On Wed, Nov 29, 2023 at 03:57:25PM +0800, Jerry Shih wrote: > On Nov 28, 2023, at 12:07, Eric Biggers <ebiggers@xxxxxxxxxx> wrote: > > On Mon, Nov 27, 2023 at 03:06:57PM +0800, Jerry Shih wrote: > >> +typedef void (*aes_xts_func)(const u8 *in, u8 *out, size_t length, > >> + const struct crypto_aes_ctx *key, u8 *iv, > >> + int update_iv); > > > > There's no need for this indirection, because the function pointer can only have > > one value. > > > > Note also that when Control Flow Integrity is enabled, assembly functions can > > only be called indirectly when they use SYM_TYPED_FUNC_START. That's another > > reason to avoid indirect calls that aren't actually necessary. > > We have two function pointers for encryption and decryption. > static int xts_encrypt(struct skcipher_request *req) > { > return xts_crypt(req, rv64i_zvbb_zvkg_zvkned_aes_xts_encrypt); > } > > static int xts_decrypt(struct skcipher_request *req) > { > return xts_crypt(req, rv64i_zvbb_zvkg_zvkned_aes_xts_decrypt); > } > The enc and dec path could be folded together into `xts_crypt()`, but we will have > additional branches for enc/decryption path if we don't want to have the indirect calls. > Use `SYM_TYPED_FUNC_START` in asm might be better. > Right. Normal branches are still more efficient and straightforward than indirect calls, though, and they don't need any special considerations for CFI. So I'd just add a 'bool encrypt' or 'bool decrypt' argument to xts_crypt(), and make xts_crypt() call the appropriate assembly function based on that. > > Did you consider writing xts_crypt() the way that arm64 and x86 do it? The > > above seems to reinvent sort of the same thing from first principles. I'm > > wondering if you should just copy the existing approach for now. Then there > > would be no need to add the scatterwalk_next() function, and also the handling > > of inputs that don't need ciphertext stealing would be a bit more streamlined. > > I will check the arm and x86's implementations. > But the `scatterwalk_next()` proposed in this series does the same thing as the > call `scatterwalk_ffwd()` in arm and x86's implementations. > The scatterwalk_ffwd() iterates from the beginning of scatterlist(O(n)), but the > scatterwalk_next() is just iterates from the end point of the last used > scatterlist(O(1)). Sure, but your scatterwalk_next() only matters when there are multiple scatterlist entries and the AES-XTS message length isn't a multiple of the AES block size. That's not an important case, so there's little need to micro-optimize it. The case that actually matters for AES-XTS is a single-entry scatterlist containing a whole number of AES blocks. - Eric