[PATCH 2/4] cache: use sendfile() instead of a pair of read() + write()

[Jason A. Donenfeld]

Excellent, thanks Sebastian! I've merged this commit.

Would you mind sending another commit where you implement this for the
read() write() situation in authenticate_post() on
http://git.zx2c4.com/cgit/tree/cgit.c#n624 ? Still bounding it to
MAX_AUTHENTICATION_POST_BYTES, but not having to copy it back to
userspace?