When introducing EC_SCALAR and EC_FELEM, I used unions as convenience
for converting to and from the byte representation. However,
type-punning with unions is not allowed in C++ and hard to use correctly
in C. As I understand the rules, they are:
- The abstract machine knows what member of union was last written to.
- In C, reading from an inactive member is defined to type-pun. In C++,
it is UB though some compilers promise the C behavior anyway.
- However, if you read or write from a *pointer* to a union member, the
strict aliasing rule applies. (A function passed two pointers of
different types otherwise needs to pessimally assume they came from
the same union.)
That last rule means the type-punning allowance doesn't apply if you
take a pointer to an inactive member, and it's common to abstract
otherwise direct accesses of members via pointers.
https://github.com/openssl/openssl/issues/18225 is an example where
similar union tricks have caused problems for OpenSSL. While we don't
have that code, EC_SCALAR and EC_FELEM play similar tricks.
We do get a second lifeline because our alternate view is a uint8_t,
which we require to be unsigned char. Strict aliasing always allows the
pointer type to be a character type, so pointer-indirected accesses of
EC_SCALAR.bytes aren't necessarily UB. But if we ever write to
EC_SCALAR.bytes directly (and we do), we'll switch the active arm and
then pointers to EC_SCALAR.words become strict aliasing violations!
This is all far too complicated to deal with. Ideally everyone would
build with -fno-strict-aliasing because no real C code actually follows
these rules. But we don't always control our downstream consumers'
CFLAGS, so let's just avoid the union. This also avoids a pitfall if we
ever move libcrypto to C++.
For p224-64.c, I just converted the representations directly, which
avoids worrying about the top 32 bits in p224_felem_to_generic. Most of
the rest was words vs. bytes conversions and boils down to a cast (we're
still dealing with a character type, at the end of the day). But I took
the opportunity to extract some more "words"-based helper functions out
of BIGNUM, so the casts would only be in one place. That too saves us
from the top bits problem in the bytes-to-words direction.
Bug: 301
Change-Id: I3285a86441daaf824a4f6862e825d463a669efdb
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/52505
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
David Benjamin
227ff6e642