Subject: [PATCH v3 1/2] help: include SHA implementation in version info

From: Justin Tobler <[email-masked]>
Date: Thu, 3 Apr 2025 09:05:28 -0500

When the `--build-options` flag is used with git-version(1), additional
information about the built version of Git is printed. During build
time, different SHA implementations may be configured, but this
information is not included in the version info.

Add the SHA implementations Git is built with to the version info by
requiring each backend to define a SHA1_BACKEND or SHA256_BACKEND symbol
as appropriate and use the value in the printed build options.

Signed-off-by: Justin Tobler <[email-masked]>
---
 Documentation/git-version.adoc | 6 ++++++
 hash.h                         | 8 ++++++++
 help.c                         | 3 +++
 3 files changed, 17 insertions(+)

diff --git a/Documentation/git-version.adoc b/Documentation/git-version.adoc
index 80fa7754a6..913ebf147d 100644
--- a/Documentation/git-version.adoc
+++ b/Documentation/git-version.adoc
@@ -22,6 +22,12 @@ OPTIONS
 --build-options::
 	Include additional information about how git was built for diagnostic
 	purposes.
++
+The libraries used to implement the SHA-1 and SHA-256 algorithms are displayed
+in the form `SHA-1: <option>` and `SHA-256: <option>`. Note that the SHA-1
+options `SHA1_APPLE`, `SHA1_OPENSSL`, and `SHA1_BLK` do not use a collision
+detection algorithm and thus may be vulnerable to known SHA-1 collision
+attacks.
 
 GIT
 ---
diff --git a/hash.h b/hash.h
index 4367acfec5..51cd0ec7b6 100644
--- a/hash.h
+++ b/hash.h
@@ -2,16 +2,20 @@
 #define HASH_H
 
 #if defined(SHA1_APPLE)
+#define SHA1_BACKEND "SHA1_APPLE (No collision detection)"
 #include <CommonCrypto/CommonDigest.h>
 #elif defined(SHA1_OPENSSL)
+#  define SHA1_BACKEND "SHA1_OPENSSL (No collision detection)"
 #  include <openssl/sha.h>
 #  if defined(OPENSSL_API_LEVEL) && OPENSSL_API_LEVEL >= 3
 #    define SHA1_NEEDS_CLONE_HELPER
 #    include "sha1/openssl.h"
 #  endif
 #elif defined(SHA1_DC)
+#define SHA1_BACKEND "SHA1_DC"
 #include "sha1dc_git.h"
 #else /* SHA1_BLK */
+#define SHA1_BACKEND "SHA1_BLK (No collision detection)"
 #include "block-sha1/sha1.h"
 #endif
 
@@ -46,17 +50,21 @@
 #endif
 
 #if defined(SHA256_NETTLE)
+#define SHA256_BACKEND "SHA256_NETTLE"
 #include "sha256/nettle.h"
 #elif defined(SHA256_GCRYPT)
+#define SHA256_BACKEND "SHA256_GCRYPT"
 #define SHA256_NEEDS_CLONE_HELPER
 #include "sha256/gcrypt.h"
 #elif defined(SHA256_OPENSSL)
+#  define SHA256_BACKEND "SHA256_OPENSSL"
 #  include <openssl/sha.h>
 #  if defined(OPENSSL_API_LEVEL) && OPENSSL_API_LEVEL >= 3
 #    define SHA256_NEEDS_CLONE_HELPER
 #    include "sha256/openssl.h"
 #  endif
 #else
+#define SHA256_BACKEND "SHA256_BLK"
 #include "sha256/block/sha256.h"
 #endif
 
diff --git a/help.c b/help.c
index c54bd9918a..991a9525db 100644
--- a/help.c
+++ b/help.c
@@ -9,6 +9,7 @@
 #include "run-command.h"
 #include "levenshtein.h"
 #include "gettext.h"
+#include "hash.h"
 #include "help.h"
 #include "command-list.h"
 #include "string-list.h"
@@ -803,6 +804,8 @@ void get_version_info(struct strbuf *buf, int show_build_options)
 #elif defined ZLIB_VERSION
 		strbuf_addf(buf, "zlib: %s\n", ZLIB_VERSION);
 #endif
+		strbuf_addf(buf, "SHA-1: %s\n", SHA1_BACKEND);
+		strbuf_addf(buf, "SHA-256: %s\n", SHA256_BACKEND);
 	}
 }
 
-- 
2.49.0