Re: [PATCH v2 04/18] crypto: crc32 - don't unnecessarily register arch algorithms

Ard Biesheuvel
2024-10-25T20:47:28Z

On Fri, 25 Oct 2024 at 21:15, Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> Instead of registering the crc32-$arch and crc32c-$arch algorithms if
> the arch-specific code was built, only register them when that code was
> built *and* is not falling back to the base implementation at runtime.
>
> This avoids confusing users like btrfs which checks the shash driver
> name to determine whether it is crc32c-generic.
>

I think we agree that 'generic' specifically means a C implementation
that is identical across all architectures, which is why I updated my
patch to export -arch instead of wrapping the C code in yet another
driver just for the fuzzing tests.

So why is this a problem? If no optimizations are available at
runtime, crc32-arch and crc32-generic are interchangeable, and so it
shouldn't matter whether you use one or the other.

You can infer from the driver name whether the C code is being used,
not whether or not the implementation is 'fast', and the btrfs hack is
already broken on arm64.

> (It would also make sense to change btrfs to test the crc32_optimization
> flags itself, so that it doesn't have to use the weird hack of parsing
> the driver name. This change still makes sense either way though.)
>

Indeed. That hack is very dubious and I'd be inclined just to ignore
this. On x86 and arm64, it shouldn't make a difference, given that
crc32-arch will be 'fast' in the vast majority of cases. On other
architectures, btrfs may use the C implementation while assuming it is
something faster, and if anyone actually notices the difference, we
can work with the btrfs devs to do something more sensible here.

> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> crypto/crc32_generic.c | 8 ++++++--
> crypto/crc32c_generic.c | 8 ++++++--
> 2 files changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/crypto/crc32_generic.c b/crypto/crc32_generic.c
> index cc064ea8240e..cecd01e4d6e6 100644
> --- a/crypto/crc32_generic.c
> +++ b/crypto/crc32_generic.c
> @@ -155,19 +155,23 @@ static struct shash_alg algs[] = {{
> .base.cra_ctxsize = sizeof(u32),
> .base.cra_module = THIS_MODULE,
> .base.cra_init = crc32_cra_init,
> }};
>
> +static int num_algs;
> +
> static int __init crc32_mod_init(void)
> {
> /* register the arch flavor only if it differs from the generic one */
> - return crypto_register_shashes(algs, 1 + IS_ENABLED(CONFIG_CRC32_ARCH));
> + num_algs = 1 + ((crc32_optimizations & CRC32_LE_OPTIMIZATION) != 0);
> +
> + return crypto_register_shashes(algs, num_algs);
> }
>
> static void __exit crc32_mod_fini(void)
> {
> - crypto_unregister_shashes(algs, 1 + IS_ENABLED(CONFIG_CRC32_ARCH));
> + crypto_unregister_shashes(algs, num_algs);
> }
>
> subsys_initcall(crc32_mod_init);
> module_exit(crc32_mod_fini);
>
> diff --git a/crypto/crc32c_generic.c b/crypto/crc32c_generic.c
> index 04b03d825cf4..47d694da9d4a 100644
> --- a/crypto/crc32c_generic.c
> +++ b/crypto/crc32c_generic.c
> @@ -195,19 +195,23 @@ static struct shash_alg algs[] = {{
> .base.cra_ctxsize = sizeof(struct chksum_ctx),
> .base.cra_module = THIS_MODULE,
> .base.cra_init = crc32c_cra_init,
> }};
>
> +static int num_algs;
> +
> static int __init crc32c_mod_init(void)
> {
> /* register the arch flavor only if it differs from the generic one */
> - return crypto_register_shashes(algs, 1 + IS_ENABLED(CONFIG_CRC32_ARCH));
> + num_algs = 1 + ((crc32_optimizations & CRC32C_OPTIMIZATION) != 0);
> +
> + return crypto_register_shashes(algs, num_algs);
> }
>
> static void __exit crc32c_mod_fini(void)
> {
> - crypto_unregister_shashes(algs, 1 + IS_ENABLED(CONFIG_CRC32_ARCH));
> + crypto_unregister_shashes(algs, num_algs);
> }
>
> subsys_initcall(crc32c_mod_init);
> module_exit(crc32c_mod_fini);
>
> --
> 2.47.0
>
>