Added CAB decompression parameter MSCABD_PARAM_SALVAGE

[micahsnyder]

Added CAB decompression parameter MSCABD_PARAM_SALVAGE which makes a best effort to extract as many files as possible from damaged, mangled, malformed or otherwise non-standard CAB archives.

[kyz]

Thanks very much for this!

For context, you and I have been discussing merging the differences between ClamAV's copy of libmspack into libmspack proper, and most of the changes are to be more forgiving about non-compliant / broken CAB files, provided it gets to the data inside them.

So, the addition of a SALVAGE mode means you can turn this feature on and get at data no matter how broken a file is. I'll also be adding this to cabextract's existing "-f" flag, which is already used to salvage broken MSZIP files.

[kyz]

I'm still not keen on this redefinition of CAB_BLOCKMAX and CAB_LENGTHMAX.

You found a non-compliant / block CAB file which declares it goes beyond these limits. Having looked at it, it does have individually-compressed MSZIP blocks that expand to >32KB, so it looks like a human created the file without fully reading the specification. However, they wouldn't get away with this trying to make LZX or Quantum compressed blocks, as unlike MSZIP these keep track of the overall offset beyond each block, and would likely break after 2GB.

I think, for the purposes of salvage mode, it should just avoid checking the size of the each block, rather than define and perform a redundant check - block size is an unsigned 16-bit integer, so will be within 0-65535 anyway.

The CAB_LENGTHMAX should stay in place, to protect LZX/Quantum decompression, and in your file's case, it will result in the same outcome: a corrupt file size says the file is >2GB in size, there aren't nearly enough blocks to provide 2GB of data to decompress, so decompressing the file will fail as soon as the blocks run out. Allowing 4GB doesn't decompress any more than allowing 2GB.

[micahsnyder]

Thanks for the collaboration, and for the in-depth analysis of the non-standard/broken CAB files. We definitely appreciate it.