Re: [PATCH] ecryptfs: streamline offset formatting in ecryptfs_derive_iv

[Eric Biggers <ebiggers@kernel.org>]

On Sun, Mar 29, 2026 at 11:23:25PM +0200, Thorsten Blum wrote:
> Use the number of characters written by scnprintf() to zero-pad the
> remaining bytes, instead of clearing the buffer first and then writing
> the offset.
[...]
> +	size_t len;
[...]
>  	memcpy(src, crypt_stat->root_iv, crypt_stat->iv_bytes);
> -	memset((src + crypt_stat->iv_bytes), 0, 16);
> -	snprintf((src + crypt_stat->iv_bytes), 16, "%lld", offset);
> +	len = scnprintf(src + crypt_stat->iv_bytes, 16, "%lld", offset) + 1;
> +	memset(src + crypt_stat->iv_bytes + len, 0, 16 - len);

This isn't exactly "streamlining" the code.  memset(p, 0, 16) tends to
get compiled into just two instructions.  In contrast, a variable-length
memset tends to be several instructions to set up, plus a call
instruction, and the instructions inside memset() itself.  scnprintf()
is also a few more instructions than snprintf().

So I'd say the old version is more "streamlined", actually.  Granted,
the difference is probably only a few cycles, but it sounds like the
motivation for this patch is that you assumed the new version is faster?

- Eric