QCryptographicHash: explicitly activate the used union member

A union member lifetime does not start automatically if one
takes a pointer to a union member and writes into the member.
Only the subscript syntax in an assignment features such
automatic lifetime start [class.union§6].

In the other cases, one is allowed to get a pointer to the
storage ([basic.life§6]) but has to start lifetime explicitly
via placement new. Hence, do so, or we end up in UB land.

We're left with the problem of reset() which may be called
multiple times. I think we can actually just create a new
object in the same storage without destroying the pre-existing
object by exploiting [basic.life§5]: reusing the storage causes
lifetime end for the old object. Moreover, since the union
is over trivial datatypes (C structs), there's no side effects
in the destructor of the old object, so there is no need
to call it.

Drive by fix, apply some DRY for SHA3.

Change-Id: Idc351568635e59d45421311f043956ab3aabf389
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Linus Jahn <lnj@kaidan.im>

1 files changed, 9 insertions, 7 deletions

diff --git a/src/corelib/tools/qcryptographichash.cpp b/src/corelib/tools/qcryptographichash.cpp
index 1d4afabedd..8c02ea21f9 100644
--- a/src/corelib/tools/qcryptographichash.cpp
+++ b/src/corelib/tools/qcryptographichash.cpp
@@ -308,6 +308,7 @@ void QCryptographicHash::reset()
 {
     switch (d->method) {
     case Sha1:
+        new (&d->sha1Context) Sha1State;
         sha1InitState(&d->sha1Context);
         break;
 #ifdef QT_CRYPTOGRAPHICHASH_ONLY_SHA1
@@ -317,38 +318,39 @@ void QCryptographicHash::reset()
         break;
 #else
     case Md4:
+        new (&d->md4Context) md4_context;
         md4_init(&d->md4Context);
         break;
     case Md5:
+        new (&d->md5Context) MD5Context;
         MD5Init(&d->md5Context);
         break;
     case Sha224:
+        new (&d->sha224Context) SHA224Context;
         SHA224Reset(&d->sha224Context);
         break;
     case Sha256:
+        new (&d->sha256Context) SHA256Context;
         SHA256Reset(&d->sha256Context);
         break;
     case Sha384:
+        new (&d->sha384Context) SHA384Context;
         SHA384Reset(&d->sha384Context);
         break;
     case Sha512:
+        new (&d->sha512Context) SHA512Context;
         SHA512Reset(&d->sha512Context);
         break;
     case RealSha3_224:
     case Keccak_224:
-        sha3Init(&d->sha3Context, 224);
-        break;
     case RealSha3_256:
     case Keccak_256:
-        sha3Init(&d->sha3Context, 256);
-        break;
     case RealSha3_384:
     case Keccak_384:
-        sha3Init(&d->sha3Context, 384);
-        break;
     case RealSha3_512:
     case Keccak_512:
-        sha3Init(&d->sha3Context, 512);
+        new (&d->sha3Context) SHA3Context;
+        sha3Init(&d->sha3Context, hashLength(d->method) * 8);
         break;
 #endif
     }