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46 lines
2.3 KiB
Plaintext
46 lines
2.3 KiB
Plaintext
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[Regarding an optimization to the bounds-checking code in the core
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NEXTBYTE macro, which code is absolutely vital to the proper processing
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of corrupt zipfiles (lack of checking can result in an infinite loop)
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but which also slows processing.]
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The key to the solution is a pair of small functions called
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defer_leftover_input() and undefer_input(). The idea is, whenever
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you are going to be processing input using NEXTBYTE, you call
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defer_leftover_input(), and whenever you are going to process input by
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any other means, such as readbuf(), ZLSEEK, or directly reading stuff
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into G.inbuf, you call undefer_input(). What defer_leftover_input()
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does is adjust G.incnt so that any data beyond the current end of file
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is not visible. undefer_input() restores it to visibility. So when
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you're calling NEXTBYTE (or NEEDBITS or READBITS), an end-of-data
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condition only occurs at the same time as an end-of-buffer condition,
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and can be handled inside readbyte() instead of needing a check in the
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NEXTBYTE macro. Note: none of this applies to fUnZip.
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In order for this to work, certain conditions have to be met:
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1) NEXTBYTE input must not be mixed with other forms of input involving
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G.inptr and G.incnt. They must be separated by defer/undefer.
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I believe this condition is fully met by simply bracketing the central
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part of extract_or_test_member with defer/undefer, around the part
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where the actual decompression is done, and another defer/undefer pair
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in decrypt() around the reading of the RAND_HEADER_LEN password bytes.
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When USE_ZLIB is defined, I think that calls of fillinbuf() must be
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bracketed by defer/undefer.
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2) G.csize must not be assumed to contain the number of bytes left to
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process, when decompressing with NEXTBYTE. Instead, it contains
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the number of bytes left after the current buffer is exausted. To
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check the number of bytes remaining, use (G.csize + G.incnt).
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I believe the only places this change was needed were in explode.c,
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mostly in the check at the end of each explode function that tests
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whether the correct number of bytes has been read. G.incnt will
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normally be zero at that time anyway. The other place is the line
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that says "bd = G.csize > 200000L ? 8 : 7;" but that's just a rough
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heuristic anyway.
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[Paul Kienitz]
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