Re: [PATCH net-next v2 2/5] net/tcp-ao: Use crypto library API instead of crypto_ahash

Ard Biesheuvel
2026-04-28T06:35:10Z

On Tue, 28 Apr 2026, at 03:24, David Laight wrote:
> On Mon, 27 Apr 2026 10:27:24 -0700
> Eric Biggers <ebiggers@kernel.org> wrote:
>
>> Currently the kernel's TCP-AO implementation does the MAC and KDF
>> computations using the crypto_ahash API. This API is inefficient and
>> difficult to use, and it has required extensive workarounds in the form
>> of per-CPU preallocated objects (tcp_sigpool) to work at all.
>>
>> Let's use lib/crypto/ instead. This means switching to straightforward
>> stack-allocated structures, virtually addressed buffers, and direct
>> function calls. It also means removing quite a bit of error handling.
>> This makes TCP-AO quite a bit faster.
>>
>> This also enables many additional cleanups, which later commits will
>> handle: removing tcp-sigpool, removing support for crypto_tfm cloning,
>> removing more error handling, and replacing more dynamically-allocated
>> buffers with stack buffers based on the now-statically-known limits.
>>
>> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
>> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ...
>> @@ -344,33 +444,26 @@ static int tcp_v4_ao_calc_key(struct tcp_ao_key *mkt, u8 *key,
>> struct kdf_input_block {
>> u8 counter;
>> u8 label[6];
>> struct tcp4_ao_context ctx;
>> __be16 outlen;
>> - } __packed * tmp;
>
> That looks a bit horrid.
> I also had a feeling that the compiler sometimes rejects non-packed structures
> inside packed ones.
> Perhaps nest the whole thing inside another structure that has an initial
> u8 pad and is marked __packed __aligned(4).
> Then the assignments to the fields of 'ctx' will be known to be aligned
> even when tcp4_ao_context is also __packed.
>

Agree with Eric that this has no bearing on this patch, but I'm not sure
I see the problem here. 'ctx' will not be packed, and appear misaligned
in struct kdf_input_block, but that would only matter if the address of
the ctx field were taken and passed to a function taking a pointer to
struct tcp4_ao_context (which would expect it to appear naturally
aligned).

Having a feeling about what the compiler sometimes rejects is not
actionable feedback - could you be more specific about which problem
you think needs to be solved here? Are you concerned about unaligned
accesses when populating the struct?