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Our git_hash_discard() is a bit hacky: it just calls git_hash_final()
into a dummy result buffer, using the side effect that each
implementation's Final() function will also free any resources.
This is probably not too terrible, since generating the final hash is
not that expensive and we'd mostly call discard on unusual or error code
paths. But we can do better by widening the platform API a bit to add an
explicit discard function.
This requires an annoying amount of boilerplate:
- Each algorithm needs a git_$ALGO_discard() wrapper that dereferences
the union'd git_hash_ctx into the type-safe field. So sha1 + sha256
+ sha1-unsafe, plus a BUG() for the unknown algo. And then these all
need to be referenced in the git_hash_algo structs.
- Platforms which don't do anything special to discard now need a
fallback function which does nothing. And we need this for each algo
(sha1, sha256, and sha1-unsafe).
- Platforms which do need to discard must define their discard
functions. This includes sha1/openssl, sha256/openssl, and
sha256/gcrypt (no sha1-unsafe here as it sits atop the sha1/openssl
functions).
- Algo selection needs to point platform_*_Discard to the appropriate
underlying macro, or indicate that the fallback should be used. We
have a similar situation for the Clone function (where a straight
memcpy() of the context struct is not enough for some platforms).
I've tied Discard to the same flag used by Clone here, since they
are basically the same problem: is the hash context a sequence of
bytes, or does it need smart copying/discarding?
It's easy to miss a case here since we don't even compile the
implementations we aren't using. I've tested with each of:
- no flags, which uses our internal sha1/sha256 implementations, both
of which exercise the noop fallback function
- OPENSSL_SHA1_UNSAFE=1, which checks that our unsafe macro
redirections work
- OPENSSL_SHA1=1, though you should not do that in real life!
- OPENSSL_SHA256=1, passes tests with GIT_TEST_DEFAULT_HASH=sha256
- GCRYPT_SHA256=1, which likewise passes
The other implementations do not set the CLONE_HELPER flag, so they
treat the context as bytes and should be fine with the fallback.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Our abstracted hash-algorithm API allows for cloning a hash context. By
default this just memcpy()s the bytes, but specific implementations can
provide a custom clone function.
Our API is based around the way that OpenSSL works, which is that you
first initialize the destination context, then copy into it. In our code
that is this:
algo->init_fn(&dst);
git_hash_clone(&dst, src);
and that translates into OpenSSL calls like:
/* init_fn */
dst->ectx = EVP_MD_CTX_new();
EVP_DigestInit_ex(dst->ectx, EVP_sha256());
/* clone */
EVP_MD_CTX_copy_ex(dst->ectx, src->ectx);
So the allocation happens in the first step, and then the clone is just
copying values (the DigestInit is initializing values that just get
overwritten, but that's not wrong, just a little inefficient).
But libgcrypt doesn't work like that! Its copy function initializes dst
from scratch. So when using the sha256 gcrypt backend, that becomes:
/* init_fn; this allocates */
gcry_md_open(&dst, GCRY_MD_SHA256);
/* clone; this also allocates, leaking the previous value! */
gcry_md_copy(&dst, src);
You can see the leaks in the test suite by running:
make \
SANITIZE=leak \
GCRYPT_SHA256=1 \
GIT_TEST_DEFAULT_SHA=256 \
test
which has many failures, as opposed to building with OPENSSL_SHA256,
which is leak-free.
The easy fix here is for the clone function to close the open context
we're about to overwrite. It's a little inefficient (we did a pointless
open in the init function), but probably not a big deal in practice.
If our API went the other way, assuming that we're always cloning into
garbage bytes, then we could be more efficient. We'd teach OpenSSL's
clone function to do its own new(), skip the DigestInit, and then copy
into it. And gcrypt could stick with just the copy() call.
But look again at the asymmetry in the very first code example. We call
the init function straight from the git_hash_algo struct, and then
subsequent calls are dispatched through our git_hash_* wrappers. If you
wanted to clone into an uninitialized destination, you'd do something
like:
algo->clone_fn(&dst, src);
instead. That would require changing all of the callers. There's not
that many of them, but I don't know that it's worth changing our calling
conventions to try to reclaim this tiny bit of efficiency.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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`gcry_md_open' allocates memory and must (like all allocation
functions) be checked for failure.
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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`gcry_md_open' needs to be paired with `gcry_md_close' to ensure
resources are released. Since our internal APIs don't have
separate close/release callbacks, sticking it into the finalization
callback seems appropriate.
Building with SANITIZE=leak and running `git fsck' on a SHA-256
repository no longer reports leaks.
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Non-static functions cause `undefined reference' errors when
building with `SANITIZE=leak' due to the lack of prototypes.
Mark all these functions as `static inline' as we do in
sha256/nettle.h to avoid the need to maintain prototypes.
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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For all of our SHA-1 implementations and most of our SHA-256
implementations, the hash context we use is a real struct. For these
implementations, it's possible to copy a hash context by making a copy
of the struct.
However, for our libgcrypt implementation, our hash context is a
pointer. Consequently, copying it does not lead to an independent hash
context like we intended.
Fortunately, however, libgcrypt provides us with a handy function to
copy hash contexts. Let's add a cloning function to the hash algorithm
API, and use it in the one place we need to make a hash context copy.
With this change, our libgcrypt SHA-256 implementation is fully
functional with all of our other hash implementations.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Generally, one gets better performance out of cryptographic routines
written in assembly than C, and this is also true for SHA-256. In
addition, most Linux distributions cannot distribute Git linked against
OpenSSL for licensing reasons.
Most systems with GnuPG will also have libgcrypt, since it is a
dependency of GnuPG. libgcrypt is also faster than the SHA1DC
implementation for messages of a few KiB and larger.
For comparison, on a Core i7-6600U, this implementation processes 16 KiB
chunks at 355 MiB/s while SHA1DC processes equivalent chunks at 337
MiB/s.
In addition, libgcrypt is licensed under the LGPL 2.1, which is
compatible with the GPL. Add an implementation of SHA-256 that uses
libgcrypt.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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