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From: "René Scharfe" <l.s.r@web.de>
To: git@vger.kernel.org
Cc: "Junio C Hamano" <gitster@pobox.com>,
	"Johannes Schindelin" <johannes.schindelin@gmx.de>,
	"Rohit Ashiwal" <rohit.ashiwal265@gmail.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Jeff King" <peff@peff.net>,
	"brian m . carlson" <sandals@crustytoothpaste.net>
Subject: [PATCH v3 4/5] archive-tar: use OS_CODE 3 (Unix) for internal gzip
Date: Sun, 12 Jun 2022 08:18:20 +0200	[thread overview]
Message-ID: <0a6877c3-07f2-b0e2-f23c-9ea4c588e8a5@web.de> (raw)
In-Reply-To: <217a2f4d-4fc2-aaed-f5c2-1b7e134b046d@web.de>

gzip(1) encodes the OS it runs on in the 10th byte of its output. It
uses the following OS_CODE values according to its tailor.h [1]:

        0 - MS-DOS
        3 - UNIX
        5 - Atari ST
        6 - OS/2
       10 - TOPS-20
       11 - Windows NT

The gzip.exe that comes with Git for Windows uses OS_CODE 3 for some
reason, so this value is used on practically all supported platforms
when generating tgz archives using gzip(1).

Zlib uses a bigger set of values according to its zutil.h [2], aligned
with section 4.4.2 of the ZIP specification, APPNOTE.txt [3]:

         0 - MS-DOS
         1 - Amiga
         3 - UNIX
         4 - VM/CMS
         5 - Atari ST
         6 - OS/2
         7 - Macintosh
         8 - Z-System
        10 - Windows NT
        11 - MVS (OS/390 - Z/OS)
        13 - Acorn Risc
        16 - BeOS
        18 - OS/400
        19 - OS X (Darwin)

Thus the internal gzip implementation in archive-tar.c sets different
OS_CODE header values on major platforms Windows and macOS.  Git for
Windows uses its own zlib-based variant since v2.20.1 by default and
thus embeds OS_CODE 10 in tgz archives.

The tar archive for a commit is generated consistently on all systems
(by the same Git version).  The OS_CODE in the gzip header does not
influence extraction.  Avoid leaking OS information and make tgz
archives constistent and reproducable (with the same Git and libz
versions) by using OS_CODE 3 everywhere.

NB: The function deflateSetHeader() was introduced by zlib 1.2.2.1,
released 2004-10-31.

At least on macOS 12.4 this produces the same output as gzip(1) for the
examples I tried:

   # before
   $ git -c tar.tgz.command='git archive gzip' archive --format=tgz v2.36.0 | shasum
   3abbffb40b7c63cf9b7d91afc682f11682f80759  -

   # with this patch
   $ git -c tar.tgz.command='git archive gzip' archive --format=tgz v2.36.0 | shasum
   dc6dc6ba9636d522799085d0d77ab6a110bcc141  -

   $ git archive --format=tar v2.36.0 | gzip -cn | shasum
   dc6dc6ba9636d522799085d0d77ab6a110bcc141  -

[1] https://git.savannah.gnu.org/cgit/gzip.git/tree/tailor.h
[2] https://github.com/madler/zlib/blob/master/zutil.h
[3] https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT

Signed-off-by: René Scharfe <l.s.r@web.de>
---
Perhaps makes sense for remote-curl as well (out of scope of this
series)?

 archive-tar.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/archive-tar.c b/archive-tar.c
index 53d0ef685c..bf7e321e0e 100644
--- a/archive-tar.c
+++ b/archive-tar.c
@@ -460,6 +460,14 @@ static void tgz_write_block(const void *data)

 static const char internal_gzip_command[] = "git archive gzip";

+static void tgz_set_os(git_zstream *strm, int os)
+{
+#if ZLIB_VERNUM >= 0x1221
+	struct gz_header_s gzhead = { .os = os };
+	deflateSetHeader(&strm->z, &gzhead);
+#endif
+}
+
 static int write_tar_filter_archive(const struct archiver *ar,
 				    struct archiver_args *args)
 {
@@ -473,6 +481,7 @@ static int write_tar_filter_archive(const struct archiver *ar,
 	if (!strcmp(ar->filter_command, internal_gzip_command)) {
 		write_block = tgz_write_block;
 		git_deflate_init_gzip(&gzstream, args->compression_level);
+		tgz_set_os(&gzstream, 3); /* Unix, for reproducibility */
 		gzstream.next_out = outbuf;
 		gzstream.avail_out = sizeof(outbuf);

--
2.36.1

  parent reply	other threads:[~2022-06-12  6:18 UTC|newest]

