Re: [PATCH v6 3/6] crypto: AF_ALG -- add asymmetric cipher interface

[Denis Kenzior <denkenz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi Stephan,

 >>
>> This brings me to another proposal for read buffer sizing: AF_ALG akcipher
>> can guarantee that partial reads (where the read buffer is shorter than
>> the output of the crypto op) will work using the same semantics as
>> SOCK_DGRAM/SOCK_SEQPACKET. With those sockets, as much data as will fit is
>> copied in to the read buffer and the remainder is discarded.
>>
>> I realize there's a performance and memory tradeoff, since the crypto
>> algorithm needs a sufficiently large output buffer that would have to be
>> created by AF_ALG akcipher. The user could manage that tradeoff by
>> providing a larger buffer (typically key_size?) if it wants to avoid
>> allocating and copying intermediate buffers inside the kernel.
>
> How shall the user know that something got truncated or that the kernel
> created memory?
>

To the former point, recall the signature of recv:
ssize_t recv(int sockfd, void *buf, size_t len, int flags);

Traditionally, userspace apps can know that the buffer provided to recv 
was too small in two ways:

The return value from recv / recvmsg was >= len.

In the case of recvmsg, the MSG_TRUNC flag is set.

To quote man recv:

"All  three calls return the length of the message on successful comple‐
tion.  If a message is too long to fit in the supplied  buffer,  excess
bytes  may  be discarded depending on the type of socket the message is
received from."

and

"MSG_TRUNC (since Linux 2.2)

	For   raw   (AF_PACKET),   Internet   datagram   (since    Linux
	2.4.27/2.6.8),  netlink  (since Linux 2.6.22), and UNIX datagram
	(since Linux 3.4) sockets: return the real length of the  packet
	or datagram, even when it was longer than the passed buffer.
"

Regards,
-Denis