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

[Ard Biesheuvel]

On Sat, 26 Oct 2024 at 06:10, Eric Biggers <ebiggers at kernel.org> wrote:
>
> On Fri, Oct 25, 2024 at 10:02:39PM +0000, Eric Biggers wrote:
> > On Fri, Oct 25, 2024 at 10:47:15PM +0200, Ard Biesheuvel wrote:
> > > On Fri, 25 Oct 2024 at 21:15, Eric Biggers <ebiggers at kernel.org> wrote:
> > > >
> > > > From: Eric Biggers <ebiggers at 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.
> >
> > Yes, we probably could get away without this.  It's never really been
> > appropriate to use the crypto driver names for anything important.  And btrfs
> > probably should just assume CRC32C == fast unconditionally, like what it does
> > with xxHash64, or even do a quick benchmark to measure the actual speed of its
> > hash algorithm (which can also be sha256 or blake2b which can be very fast too).
> >
> > Besides the btrfs case, my concern was there may be advice floating around about
> > checking /proc/crypto to check what optimized code is being used.  Having
> > crc32-$arch potentially be running the generic code would make that misleading.
> > It might make sense to keep it working similar to how it did before.
> >
> > But I do agree that we could probably get away without this.
>
> While testing this patchset I notice that none of the crypto API drivers for
> crc32 or crc32c even need to be loaded on my system anymore, as everything on my
> system that uses those algorithms (such as ext4) just uses the library APIs now.
> That makes the "check /proc/crypto" trick stop working anyway.
>
> I think you're right that we shouldn't bother with patches 3-4, and I'll plan to
> go back to leaving them out in the next version, unless someone yells.
>

Agreed.

If we need to make this distinction, it might be cleaner to use the
static_call API instead, e.g.,

+DECLARE_STATIC_CALL(crc32_le_arch, crc32_le_base);
+
 static inline u32 __pure crc32_le(u32 crc, const u8 *p, size_t len)
 {
        if (IS_ENABLED(CONFIG_CRC32_ARCH))
-               return crc32_le_arch(crc, p, len);
+               return static_call(crc32_le_arch)(crc, p, len);
        return crc32_le_base(crc, p, len);
 }

and use static_call_update() to update the target if the feature is
supported. Then, we could check in the driver whether the static call
points to the default or not:

+static bool have_arch;
+
 static int __init crc32_mod_init(void)
 {
+       have_arch = IS_ENABLED(CONFIG_CRC32_ARCH) &&
+                   static_call_query(crc32_le_arch) != crc32_le_base;
+
        /* register the arch flavor only if it differs from the generic one */
-       return crypto_register_shashes(algs, 1 + IS_ENABLED(CONFIG_CRC32_ARCH));
+       return crypto_register_shashes(algs, 1 + have_arch);
 }