getsockopt/setsockopt with SO_RCVBUF and SO_SNDBUF "non-standard" behaviour

[Eugen Dedu]

Hi all,

I looked on Internet and at the old thread
http://lkml.indiana.edu/hypermail/linux/kernel/0108.0/0275.html, but the
issue is still not settled as far as I see.

I need to have the highest memory available for snd/rcv buffer and I
need to know/confirm how much it allocated for my process (how much I
can use).

So with Linux we need to do something like:
setsockopt (..., SO_RCVBUF, 256000, ...)
getsockopt (..., SO_RCVBUF, &i, ...)
i /= 2;

where i is the size I am looking for.

Now, to make this code work for other OSes it should be changed to:
setsockopt (..., SO_RCVBUF, 256000, ...)
getsockopt (..., SO_RCVBUF, &i, ...)
#ifdef LINUX
i /= 2;
#endif

First question, is this code correct? If not, what code gives the
amount of memory useable for my process?

Second, it seems to me that linux is definitely "non-standard" here.
Saying that linux uses twice as memory has nothing to do with that,
since getsockopt should return what the application can count on, not
what is the internal use. It is like a hypothetical malloc (10) would
return not 10, but 20 (including meta-information). Is that right?

Cheers,
--
Eugen Dedu
http://eugen.dedu.free.fr
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[Eugen Dedu]

Any idea?

[Eric Dumazet]

That the way it's done on linux since day 0

You can probably find a lot of pages on the web explaining the
rationale.

If your application handles UDP frames, what SO_RCVBUF should count ?

If its the amount of payload bytes, you could have a pathological
situation where an attacker sends 1-byte UDP frames fast enough and
could consume a lot of kernel memory.

Each frame consumes a fair amount of kernel memory (between 512 bytes
and 8 Kbytes depending on the driver).

So linux says : If user expect to receive XXXX bytes, set a limit of
_kernel_ memory used to store these bytes, and use an estimation of 100%
of overhead. That is : allow 2*XXXX bytes to be allocated for socket
receive buffers.

[Rick Jones]

On 07/18/2012 09:11 AM, Eric Dumazet wrote:
>
> That the way it's done on linux since day 0
>
> You can probably find a lot of pages on the web explaining the
> rationale.
>
> If your application handles UDP frames, what SO_RCVBUF should count ?
>
> If its the amount of payload bytes, you could have a pathological
> situation where an attacker sends 1-byte UDP frames fast enough and
> could consume a lot of kernel memory.
>
> Each frame consumes a fair amount of kernel memory (between 512 bytes
> and 8 Kbytes depending on the driver).
>
> So linux says : If user expect to receive XXXX bytes, set a limit of
> _kernel_ memory used to store these bytes, and use an estimation of 100%
> of overhead. That is : allow 2*XXXX bytes to be allocated for socket
> receive buffers.

Expanding on/rewording that, in a setsockopt() call SO_RCVBUF specifies
the data bytes and gets doubled to become the kernel/overhead byte
limit. Unless the doubling would be greater than net.core.rmem_max, in
which case the limit becomes net.core.rmem_max.

But on getsockopt() SO_RCVBUF is always the kernel/overhead byte limit.

In one call it is fish. In the other it is fowl.

Other stacks appear to keep their kernel/overhead limit quiet, keeping
SO_RCVBUF an expression of a data limit in both setsockopt() and
getsockopt(). With those stacks, there is I suppose the possible source
of confusion when/if someone tests the queuing to a socket, sends "high
overhead" packets and doesn't get to SO_RCVBUF worth of data though I
don't recall encountering that in my "pre-linux" time.

The sometimes fish, sometimes fowl version (along with the auto tuning
when one doesn't make setsockopt() calls) gave me fits in netperf for
years until I finally relented and split the socket buffer size
variables into three - what netperf's user requested via the command
line, what it was right after the socket was created, and what it was at
the end of the data phase of the test.

rick jones

[Eugen Dedu]

Thank you to both for the answers. As I understand, it it is impossible
(or not practical) to fulfill sometimes user requirements on buff size,
since if only 1-byte udp packets arrive and are not consumed by
application, the memory needed by linux is say 1000 greater, which of
course is not available. Other OSes have the same problem (see above
"doesn't get to SO_RCVBUF worth of data"), except that they return the
same value in getsockopt as setsockopt. However, note that with linux
the confusion is still possible, even if it appears more rarely.

> The sometimes fish, sometimes fowl version (along with the auto tuning
> when one doesn't make setsockopt() calls) gave me fits in netperf for
> years until I finally relented and split the socket buffer size
> variables into three - what netperf's user requested via the command
> line, what it was right after the socket was created, and what it was at
> the end of the data phase of the test.

--
Eugen