Re: [PATCH bpf-next v3 2/6] crypto: Add BPF hash algorithm type registration module

[Song Liu]

On Sun, Dec 7, 2025 at 7:01 PM Daniel Hodges <g...@danielhodges.dev> wrote:
>
> Add bpf_crypto_shash module that registers a hash type with the BPF
> crypto infrastructure, enabling BPF programs to access kernel hash
> algorithms through a unified interface.
>
> Update the bpf_crypto_type interface with hash-specific callbacks:
>    - alloc_tfm: Allocates crypto_shash context with proper descriptor size
>    - free_tfm: Releases hash transform and context memory
>    - has_algo: Checks algorithm availability via crypto_has_shash()
>    - hash: Performs single-shot hashing via crypto_shash_digest()
>    - digestsize: Returns the output size for the hash algorithm
>    - get_flags: Exposes transform flags to BPF programs
>
> Update bpf_shash_ctx to contain crypto_shash transform and shash_desc
> descriptor to accommodate algorithm-specific descriptor requirements.
>
> Signed-off-by: Daniel Hodges <g...@danielhodges.dev>
> ---
>  crypto/Makefile           |  3 ++
>  crypto/bpf_crypto_shash.c | 95 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 98 insertions(+)
>  create mode 100644 crypto/bpf_crypto_shash.c
>
> diff --git a/crypto/Makefile b/crypto/Makefile
> index 16a35649dd91..853dff375906 100644
> --- a/crypto/Makefile
> +++ b/crypto/Makefile
> @@ -30,6 +30,9 @@ obj-$(CONFIG_CRYPTO_ECHAINIV) += echainiv.o
>  crypto_hash-y += ahash.o
>  crypto_hash-y += shash.o
>  obj-$(CONFIG_CRYPTO_HASH2) += crypto_hash.o
> +ifeq ($(CONFIG_BPF_SYSCALL),y)
> +obj-$(CONFIG_CRYPTO_HASH2) += bpf_crypto_shash.o
> +endif
>
>  obj-$(CONFIG_CRYPTO_AKCIPHER2) += akcipher.o
>  obj-$(CONFIG_CRYPTO_SIG2) += sig.o
> diff --git a/crypto/bpf_crypto_shash.c b/crypto/bpf_crypto_shash.c
> new file mode 100644
> index 000000000000..95c178ec0ce8
> --- /dev/null
> +++ b/crypto/bpf_crypto_shash.c
> @@ -0,0 +1,95 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +#include <linux/types.h>
> +#include <linux/module.h>
> +#include <linux/bpf_crypto.h>
> +#include <crypto/hash.h>
> +
> +struct bpf_shash_ctx {
> +       struct crypto_shash *tfm;
> +       struct shash_desc desc;
> +};
> +
> +static void *bpf_crypto_shash_alloc_tfm(const char *algo)
> +{
> +       struct bpf_shash_ctx *ctx;
> +       struct crypto_shash *tfm;
> +
> +       tfm = crypto_alloc_shash(algo, 0, 0);
> +       if (IS_ERR(tfm))
> +               return tfm;
> +
> +       ctx = kzalloc(sizeof(*ctx) + crypto_shash_descsize(tfm), GFP_KERNEL);
> +       if (!ctx) {
> +               crypto_free_shash(tfm);
> +               return ERR_PTR(-ENOMEM);
> +       }
> +
> +       ctx->tfm = tfm;
> +       ctx->desc.tfm = tfm;
> +
> +       return ctx;
> +}

What if we let bpf_crypto_shash_alloc_tfm() return a "struct shash_desc"?
shash_desc->tfm is already struct crypto_shash. This way, we don't need
bpf_shash_ctx any more. Would this work?

Thanks,
Song