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hash.h: scaffolding for _unsafe hashing variants

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Git's default SHA-1 implementation is collision-detecting, which hardens
us against known SHA-1 attacks against Git objects. This makes Git
object writes safer at the expense of some speed when hashing through
the collision-detecting implementation, which is slower than
non-collision detecting alternatives.

Prepare for loading a separate "unsafe" SHA-1 implementation that can be
used for non-cryptographic purposes, like computing the checksum of
files that use the hashwrite() API.

This commit does not actually introduce any new compile-time knobs to
control which implementation is used as the unsafe SHA-1 variant, but
does add scaffolding so that the "git_hash_algo" structure has five new
function pointers which are "unsafe" variants of the five existing
hashing-related function pointers:

  - git_hash_init_fn unsafe_init_fn
  - git_hash_clone_fn unsafe_clone_fn
  - git_hash_update_fn unsafe_update_fn
  - git_hash_final_fn unsafe_final_fn
  - git_hash_final_oid_fn unsafe_final_oid_fn

The following commit will introduce compile-time knobs to specify which
SHA-1 implementation is used for non-cryptographic uses.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Taylor Blau 2024-09-26 11:22:47 -04:00 committed by Junio C Hamano
parent 4c61a1d040
commit 253ed9ecff
2 changed files with 84 additions and 0 deletions
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42
hash.h
View file

@ -44,14 +44,32 @@
#define platform_SHA1_Final SHA1_Final
#endif
#ifndef platform_SHA_CTX_unsafe
# define platform_SHA_CTX_unsafe platform_SHA_CTX
# define platform_SHA1_Init_unsafe platform_SHA1_Init
# define platform_SHA1_Update_unsafe platform_SHA1_Update
# define platform_SHA1_Final_unsafe platform_SHA1_Final
# ifdef platform_SHA1_Clone
# define platform_SHA1_Clone_unsafe platform_SHA1_Clone
# endif
#endif
#define git_SHA_CTX platform_SHA_CTX
#define git_SHA1_Init platform_SHA1_Init
#define git_SHA1_Update platform_SHA1_Update
#define git_SHA1_Final platform_SHA1_Final
#define git_SHA_CTX_unsafe platform_SHA_CTX_unsafe
#define git_SHA1_Init_unsafe platform_SHA1_Init_unsafe
#define git_SHA1_Update_unsafe platform_SHA1_Update_unsafe
#define git_SHA1_Final_unsafe platform_SHA1_Final_unsafe
#ifdef platform_SHA1_Clone
#define git_SHA1_Clone platform_SHA1_Clone
#endif
#ifdef platform_SHA1_Clone_unsafe
# define git_SHA1_Clone_unsafe platform_SHA1_Clone_unsafe
#endif
#ifndef platform_SHA256_CTX
#define platform_SHA256_CTX SHA256_CTX
@ -81,6 +99,13 @@ static inline void git_SHA1_Clone(git_SHA_CTX *dst, const git_SHA_CTX *src)
memcpy(dst, src, sizeof(*dst));
}
#endif
#ifndef SHA1_NEEDS_CLONE_HELPER_UNSAFE
static inline void git_SHA1_Clone_unsafe(git_SHA_CTX_unsafe *dst,
const git_SHA_CTX_unsafe *src)
{
memcpy(dst, src, sizeof(*dst));
}
#endif
#ifndef SHA256_NEEDS_CLONE_HELPER
static inline void git_SHA256_Clone(git_SHA256_CTX *dst, const git_SHA256_CTX *src)
@ -178,6 +203,8 @@ enum get_oid_result {
/* A suitably aligned type for stack allocations of hash contexts. */
union git_hash_ctx {
git_SHA_CTX sha1;
git_SHA_CTX_unsafe sha1_unsafe;
git_SHA256_CTX sha256;
};
typedef union git_hash_ctx git_hash_ctx;
@ -222,6 +249,21 @@ struct git_hash_algo {
/* The hash finalization function for object IDs. */
git_hash_final_oid_fn final_oid_fn;
/* The non-cryptographic hash initialization function. */
git_hash_init_fn unsafe_init_fn;
/* The non-cryptographic hash context cloning function. */
git_hash_clone_fn unsafe_clone_fn;
/* The non-cryptographic hash update function. */
git_hash_update_fn unsafe_update_fn;
/* The non-cryptographic hash finalization function. */
git_hash_final_fn unsafe_final_fn;
/* The non-cryptographic hash finalization function. */
git_hash_final_oid_fn unsafe_final_oid_fn;
/* The OID of the empty tree. */
const struct object_id *empty_tree;

