diff options
| author | Roberto Tyley <roberto.tyley@guardian.co.uk> | 2011-08-07 19:46:13 +0100 |
|---|---|---|
| committer | Junio C Hamano <gitster@pobox.com> | 2011-08-11 13:02:47 -0700 |
| commit | 7f684a2aff636f44a5063c05cf84c0db0460ae1a (patch) | |
| tree | 3d7b763261b6aeb1e01cbb2b4a8bdc144c9f029a | |
| parent | e9e0643fe64b613b6596b79b59df8ff10746f1a1 (diff) | |
| download | git-7f684a2aff636f44a5063c05cf84c0db0460ae1a.tar.gz git-7f684a2aff636f44a5063c05cf84c0db0460ae1a.tar.xz | |
Tolerate zlib deflation with window size < 32Kb
Git currently reports loose objects as 'corrupt' if they've been
deflated using a window size less than 32Kb, because the
experimental_loose_object() function doesn't recognise the header
byte as a zlib header. This patch makes the function tolerant of
all valid window sizes (15-bit to 8-bit) - but doesn't sacrifice
it's accuracy in distingushing the standard loose-object format
from the experimental (now abandoned) format.
On memory constrained systems zlib may use a much smaller window
size - working on Agit, I found that Android uses a 4KB window;
giving a header byte of 0x48, not 0x78. Consequently all loose
objects generated appear 'corrupt', which is why Agit is a read-only
Git client at this time - I don't want my client to generate Git
repos that other clients treat as broken :(
This patch makes Git tolerant of different deflate settings - it
might appear that it changes experimental_loose_object() to the point
where it could incorrectly identify the experimental format as the
standard one, but the two criteria (bitmask & checksum) can only
give a false result for an experimental object where both of the
following are true:
1) object size is exactly 8 bytes when uncompressed (bitmask)
2) [single-byte in-pack git type&size header] * 256
+ [1st byte of the following zlib header] % 31 = 0 (checksum)
As it happens, for all possible combinations of valid object type
(1-4) and window bits (0-7), the only time when the checksum will be
divisible by 31 is for 0x1838 - ie object type *1*, a Commit - which,
due the fields all Commit objects must contain, could never be as
small as 8 bytes in size.
Given this, the combination of the two criteria (bitmask & checksum)
always correctly determines the buffer format, and is more tolerant
than the previous version.
The alternative to this patch is simply removing support for the
experimental format, which I am also totally cool with.
References:
Android uses a 4KB window for deflation:
http://android.git.kernel.org/?p=platform/libcore.git;a=blob;f=luni/src/main/native/java_util_zip_Deflater.cpp;h=c0b2feff196e63a7b85d97cf9ae5bb2583409c28;hb=refs/heads/gingerbread#l53
Code snippet searching for false positives with the zlib checksum:
https://gist.github.com/1118177
Signed-off-by: Roberto Tyley <roberto.tyley@guardian.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
19 files changed, 97 insertions, 6 deletions
diff --git a/sha1_file.c b/sha1_file.c index 697f4a43c..475d215c1 100644 --- a/sha1_file.c +++ b/sha1_file.c @@ -1217,14 +1217,34 @@ static int experimental_loose_object(unsigned char *map) unsigned int word; /* - * Is it a zlib-compressed buffer? If so, the first byte - * must be 0x78 (15-bit window size, deflated), and the - * first 16-bit word is evenly divisible by 31. If so, - * we are looking at the official format, not the experimental - * one. + * We must determine if the buffer contains the standard + * zlib-deflated stream or the experimental format based + * on the in-pack object format. Compare the header byte + * for each format: + * + * RFC1950 zlib w/ deflate : 0www1000 : 0 <= www <= 7 + * Experimental pack-based : Stttssss : ttt = 1,2,3,4 + * + * If bit 7 is clear and bits 0-3 equal 8, the buffer MUST be + * in standard loose-object format, UNLESS it is a Git-pack + * format object *exactly* 8 bytes in size when inflated. + * + * However, RFC1950 also specifies that the 1st 16-bit word + * must be divisible by 31 - this checksum tells us our buffer + * is in the standard format, giving a false positive only if + * the 1st word of the Git-pack format object happens to be + * divisible by 31, ie: + * ((byte0 * 256) + byte1) % 31 = 0 + * => 0ttt10000www1000 % 31 = 0 + * + * As it happens, this case can only arise for www=3 & ttt=1 + * - ie, a Commit object, which would have to be 8 bytes in + * size. As no Commit can be that small, we find that the + * combination of these two criteria (bitmask & checksum) + * can always correctly determine the buffer format. */ word = (map[0] << 8) + map[1]; - if (map[0] == 0x78 && !(word % 31)) + if ((map[0] & 0x8F) == 0x08 && !(word % 31)) return 0; else return 1; diff --git a/t/t1013-loose-object-format.sh b/t/t1013-loose-object-format.sh new file mode 100755 index 000000000..f45702c45 --- /dev/null +++ b/t/t1013-loose-object-format.sh @@ -0,0 +1,68 @@ +#!