Cast |a| and |b| to integers. In C, pointer comparisons between unrelated
+   * objects are undefined whereas pointer to integer conversions are merely
+   * implementation-defined. We assume the implementation defined it in a sane
+   * way. */
+  uintptr_t a_u = (uintptr_t)a;
+  uintptr_t b_u = (uintptr_t)b;
+  return a_u + a_len > b_u && b_u + b_len > a_u;
+}
+
+
 /* Constant-time utility functions.
  *
  * The following methods return a bitmask of all ones (0xff...f) for true and 0