SSHBUF_SIZE_INC
sz = roundup(1223 + SSHBUF_SIZE_INC * 3, SSHBUF_SIZE_INC);
ASSERT_SIZE_T_EQ(sshbuf_alloc(p1) % SSHBUF_SIZE_INC, 0);
rlen = ROUNDUP(buf->size, SSHBUF_SIZE_INC);
rlen = ROUNDUP(buf->alloc + need, SSHBUF_SIZE_INC);