|author||David Benjamin <email@example.com>||Thu May 05 08:43:27 2022 -0400|
|committer||Boringssl LUCI CQ <firstname.lastname@example.org>||Tue May 10 17:22:42 2022 +0000|
Remove unions in EC_SCALAR and EC_FELEM. 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 <email@example.com> Reviewed-by: Bob Beck <firstname.lastname@example.org>
BoringSSL is a fork of OpenSSL that is designed to meet Google's needs.
Although BoringSSL is an open source project, it is not intended for general use, as OpenSSL is. We don't recommend that third parties depend upon it. Doing so is likely to be frustrating because there are no guarantees of API or ABI stability.
Programs ship their own copies of BoringSSL when they use it and we update everything as needed when deciding to make API changes. This allows us to mostly avoid compromises in the name of compatibility. It works for us, but it may not work for you.
BoringSSL arose because Google used OpenSSL for many years in various ways and, over time, built up a large number of patches that were maintained while tracking upstream OpenSSL. As Google's product portfolio became more complex, more copies of OpenSSL sprung up and the effort involved in maintaining all these patches in multiple places was growing steadily.
Currently BoringSSL is the SSL library in Chrome/Chromium, Android (but it's not part of the NDK) and a number of other apps/programs.
There are other files in this directory which might be helpful: