On 10/21/24 2:21 PM, Puranjay Mohan wrote:
1. Optimization
------------
The current implementation copies the 'from' and 'to' buffers to a
scratchpad and it takes the bitwise NOT of 'from' buffer while copying.
In the next step csum_partial() is called with this scratchpad.
so, mathematically, the current implementation is doing:
result = csum(to - from)
Here, 'to' and '~ from' are copied in to the scratchpad buffer, we need
it in the scratchpad buffer because csum_partial() takes a single
contiguous buffer and not two disjoint buffers like 'to' and 'from'.
We can re write this equation to:
result = csum(to) - csum(from)
using the distributive property of csum().
this allows 'to' and 'from' to be at different locations and therefore
this scratchpad and copying is not needed.
This in C code will look like:
result = csum_sub(csum_partial(to, to_size, seed),
csum_partial(from, from_size, 0));
2. Homogenization
--------------
The bpf_csum_diff() helper calls csum_partial() which is implemented by
some architectures like arm and x86 but other architectures rely on the
generic implementation in lib/checksum.c
The generic implementation in lib/checksum.c returns a 16 bit value but
the arch specific implementations can return more than 16 bits, this
works out in most places because before the result is used, it is passed
through csum_fold() that turns it into a 16-bit value.
bpf_csum_diff() directly returns the value from csum_partial() and
therefore the returned values could be different on different
architectures. see discussion in [1]:
for the int value 28 the calculated checksums are:
x86 : -29 : 0xffffffe3
generic (arm64, riscv) : 65507 : 0x0000ffe3
arm : 131042 : 0x0001ffe2
Pass the result of bpf_csum_diff() through from32to16() before returning
to homogenize this result for all architectures.
NOTE: from32to16() is used instead of csum_fold() because csum_fold()
does from32to16() + bitwise NOT of the result, which is not what we want
to do here.
[1] https://lore.kernel.org/bpf/CAJ+HfNiQbOcqCLxFUP2FMm5QrLXUUaj852Fxe3hn_2JNiucn6g@xxxxxxxxxxxxxx/
Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
Thanks for looking into this!
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
