So far, the documentation of memory_cleanse() is a verbatim copy of
the commit message in BoringSSL, where this code was originally
written. However, our code evolved since then, and the commit message
is not particularly helpful in the code but is rather of historical
interested in BoringSSL only.
This commit improves improves the comments around memory_cleanse()
and gives a better rationale for the method that we use. This commit
touches only comments.
Commit fbf327b13868861c2877c5754caf5a9816f2603c ("Minimal code
changes to allow msvc compilation.") was indeed minimal in terms
of lines touched. But as a result of that minimalism it changed the
logic in memory_cleanse() to first call std::memset() and then
additionally the MSVC-specific SecureZeroMemory() function, and it
also moved a comment to the wrong location.
This commit removes the superfluous call to std::memset() on MSVC
and ensures that the comment is in the right position again.
The implementation we currently use from OpenSSL prevents the compiler from optimizing away clensing operations on blocks of memory that are about to be released, but this protection is not extended to link-time optimization. This commit copies the solution cooked up by Google compiler engineers which uses inline assembly directives to instruct the compiler not to optimize out the call under any circumstances. As the code is in-lined, this has the added advantage of removing one more OpenSSL dependency.
Regarding license compatibility, Google's contributions to BoringSSL library, including this code, is made available under the ISC license, which is MIT compatible.
BoringSSL git commit: ad1907fe73334d6c696c8539646c21b11178f20f
This makes it easier for us to replace it if desired, since it's now only in
one spot. Also, it avoids the openssl include from allocators.h, which
essentially forced openssl to be included from every compilation unit.
Jack Grigg
Tim Ruffing
Aaron Clauson
Adam Langley
Daira Hopwood
Cory Fields