Thread overview: 74+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-12 23:04 [PATCH 0/2] Avoid spawning gzip in git archive Johannes Schindelin via GitGitGadget
2019-04-12 23:04 ` [PATCH 1/2] archive: replace write_or_die() calls with write_block_or_die() Rohit Ashiwal via GitGitGadget
2019-04-13  1:34   ` Jeff King
2019-04-13  5:51     ` Junio C Hamano
2019-04-14  4:36       ` Rohit Ashiwal
2019-04-26 14:29       ` Johannes Schindelin
2019-04-26 23:44         ` Junio C Hamano
2019-04-29 21:32           ` Johannes Schindelin
2019-05-01 18:09             ` Jeff King
2019-05-02 20:29               ` René Scharfe
2019-05-05  5:25               ` Junio C Hamano
2019-05-06  5:07                 ` Jeff King
2019-04-14  4:34     ` Rohit Ashiwal
2019-04-14 10:33       ` Junio C Hamano
2019-04-26 14:28     ` Johannes Schindelin
2019-05-01 18:07       ` Jeff King
2019-04-12 23:04 ` [PATCH 2/2] archive: avoid spawning `gzip` Rohit Ashiwal via GitGitGadget
2019-04-13  1:51   ` Jeff King
2019-04-13 22:01     ` René Scharfe
2019-04-15 21:35       ` Jeff King
2019-04-26 14:51         ` Johannes Schindelin
2019-04-27  9:59           ` René Scharfe
2019-04-27 17:39             ` René Scharfe
2019-04-29 21:25               ` Johannes Schindelin
2019-05-01 17:45                 ` René Scharfe
2019-05-01 18:18                   ` Jeff King
2019-06-10 10:44                     ` René Scharfe
2019-06-13 19:16                       ` Jeff King
2019-04-13 22:16     ` brian m. carlson
2019-04-15 21:36       ` Jeff King
2019-04-26 14:54       ` Johannes Schindelin
2019-05-02 20:20         ` Ævar Arnfjörð Bjarmason
2019-05-03 20:49           ` Johannes Schindelin
2019-05-03 20:52             ` Jeff King
2019-04-26 14:47     ` Johannes Schindelin
     [not found] ` <pull.145.v2.git.gitgitgadget@gmail.com>
     [not found]   ` <4ea94a8784876c3a19e387537edd81a957fc692c.1556321244.git.gitgitgadget@gmail.com>
2019-05-02 20:29     ` [PATCH v2 3/4] archive: optionally use zlib directly for gzip compression René Scharfe
     [not found]   ` <ac2b2488a1b42b3caf8a84594c48eca796748e59.1556321244.git.gitgitgadget@gmail.com>
2019-05-02 20:30     ` [PATCH v2 2/4] archive-tar: mark RECORDSIZE/BLOCKSIZE as unsigned René Scharfe
2019-05-08 11:45       ` Johannes Schindelin
2019-05-08 23:04         ` Jeff King
2019-05-09 14:06           ` Johannes Schindelin
2019-05-09 18:38             ` Jeff King
2019-05-10 17:18               ` René Scharfe
2019-05-10 21:20                 ` Jeff King
2022-06-12  6:00 ` [PATCH v3 0/5] Avoid spawning gzip in git archive René Scharfe
2022-06-12  6:03   ` [PATCH v3 1/5] archive: rename archiver data field to filter_command René Scharfe
2022-06-12  6:05   ` [PATCH v3 2/5] archive-tar: factor out write_block() René Scharfe
2022-06-12  6:08   ` [PATCH v3 3/5] archive-tar: add internal gzip implementation René Scharfe
2022-06-13 19:10     ` Junio C Hamano
2022-06-12  6:18   ` René Scharfe [this message]
2022-06-12  6:19   ` [PATCH v3 5/5] archive-tar: use internal gzip by default René Scharfe
2022-06-13 21:55     ` Junio C Hamano
2022-06-14 11:27       ` Johannes Schindelin
2022-06-14 15:47         ` René Scharfe
2022-06-14 15:56           ` René Scharfe
2022-06-14 16:29           ` Johannes Schindelin
2022-06-14 20:04             ` René Scharfe
2022-06-15 16:41               ` Junio C Hamano
2022-06-14 11:28   ` [PATCH v3 0/5] Avoid spawning gzip in git archive Johannes Schindelin
2022-06-14 20:05     ` René Scharfe
2022-06-30 18:55       ` Johannes Schindelin
2022-07-01 16:05         ` Johannes Schindelin
2022-07-01 16:27           ` Jeff King
2022-07-01 17:47             ` Junio C Hamano
2022-06-15 16:53 ` [PATCH v4 0/6] " René Scharfe
2022-06-15 16:58   ` [PATCH v4 1/6] archive: update format documentation René Scharfe
2022-06-15 16:59   ` [PATCH v4 2/6] archive: rename archiver data field to filter_command René Scharfe
2022-06-15 17:01   ` [PATCH v4 3/6] archive-tar: factor out write_block() René Scharfe
2022-06-15 17:02   ` [PATCH v4 4/6] archive-tar: add internal gzip implementation René Scharfe
2022-06-15 20:32     ` Ævar Arnfjörð Bjarmason
2022-06-16 18:55       ` René Scharfe
2022-06-24 11:13         ` Ævar Arnfjörð Bjarmason
2022-06-24 20:24           ` René Scharfe
2022-06-15 17:04   ` [PATCH v4 5/6] archive-tar: use OS_CODE 3 (Unix) for internal gzip René Scharfe
2022-06-15 17:05   ` [PATCH v4 6/6] archive-tar: use internal gzip by default René Scharfe

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