6 ms·

Of course, but I would still strongly advise against this. If you really cannot avoid implementing something like this, you should inspect the 7-Zip code in or

Of course, but I would still strongly advise against this. If you really cannot avoid implementing something like this, you should inspect the 7-Zip code in order to be 100% sure that the magic number detection in your filter is identical (or matches a superset) to the one from 7-Zip.

codetrotter 8y ago
> you should inspect the 7-Zip code in order to be 100% sure that the magic number detection in your filter is identical (or matches a superset) to the one from 7-Zip CPP/7zip/Archive/Rar/RarHandler.cpp: #define SIGNATURE { 0x52 , 0x61, 0x72, 0x21, 0x1a, 0x07, 0x00 } CPP/7zip/UI/Common/OpenArchive.cpp: const Byte kRarHeader[] = { 0x52 , 0x61, 0x72, 0x21, 0x1a, 0x07, 0x00 }; CPP/7zip/Archive/Rar/Rar5Handler.cpp: #define SIGNATURE { 0x52 , 0x61, 0x72, 0x21, 0x1a, 0x07, 0x01, 0 } Those are the two magic numbers for RAR archive version 1.50 onwards and RAR archive version 5.0 onwards respectively, and those are the places they are referenced in the 7-zip source code. I looked at the source archive of the 18.05 version, downloaded from https://www.7-zip.org/a/7z1805-src.7z https://www.7-zip.org/a/7z1805-src.7z. I guess if you wanted to be really rigorous you'd look at previous versions as well. If the 7-zip project makes use of a version control system and that vcs supports the equivalent of git blame then that should not be too difficult of a task for whoever wanted to go to that extent of investigation. And here is where one copy of the 'file' command identifies the same magic numbers as RAR archives: https://github.com/file/file/blob/f0a725a13fe0c1b046d8e0705748415a80286e92/magic/Magdir/archive#L985 https://github.com/file/file/blob/f0a725a13fe0c1b046d8e07057... 0 string Rar!\x1a\7\0 RAR archive data https://github.com/file/file/blob/f0a725a13fe0c1b046d8e0705748415a80286e92/magic/Magdir/archive#L997 https://github.com/file/file/blob/f0a725a13fe0c1b046d8e07057... 0 string Rar!\x1a\7\1\0 RAR archive data, v5 :)
- landave 8y ago
  You missed my point. It is trivial to find out what the magic number is. What is more important though: How exactly is the magic number matched? From what you have written, one might be tempted to simply check whether a file begins with this magic number. And this would be wrong. If you take a look at the matching in CPP/7zip/Archive/Rar/RarHandler.cpp: Byte marker[NHeader::kMarkerSize]; RINOK(ReadStream_FALSE(stream, marker, NHeader::kMarkerSize)); if (memcmp(marker, kMarker, NHeader::kMarkerSize) == 0) m_Position += NHeader::kMarkerSize; else { if (searchHeaderSizeLimit && *searchHeaderSizeLimit == 0) return S_FALSE; RINOK(stream->Seek(m_StreamStartPosition, STREAM_SEEK_SET, NULL)); RINOK(FindSignatureInStream(stream, kMarker, NHeader::kMarkerSize, searchHeaderSizeLimit, arcStartPos)); m_Position = arcStartPos + NHeader::kMarkerSize; RINOK(stream->Seek(m_Position, STREAM_SEEK_SET, NULL)); } 7-Zip finds the magic number if it appears within some searchHeaderSizeLimit, i.e., the file does not need to start (at offset 0) with the magic number. For example, 7-Zip will extract a RAR file which begins with [00 52 61 72 21 1A 07 00] (instead of [52 61 72 21 1A 07 00]) just fine.
  - codetrotter 8y ago
    Oh, I did not expect that. Yes you are right, one must be careful about these things.

codetrotter 8y ago

> you should inspect the 7-Zip code in order to be 100% sure that the magic number detection in your filter is identical (or matches a superset) to the one from 7-Zip CPP/7zip/Archive/Rar/RarHandler.cpp: #define SIGNATURE { 0x52 , 0x61, 0x72, 0x21, 0x1a, 0x07, 0x00 } CPP/7zip/UI/Common/OpenArchive.cpp: const Byte kRarHeader[] = { 0x52 , 0x61, 0x72, 0x21, 0x1a, 0x07, 0x00 }; CPP/7zip/Archive/Rar/Rar5Handler.cpp: #define SIGNATURE { 0x52 , 0x61, 0x72, 0x21, 0x1a, 0x07, 0x01, 0 } Those are the two magic numbers for RAR archive version 1.50 onwards and RAR archive version 5.0 onwards respectively, and those are the places they are referenced in the 7-zip source code. I looked at the source archive of the 18.05 version, downloaded from https://www.7-zip.org/a/7z1805-src.7z https://www.7-zip.org/a/7z1805-src.7z. I guess if you wanted to be really rigorous you'd look at previous versions as well. If the 7-zip project makes use of a version control system and that vcs supports the equivalent of git blame then that should not be too difficult of a task for whoever wanted to go to that extent of investigation. And here is where one copy of the 'file' command identifies the same magic numbers as RAR archives: https://github.com/file/file/blob/f0a725a13fe0c1b046d8e0705748415a80286e92/magic/Magdir/archive#L985 https://github.com/file/file/blob/f0a725a13fe0c1b046d8e07057... 0 string Rar!\x1a\7\0 RAR archive data https://github.com/file/file/blob/f0a725a13fe0c1b046d8e0705748415a80286e92/magic/Magdir/archive#L997 https://github.com/file/file/blob/f0a725a13fe0c1b046d8e07057... 0 string Rar!\x1a\7\1\0 RAR archive data, v5 :)

landave 8y ago
You missed my point. It is trivial to find out what the magic number is. What is more important though: How exactly is the magic number matched? From what you have written, one might be tempted to simply check whether a file begins with this magic number. And this would be wrong. If you take a look at the matching in CPP/7zip/Archive/Rar/RarHandler.cpp: Byte marker[NHeader::kMarkerSize]; RINOK(ReadStream_FALSE(stream, marker, NHeader::kMarkerSize)); if (memcmp(marker, kMarker, NHeader::kMarkerSize) == 0) m_Position += NHeader::kMarkerSize; else { if (searchHeaderSizeLimit && *searchHeaderSizeLimit == 0) return S_FALSE; RINOK(stream->Seek(m_StreamStartPosition, STREAM_SEEK_SET, NULL)); RINOK(FindSignatureInStream(stream, kMarker, NHeader::kMarkerSize, searchHeaderSizeLimit, arcStartPos)); m_Position = arcStartPos + NHeader::kMarkerSize; RINOK(stream->Seek(m_Position, STREAM_SEEK_SET, NULL)); } 7-Zip finds the magic number if it appears within some searchHeaderSizeLimit, i.e., the file does not need to start (at offset 0) with the magic number. For example, 7-Zip will extract a RAR file which begins with [00 52 61 72 21 1A 07 00] (instead of [52 61 72 21 1A 07 00]) just fine.
- codetrotter 8y ago
  Oh, I did not expect that. Yes you are right, one must be careful about these things.