42
object-file.c
View file

@ -115,6 +115,33 @@ static void git_hash_sha1_final_oid(struct object_id *oid, git_hash_ctx *ctx)
oid->algo = GIT_HASH_SHA1;
}
static void git_hash_sha1_init_unsafe(git_hash_ctx *ctx)
{
git_SHA1_Init_unsafe(&ctx->sha1_unsafe);
}
static void git_hash_sha1_clone_unsafe(git_hash_ctx *dst, const git_hash_ctx *src)
{
git_SHA1_Clone_unsafe(&dst->sha1_unsafe, &src->sha1_unsafe);
}
static void git_hash_sha1_update_unsafe(git_hash_ctx *ctx, const void *data,
size_t len)
{
git_SHA1_Update_unsafe(&ctx->sha1_unsafe, data, len);
}
static void git_hash_sha1_final_unsafe(unsigned char *hash, git_hash_ctx *ctx)
{
git_SHA1_Final_unsafe(hash, &ctx->sha1_unsafe);
}
static void git_hash_sha1_final_oid_unsafe(struct object_id *oid, git_hash_ctx *ctx)
{
git_SHA1_Final_unsafe(oid->hash, &ctx->sha1_unsafe);
memset(oid->hash + GIT_SHA1_RAWSZ, 0, GIT_MAX_RAWSZ - GIT_SHA1_RAWSZ);
oid->algo = GIT_HASH_SHA1;
}
static void git_hash_sha256_init(git_hash_ctx *ctx)
{
@ -189,6 +216,11 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
.update_fn = git_hash_unknown_update,
.final_fn = git_hash_unknown_final,
.final_oid_fn = git_hash_unknown_final_oid,
.unsafe_init_fn = git_hash_unknown_init,
.unsafe_clone_fn = git_hash_unknown_clone,
.unsafe_update_fn = git_hash_unknown_update,
.unsafe_final_fn = git_hash_unknown_final,
.unsafe_final_oid_fn = git_hash_unknown_final_oid,
.empty_tree = NULL,
.empty_blob = NULL,
.null_oid = NULL,
@ -204,6 +236,11 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
.update_fn = git_hash_sha1_update,
.final_fn = git_hash_sha1_final,
.final_oid_fn = git_hash_sha1_final_oid,
.unsafe_init_fn = git_hash_sha1_init_unsafe,
.unsafe_clone_fn = git_hash_sha1_clone_unsafe,
.unsafe_update_fn = git_hash_sha1_update_unsafe,
.unsafe_final_fn = git_hash_sha1_final_unsafe,
.unsafe_final_oid_fn = git_hash_sha1_final_oid_unsafe,
.empty_tree = &empty_tree_oid,
.empty_blob = &empty_blob_oid,
.null_oid = &null_oid_sha1,
@ -219,6 +256,11 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
.update_fn = git_hash_sha256_update,
.final_fn = git_hash_sha256_final,
.final_oid_fn = git_hash_sha256_final_oid,
.unsafe_init_fn = git_hash_sha256_init,
.unsafe_clone_fn = git_hash_sha256_clone,
.unsafe_update_fn = git_hash_sha256_update,
.unsafe_final_fn = git_hash_sha256_final,
.unsafe_final_oid_fn = git_hash_sha256_final_oid,
.empty_tree = &empty_tree_oid_sha256,
.empty_blob = &empty_blob_oid_sha256,
.null_oid = &null_oid_sha256,