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Look ma, without libraries!
Written while listening to āTwisted Logicā by Coldplay.
Published: 2021-08-19
Tagged with:
Part two: āExtracting files from zipsā
The code for the whole series; see āzipls.cā for this post in particular.
Edit 2021/08/20: some edits to improve the code and the commentary.
Edit 2021/08/21: stricter while condition for ālsā and added links to the code
Disclaimer: before today I didnāt knew anything about how zip files are structured, so take everything here with a huge grain of salt. The good news is that the code I wrote seems to be coherent with what Iāve read online and to actually work against some zips files I had around.
Background: Iād like to add support for gempubs to Telescope, the Gemini client Iām writing. gempubs are basically a directory of text/gemini files plus other assets (metadata.txt and images presumably) all zipped in a single archive.
gempub: a new eBook format based on text/gemini
There are a lot of libraries to handle zip files, but I decided to give it a shot a writing something from scratch. After all, I donāt need to edit zips or do fancy stuff, I only need to read files from the archive, thatās all.
To start, in this entry weāll only see how to dump the list of files in a zip archive. Maybe future entries will deal with more zip stuff.
From what Iāve gathered from APPNOTE.TXT and other sources, a zip file is a sequence of file ārecordsā (a header followed by the file content) and a trailing ācentral directoryā that holds the information about all the files.
Having the central directory at the end of the file instead that at the beginning seems to be a choice to waste people time^W^W^W allow embedding zips into other file formats, such as GIFs or EXE. I guess in some cases this may be an invaluable property, I just fail to see where, but anyway.
Edit 2021/08/20: Another advantage of having the central directory at the end is that is probably possible to build up a zip on-the-fly, maybe outputting to standard output or to a similar non-seekable device, without having to build all the zip in memory first.
One may think that itās possible to scan a zip by reading these ārecordsā, but itās not the case unfortunately: the only source of truth for the actual files stored in the archive is the central directory. Applications that modify the zip may reuse or leave dummy file headers around, especially if they delete or replace files.
To aggravate the situation, itās not obvious how to find the start of the central directory. Zip are truly wonderful, huh? I guess that adding a trailing 4-byte offset that points to the start of the central directory wouldnāt be bad, but weāre a bit too late.
The central directory is a sequence of record that identifies the files in the archive followed by a digital signature, two ZIP64 fields and the end of the central directory record. I still havenāt wrapped my head around the digital signature and the ZIP64 fields, but they donāt seem necessary to access the list of files.
The last part of the central directory, the end record, contains a handy pointer to the start of the content directory. Unfortunately, it also contains a trailing variable-width comment area that complicate things a bit.
But enough with the talks, letās jump to the code. Since Telescope is written in C, the small toy program object of this entry will also be written in C. The main function is pretty straightforward:
int
main(int argc, char **argv)
{
int fd;
void *zip, *cd;
size_t len;
if (argc != 2)
errx(1, "missing file to inspect");
if ((fd = open(argv[1], O_RDONLY)) == -1)
err(1, "open %s", argv[1]);
zip = map_file(fd, &len);
if ((cd = find_central_directory(zip, len)) == NULL)
errx(1, "can't find central directory");
ls(zip, len, cd);
munmap(zip, len);
close(fd);
return 0;
}
I think it would be easier for us to just mmap(2) the file into memory rather than moving back and forward by means of lseek(2). map_file is a thin wrapper around mmap(2):
void *
map_file(int fd, size_t *len)
{
off_t jump;
void *addr;
if ((jump = lseek(fd, 0, SEEK_END)) == -1)
err(1, "lseek");
if (lseek(fd, 0, SEEK_SET) == -1)
err(1, "lseek");
if ((addr = mmap(NULL, jump, PROT_READ, MAP_PRIVATE, fd, 0))
== MAP_FAILED)
err(1, "mmap");
*len = jump;
return addr;
}
Just as we were discussing before, to locate the central directory we must first locate the āend of central directory recordā. Its structure is as follows (the numbers inside the brackets indicates the byte count)
signature[4] disk_number[2] disk_cd_number[2] disk_entries[2] total_entrie[2] central_directory_size[4] cd_offset[4] comment_len[2] commentā¦
The signature is always ā\x50\x4b\x05\x06ā, which helps in finding the record. We still need to be careful, since I havenāt seen anywhere that the signature MUST NOT appear inside the comment.
To be sure that weāve actually found the real start of the end record, thereās a explicit check: the comment length plus the size of the non-variable part of the header must be equal to how far we have travelled from the end of the file. Granted, this is not completely bulletproof, since a specially-crafted comment may appear like a proper end of central directory record, but Iām not sure what could we do better to protect against faulty files.