/bin/sh +# +# Copyright (c) 2011 Roberto Tyley +# + +test_description='Correctly identify and parse loose object headers + +There are two file formats for loose objects - the original standard +format, and the experimental format introduced with Git v1.4.3, later +deprecated with v1.5.3. Although Git no longer writes the +experimental format, objects in both formats must be read, with the +format for a given file being determined by the header. + +Detecting file format based on header is not entirely trivial, not +least because the first byte of a zlib-deflated stream will vary +depending on how much memory was allocated for the deflation window +buffer when the object was written out (for example 4KB on Android, +rather that 32KB on a normal PC). + +The loose objects used as test vectors have been generated with the +following Git versions: + +standard format: Git v1.7.4.1 +experimental format: Git v1.4.3 (legacyheaders=false) +standard format, deflated with 4KB window size: Agit/JGit on Android +' + +. ./test-lib.sh +LF=' +' + +assert_blob_equals() { + printf "%s" "$2" >expected && + git cat-file -p "$1" >actual && + test_cmp expected actual +} + +test_expect_success setup ' + cp -R "$TEST_DIRECTORY/t1013/objects" .git/ + git --version +' + +test_expect_success 'read standard-format loose objects' ' + git cat-file tag 8d4e360d6c70fbd72411991c02a09c442cf7a9fa && + git cat-file commit 6baee0540ea990d9761a3eb9ab183003a71c3696 && + git ls-tree 7a37b887a73791d12d26c0d3e39568a8fb0fa6e8 && + assert_blob_equals "257cc5642cb1a054f08cc83f2d943e56fd3ebe99" "foo$LF" +' + +test_expect_success 'read experimental-format loose objects' ' + git cat-file tag 76e7fa9941f4d5f97f64fea65a2cba436bc79cbb && + git cat-file commit 7875c6237d3fcdd0ac2f0decc7d3fa6a50b66c09 && + git ls-tree 95b1625de3ba8b2214d1e0d0591138aea733f64f && + assert_blob_equals "2e65efe2a145dda7ee51d1741299f848e5bf752e" "a" && + assert_blob_equals "9ae9e86b7bd6cb1472d9373702d8249973da0832" "ab" && + assert_blob_equals "85df50785d62d3b05ab03d9cbf7e4a0b49449730" "abcd" && + assert_blob_equals "1656f9233d999f61ef23ef390b9c71d75399f435" "abcdefgh" && + assert_blob_equals "1e72a6b2c4a577ab0338860fa9fe87f761fc9bbd" "abcdefghi" && + assert_blob_equals "70e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd" "abcdefghijklmnop" && + assert_blob_equals "bd15045f6ce8ff75747562173640456a394412c8" "abcdefghijklmnopqrstuvwx" +' + +test_expect_success 'read standard-format objects deflated with smaller window buffer' ' + git cat-file tag f816d5255855ac160652ee5253b06cd8ee14165a && + git cat-file tag 149cedb5c46929d18e0f118e9fa31927487af3b6 +' + +test_done diff --git a/t/t1013/objects/14/9cedb5c46929d18e0f118e9fa31927487af3b6 b/t/t1013/objects/14/9cedb5c46929d18e0f118e9fa31927487af3b6 Binary files differnew file mode 100644 index 000000000..472fd1458 --- /dev/null +++ b/t/t1013/objects/14/9cedb5c46929d18e0f118e9fa31927487af3b6 diff --git a/t/t1013/objects/16/56f9233d999f61ef23ef390b9c71d75399f435 b/t/t1013/objects/16/56f9233d999f61ef23ef390b9c71d75399f435 Binary files differnew file mode 100644 index 000000000..c379d74ae --- /dev/null +++ b/t/t1013/objects/16/56f9233d999f61ef23ef390b9c71d75399f435 diff --git a/t/t1013/objects/1e/72a6b2c4a577ab0338860fa9fe87f761fc9bbd b/t/t1013/objects/1e/72a6b2c4a577ab0338860fa9fe87f761fc9bbd Binary files differnew file mode 100644 index 000000000..93706305b --- /dev/null +++ b/t/t1013/objects/1e/72a6b2c4a577ab0338860fa9fe87f761fc9bbd diff --git a/t/t1013/objects/25/7cc5642cb1a054f08cc83f2d943e56fd3ebe99 b/t/t1013/objects/25/7cc5642cb1a054f08cc83f2d943e56fd3ebe99 Binary files differnew file mode 100644 index 000000000..bdcf704c9 --- /dev/null +++ b/t/t1013/objects/25/7cc5642cb1a054f08cc83f2d943e56fd3ebe99 diff --git a/t/t1013/objects/2e/65efe2a145dda7ee51d1741299f848e5bf752e b/t/t1013/objects/2e/65efe2a145dda7ee51d1741299f848e5bf752e Binary files differnew file mode 100644 index 000000000..ad62c43e4 --- /dev/null +++ b/t/t1013/objects/2e/65efe2a145dda7ee51d1741299f848e5bf752e diff --git a/t/t1013/objects/6b/aee0540ea990d9761a3eb9ab183003a71c3696 b/t/t1013/objects/6b/aee0540ea990d9761a3eb9ab183003a71c3696 Binary files differnew file mode 100644 index 000000000..3d2f0337d --- /dev/null +++ b/t/t1013/objects/6b/aee0540ea990d9761a3eb9ab183003a71c3696 diff --git a/t/t1013/objects/70/e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd b/t/t1013/objects/70/e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd Binary files differnew file mode 100644 index 000000000..b3f71a6ee --- /dev/null +++ b/t/t1013/objects/70/e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd diff --git a/t/t1013/objects/76/e7fa9941f4d5f97f64fea65a2cba436bc79cbb b/t/t1013/objects/76/e7fa9941f4d5f97f64fea65a2cba436bc79cbb new file mode 100644 index 000000000..af4e9a7b0 --- /dev/null +++ b/t/t1013/objects/76/e7fa9941f4d5f97f64fea65a2cba436bc79cbb @@ -0,0 +1,2 @@ + x%A0@}O1{cSZ(νthZޠ?
m6di9GhبZR'QR pqL9=gsIoopeϫ_1$*SiNwpPRB +[(d-L9
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Ie &* G^D҆wUҗ S4 ,f[VAۺ x6[wtGLu? @"g{+by%M
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