Thanks to the introduction of BPF loops and increased verifier complexity
limits we are now able to implement simple Bitcoin miner in XDP. The program
assumes hard-coded input format, runs main loop 16 times, and if the solution
is found, sends it back. Having more than 16 loop iterations is still
problematic; bpf_loop might be the way to go beyond this toy example.
make
./mine
The above will run a test around XDP program by feeding it a solved
block 123 with a nounce adjusted back by 15 iterations. That's
enough for the program to try them all and eventually find a
solution.
$ make -s check
cc test_sha256.c && ./a.out
ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
85e655d6417a17953363376a624cde5c76e09589cac5f811cc4b32c1f20e533a
248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1
ok
cc test_mine.c && ./a.out
mine() = 15
00000000a3bbe4fd1da16a29dbdaba01cc35d6fc74ee17f794cf3aab94f7aaa0
ok
rm a.out
$ make -s
$ ./mine
returned XDP_TX and correct 4094077204 nonce
$ cat /sys/kernel/debug/tracing/trace_maker
...
mine-216 [001] b..11 6.270270: bpf_trace_printk: found on 15th iteration
Block header is 80 bytes that contain the following:
+---------+-----------------+----------------+------+------------+-------+
| Version | Prev Block Hash | TX Merkle Root | Time | Difficulty | Nonce |
+---------+-----------------+----------------+------+------------+-------+
Mining works by iterating nonce by one, calling sha256 over 80 bytes
of block header, then calling sha256 again over previous result. (I'm
simplifying here a lot because nonce is only 32 bits and it's
possible to exhaust all nonces without finding the block)
The resulting checksum is treated as a big number and if this big number is less then the predefined difficulty (e.g. has a lot of leading zeroes), we've mined a new block.
SHA256 is very naive and unoptimized version from the Wikipedia page.
There is a userspace test program that runs a bunch of test vectors from https://csrc.nist.gov/csrc/media/publications/fips/180/2/archive/2002-08-01/documents/fips180-2withchangenotice.pdf