Re: [PATCH v3 1/2] lib/crypto: arm64: Fix lost Poly1305 carry when resuming NEON state

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Oct 09, 2026 at 02:53:53PM +0200, Eric Biggers wrote:
> represents 2^128 + 4, so the conversion must produce the base 2^64 limbs
> h0 = 4, h1 = 0, h2 = 1.  The low 64-bit word sums to 2^64 + 4 and the
> middle 64-bit word (including the carry from the low word) sums to 2^64,
> so both carry out.  However, the carry out of the middle word goes to
> d2, leaving h2 = 0 and the value 4 instead of 2^128 + 4.

Going to change it to this instead, which should be clearer:

    represents 4 + 2^26*2^26 + (2^26-1)*2^52 + (2^26-1)*2^78 +
    (2^24-1)*2^104 = 2^128 + 4, despite the bit representing the 2^128
    term (bit 24 of h4) not being set; the extra comes from the h1 limb
    being above the normal base 2^26 range.  When this value is
    converted into the non-redundant base 2^64 form, a carry into the
    new 2^128 bit (in bit 0 of h2) is necessary to preserve the value.

> [EB: Rewrote the commit message to more clearly describe the bug and
>  its impact, and removed a lot of pointless LLM-generated text.]

Changed to

 [EB: Rewrote the commit message to more clearly describe the bug and
  its impact, and simplified explanation about why there's a carry bit.]

since Jérémy said it actually wasn't LLM generated.

- Eric