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Parsing a git HTTP protocol response




Hi all,

I am writing an implementation of the git HTTP pack protocol in C. It
just does a request to clone a repository. It works pretty well for
small repositories, but seems to fail on larger repositories and I do
not understand why.

All that my code does is send a hard-coded "want" request. As I
understand it, responses come with a four-byte size prefix, then the
data of the size - 4. The first four bytes are the size of the object
in ASCII and after that size, a new object should begin by specifying
its size. The final "0000" string should signify the end.

On small repositories my code works fine. But when use a large git
repository, I hit an object size that cannot be interpreted by ASCII
into a number. In particular, right before the crash I am pretty
consistently getting a size starting with 0x32,0x00 or 0x32,0x30,0x00
(note, it starts with 0x32 and has 0x00 in there). This is not a
readable four byte ascii value, likely an erroneous size value, which
causes the next object size calculation to land incorrectly and
subsequently the program to malfunction.

My questions:
1. Am I misunderstanding the protocol?
2. Does 0x32 signify something?
3. Also, where can I see in the source code git parses the data it
receives from a "want" request?

If you would like to see my code, it is located here:
http://git.farhan.codes/farhan/post
To compile to Linux run: gcc post.c main.c -lcurl -o post
To compile on FreeBSD you can use the Makefile or: cc -L
/usr/local/lib -I /usr/local/include -lcurl main.c post.c -o post
In both cases you need to have libcurl installed.

To run do: ./post [a git http url] [a commit value]
ie: ./post http://git.farhan.codes/farhan/openbsd
373dd5ba116d42cddf1fdcb58b578a4274f6d589

I have a lot of debugging printf() messages, but it eventually hits a
point where you see this:

========Start=========
Current stats: Current state: 999 Received: 1448
We have enough bytes, moving forward
== New Object
No Error, object is 32 bytes
Size bytes: 32 30 00 00

The program interprets the size of {0x32,0x30,0x00,0x00} to be "20"
which in decimal is 32, causing the next read to fail.
This problem repeats on a few different repositories.

Any assistance is welcome, I am very stuck on how the HTTP git protocol works.
Thanks,
--
Farhan Khan
PGP Fingerprint: B28D 2726 E2BC A97E 3854 5ABE 9A9F 00BC D525 16EE