Subject: Re: [PATCH] hash: Allow building with the external sha1dc library

From: Junio C Hamano <[email-masked]>
Date: Tue, 25 Jul 2017 12:06:11 -0700

Takashi Iwai <[email-masked]> writes:

> Some distros provide SHA1 collision detect code as a shared library.
> It's the very same code as we have in git tree, and git can link with
> it as well; at least, it may make maintenance easier, according to our
> security guys.
>
> This patch allows user to build git linking with the external sha1dc
> library instead of the built-in sha1dc code.  User needs to define
> DC_SHA1_EXTERNAL explicitly.  As default, the built-in sha1dc code is
> used like before.
>
> Signed-off-by: Takashi Iwai <[email-masked]>
> ---

I do not have such an environment to test this patch, but it looks
like a very sensible thing to do.  Will queue; thanks.

>  Makefile         | 12 ++++++++++++
>  hash.h           |  4 +++-
>  sha1dc_git_ext.c | 11 +++++++++++
>  sha1dc_git_ext.h | 25 +++++++++++++++++++++++++
>  4 files changed, 51 insertions(+), 1 deletion(-)
>  create mode 100644 sha1dc_git_ext.c
>  create mode 100644 sha1dc_git_ext.h
>
> diff --git a/Makefile b/Makefile
> index 461c845d33cb..f1a262d56254 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -162,6 +162,12 @@ all::
>  # algorithm. This is slower, but may detect attempted collision attacks.
>  # Takes priority over other *_SHA1 knobs.
>  #
> +# Define DC_SHA1_EXTERNAL in addition to DC_SHA1 if you want to build / link
> +# git with the external sha1collisiondetection library.
> +# Without this option, i.e. the default behavior is to build git with its
> +# own sha1dc code.  If any extra linker option is required, define them in
> +# DC_SHA1_LINK variable in addition.
> +#
>  # Define DC_SHA1_SUBMODULE in addition to DC_SHA1 to use the
>  # sha1collisiondetection shipped as a submodule instead of the
>  # non-submodule copy in sha1dc/. This is an experimental option used
> @@ -1472,6 +1478,11 @@ ifdef APPLE_COMMON_CRYPTO
>  	BASIC_CFLAGS += -DSHA1_APPLE
>  else
>  	DC_SHA1 := YesPlease
> +ifdef DC_SHA1_EXTERNAL
> +	LIB_OBJS += sha1dc_git_ext.o
> +	BASIC_CFLAGS += -DSHA1_DC -DDC_SHA1_EXTERNAL
> +	EXTLIBS += $(DC_SHA1_LINK) -lsha1detectcoll
> +else
>  ifdef DC_SHA1_SUBMODULE
>  	LIB_OBJS += sha1collisiondetection/lib/sha1.o
>  	LIB_OBJS += sha1collisiondetection/lib/ubc_check.o
> @@ -1492,6 +1503,7 @@ endif
>  endif
>  endif
>  endif
> +endif
>  
>  ifdef SHA1_MAX_BLOCK_SIZE
>  	LIB_OBJS += compat/sha1-chunked.o
> diff --git a/hash.h b/hash.h
> index bef3e630a093..dce327d58d07 100644
> --- a/hash.h
> +++ b/hash.h
> @@ -8,7 +8,9 @@
>  #elif defined(SHA1_OPENSSL)
>  #include <openssl/sha.h>
>  #elif defined(SHA1_DC)
> -#ifdef DC_SHA1_SUBMODULE
> +#if defined(DC_SHA1_EXTERNAL)
> +#include "sha1dc_git_ext.h"
> +#elif defined(DC_SHA1_SUBMODULE)
>  #include "sha1collisiondetection/lib/sha1.h"
>  #else
>  #include "sha1dc/sha1.h"
> diff --git a/sha1dc_git_ext.c b/sha1dc_git_ext.c
> new file mode 100644
> index 000000000000..359439fc3d93
> --- /dev/null
> +++ b/sha1dc_git_ext.c
> @@ -0,0 +1,11 @@
> +/* Only for DC_SHA1_EXTERNAL; sharing the same hooks as built-in sha1dc */
> +
> +#include "cache.h"
> +#include <sha1.h>
> +#include "sha1dc_git.c"
> +
> +void git_SHA1DCInit(SHA1_CTX *ctx)
> +{
> +	SHA1DCInit(ctx);
> +	SHA1DCSetSafeHash(ctx, 0);
> +}
> diff --git a/sha1dc_git_ext.h b/sha1dc_git_ext.h
> new file mode 100644
> index 000000000000..d0ea8ce518db
> --- /dev/null
> +++ b/sha1dc_git_ext.h
> @@ -0,0 +1,25 @@
> +/*
> + * This file is included by hash.h for DC_SHA1_EXTERNAL
> + */
> +
> +#include <sha1.h>
> +
> +/*
> + * Same as SHA1DCInit, but with default save_hash=0
> + */
> +void git_SHA1DCInit(SHA1_CTX *);
> +
> +/*
> + * Same as SHA1DCFinal, but convert collision attack case into a verbose die().
> + */
> +void git_SHA1DCFinal(unsigned char [20], SHA1_CTX *);
> +
> +/*
> + * Same as SHA1DCUpdate, but adjust types to match git's usual interface.
> + */
> +void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *data, unsigned long len);
> +
> +#define platform_SHA_CTX SHA1_CTX
> +#define platform_SHA1_Init git_SHA1DCInit
> +#define platform_SHA1_Update git_SHA1DCUpdate
> +#define platform_SHA1_Final git_SHA1DCFinal