Side note: as always, Iām treating these files as untrusted and do all the possible checks. You donāt want a malformed file to crash your program, donāt you?
One last thing: Iām totally fine with a very light and sparse usage of gotos. In find_central_directory Iām using a āgoto againā when we find a false signature inside a comment. A while loop would also do that, but itād be a bit uglier.
void *
find_central_directory(uint8_t *addr, size_t len)
{
uint32_t offset;
uint16_t clen;
uint8_t *p, *end;
/*
* At -22 bytes from the end there is the end of the central
* directory assuming an empty comment. It's a sensible place
* from which start.
*/
if (len < 22)
return NULL;
end = addr + len;
p = end - 22;
again:
for (; p > addr; --p)
if (memcmp(p, "\x50\x4b\x05\x06", 4) == 0)
break;
if (p == addr)
return NULL;
/* read comment length */
memcpy(&clen, p + 20, sizeof(clen));
clen = le16toh(clen);
/* false signature inside a comment? */
if (clen + 22 != end - p) {
p--;
goto again;
}
/* read the offset for the central directory */
memcpy(&offset, p + 16, sizeof(offset));
offset = le32toh(offset);
if (addr + offset > p)
return NULL;
return addr + offset;
}
Edit 2021/08/20: thereās a space for a little optimisation: the end record MUST be in the last 64kb (plus some bytes), so for big files thereās no need to continue searching back until the start. Why 64kb? The comment length is a 16 bit integer, so the biggest end of record possible is 22 bytes plus 64kb of comment.
If everything went well, weāve found the pointer to the start of the central directory. Itās made by a sequence of file header records:
signature[4] version[2] vers_needed[2] flags[2] compression[2] mod_time[2] mod_date[2] crc32[4] compressed_size[4] uncompressed_size[4] filename_len[2] extra_field_len[2] file_comment_len[2] disk_number[2] internal_attrs[2] offset[4] filenameā¦ extra_fieldā¦ file_commentā¦
The signature field is always "\x50\x4b\x01\x02", which is different from the end record and the other records fortunately. To list the files we just have to read the file headers record until we find one with a different signature:
void
ls(uint8_t *zip, size_t len, uint8_t *cd)
{
uint32_t offset;
uint16_t flen, xlen, clen;
uint8_t *end;
char filename[PATH_MAX];
end = zip + len;
while (cd < end - 46 && memcmp(cd, "\x50\x4b\x01\x02", 4) == 0) {
memcpy(&flen, cd + 28, sizeof(flen));
memcpy(&xlen, cd + 28 + 2, sizeof(xlen));
memcpy(&clen, cd + 28 + 2 + 2, sizeof(xlen));
flen = le16toh(flen);
xlen = le16toh(xlen);
clen = le16toh(clen);
memcpy(&offset, cd + 42, sizeof(offset));
offset = le32toh(offset);
memset(filename, 0, sizeof(filename));
memcpy(filename, cd + 46, MIN(sizeof(filename)-1, flen));
printf("%s [%d]\n", filename, offset);
cd += 46 + flen + xlen + clen;
}
}
As always, there are some magic numbers hardcoded, a real program would probably have some constants defined, but for this simple toy program Iām fine with things as is. Also, note the pedantry in ensuring we donāt end up reading out-of-bounds in the while condition, I donāt want faulty zip files to cause invalid memory access.
Now, to compile it and run:
% cc zipls.c -o zipls && ./zipls star_maker_olaf_stapledon.gpub 0_preface.gmi [0] chapter_1_1_the_starting_point.gmi [2957] chapter_1_2_earth_among_the_stars.gmi [6932] chapter_2_1_interstellar_travel.gmi [11041] chapter_3_1_on_the_other_earth.gmi [20382] ā¦
and voila, it works!
To conclude this entry, one of the things that Iām still not sure about is the endiannes of the numbers. Iām guessing they should be little endian, but itās always that or only because the zip files were produced on a little endian machine?
Edit 2021/08/20: The majority of the number are stored in little-endian. There are some exception, so check the documentation, but is mostly for fields like the MSDOS-like time and date and stuff like that. The code was updated with the calls to leXYtoh() from āendian.hā.
Otherwise Iām pretty happy with the result. In a short time I went from knowing nothing about zips to being able to at least inspect them, using only the C standard library (well, assuming POSIX). Iāll leave the files decoding for a next time.
-- text: CC0 1.0; code: public domain (unless specified otherwise). No copyright here.