From 7f684a2aff636f44a5063c05cf84c0db0460ae1a Mon Sep 17 00:00:00 2001 From: Roberto Tyley Date: Sun, 7 Aug 2011 19:46:13 +0100 Subject: [PATCH] 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 Signed-off-by: Junio C Hamano --- sha1_file.c | 32 +++++++-- t/t1013-loose-object-format.sh | 68 ++++++++++++++++++ .../14/9cedb5c46929d18e0f118e9fa31927487af3b6 | Bin 0 -> 117 bytes .../16/56f9233d999f61ef23ef390b9c71d75399f435 | Bin 0 -> 17 bytes .../1e/72a6b2c4a577ab0338860fa9fe87f761fc9bbd | Bin 0 -> 18 bytes .../25/7cc5642cb1a054f08cc83f2d943e56fd3ebe99 | Bin 0 -> 19 bytes .../2e/65efe2a145dda7ee51d1741299f848e5bf752e | Bin 0 -> 10 bytes .../6b/aee0540ea990d9761a3eb9ab183003a71c3696 | Bin 0 -> 181 bytes .../70/e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd | Bin 0 -> 26 bytes .../76/e7fa9941f4d5f97f64fea65a2cba436bc79cbb | 2 + .../78/75c6237d3fcdd0ac2f0decc7d3fa6a50b66c09 | Bin 0 -> 139 bytes .../7a/37b887a73791d12d26c0d3e39568a8fb0fa6e8 | Bin 0 -> 54 bytes .../85/df50785d62d3b05ab03d9cbf7e4a0b49449730 | Bin 0 -> 13 bytes .../8d/4e360d6c70fbd72411991c02a09c442cf7a9fa | Bin 0 -> 156 bytes .../95/b1625de3ba8b2214d1e0d0591138aea733f64f | Bin 0 -> 252 bytes .../9a/e9e86b7bd6cb1472d9373702d8249973da0832 | Bin 0 -> 11 bytes .../bd/15045f6ce8ff75747562173640456a394412c8 | Bin 0 -> 34 bytes .../e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391 | Bin 0 -> 9 bytes .../f8/16d5255855ac160652ee5253b06cd8ee14165a | 1 + 19 files changed, 97 insertions(+), 6 deletions(-) create mode 100755 t/t1013-loose-object-format.sh create mode 100644 t/t1013/objects/14/9cedb5c46929d18e0f118e9fa31927487af3b6 create mode 100644 t/t1013/objects/16/56f9233d999f61ef23ef390b9c71d75399f435 create mode 100644 t/t1013/objects/1e/72a6b2c4a577ab0338860fa9fe87f761fc9bbd create mode 100644 t/t1013/objects/25/7cc5642cb1a054f08cc83f2d943e56fd3ebe99 create mode 100644 t/t1013/objects/2e/65efe2a145dda7ee51d1741299f848e5bf752e create mode 100644 t/t1013/objects/6b/aee0540ea990d9761a3eb9ab183003a71c3696 create mode 100644 t/t1013/objects/70/e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd create mode 100644 t/t1013/objects/76/e7fa9941f4d5f97f64fea65a2cba436bc79cbb create mode 100644 t/t1013/objects/78/75c6237d3fcdd0ac2f0decc7d3fa6a50b66c09 create mode 100644 t/t1013/objects/7a/37b887a73791d12d26c0d3e39568a8fb0fa6e8 create mode 100644 t/t1013/objects/85/df50785d62d3b05ab03d9cbf7e4a0b49449730 create mode 100644 t/t1013/objects/8d/4e360d6c70fbd72411991c02a09c442cf7a9fa create mode 100644 t/t1013/objects/95/b1625de3ba8b2214d1e0d0591138aea733f64f create mode 100644 t/t1013/objects/9a/e9e86b7bd6cb1472d9373702d8249973da0832 create mode 100644 t/t1013/objects/bd/15045f6ce8ff75747562173640456a394412c8 create mode 100644 t/t1013/objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391 create mode 100644 t/t1013/objects/f8/16d5255855ac160652ee5253b06cd8ee14165a 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$LF" +' + +test_expect_success 'read experimental-format loose objects' ' + git cat-file tag 76e7fa9941f4d5f97f64fea65a2cba436bc79cbb && + git cat-file commit 7875c6237d3fcdd0ac2f0decc7d3fa6a50b66c09 && + git ls-tree 95b1625de3ba8b2214d1e0d0591138aea733f64f && + assert_blob_equals "2e65efe2a145dda7ee51d1741299f848e5bf752e" "a" && + assert_blob_equals "9ae9e86b7bd6cb1472d9373702d8249973da0832" "ab" && + assert_blob_equals "85df50785d62d3b05ab03d9cbf7e4a0b49449730" "abcd" && + assert_blob_equals "1656f9233d999f61ef23ef390b9c71d75399f435" "abcdefgh" && + assert_blob_equals "1e72a6b2c4a577ab0338860fa9fe87f761fc9bbd" "abcdefghi" && + assert_blob_equals "70e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd" "abcdefghijklmnop" && + assert_blob_equals "bd15045f6ce8ff75747562173640456a394412c8" "abcdefghijklmnopqrstuvwx" +' + +test_expect_success 'read standard-format objects deflated with smaller window buffer' ' + git cat-file tag f816d5255855ac160652ee5253b06cd8ee14165a && + git cat-file tag 149cedb5c46929d18e0f118e9fa31927487af3b6 +' + +test_done diff --git a/t/t1013/objects/14/9cedb5c46929d18e0f118e9fa31927487af3b6 b/t/t1013/objects/14/9cedb5c46929d18e0f118e9fa31927487af3b6 new file mode 100644 index 0000000000000000000000000000000000000000..472fd1458e03e47136416bce60d6e7c893a468ab GIT binary patch literal 117 zcmV-*0E+)ei51K-4#F@DKvCwL!aJ&DH^jjbLR`g3tWwmHs(9h{lzp+xy&xqlfV?lkEVv$I`1V X&;Ic{P^6Jl5+pbyA%^e+FOeitJb^w> literal 0 HcmV?d00001 diff --git a/t/t1013/objects/16/56f9233d999f61ef23ef390b9c71d75399f435 b/t/t1013/objects/16/56f9233d999f61ef23ef390b9c71d75399f435 new file mode 100644 index 0000000000000000000000000000000000000000..c379d74ae2b40faae9c31cb7478bc7f42a6fb13c GIT binary 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