linux-kernel - Re: [PATCH] vsock: Handle compat 32-bit timeout

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Date:   Wed, 06 Oct 2021 09:13:37 +0100
From:   Richard Palethorpe <rpalethorpe@...e.de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Arseny Krasnov <arseny.krasnov@...persky.com>,
        Colin Ian King <colin.king@...onical.com>,
        Norbert Slusarek <nslusarek@....net>,
        Andra Paraschiv <andraprs@...zon.com>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, rpalethorpe@...hiejp.com
Subject: Re: [PATCH] vsock: Handle compat 32-bit timeout

Hello Arnd,

Arnd Bergmann <arnd@...db.de> writes:

> On Wed, Oct 6, 2021 at 9:48 AM Richard Palethorpe <rpalethorpe@...e.com> wrote:
>>
>> Allow 32-bit timevals to be used with a 64-bit kernel.
>>
>> This allows the LTP regression test vsock01 to run without
>> modification in 32-bit compat mode.
>>
>> Fixes: fe0c72f3db11 ("socket: move compat timeout handling into sock.c")
>> Signed-off-by: Richard Palethorpe <rpalethorpe@...e.com>
>>
>> ---
>>
>> This is one of those fixes where I am not sure if we should just
>> change the test instead. Because it's not clear if someone is likely
>> to use vsock's in 32-bit compat mode?
>
> We try very hard to ensure that compat mode works for every interface,
> so it should be fixed in the kernel. Running compat mode is common
> on memory-restricted machines, e.g. on cloud platforms and on deeply
> embedded systems.

Thanks!

>
> However, I think fixing the SO_VM_SOCKETS_CONNECT_TIMEOUT
> to support 64-bit timeouts would actually be more important here. I think
> what you need to do is to define the macro the same way
> as the SO_TIMESTAMP one:
>
> #define SO_RCVTIMEO (sizeof(time_t) == sizeof(__kernel_long_t) ? \
>              SO_RCVTIMEO_OLD : SO_RCVTIMEO_NEW)
> #define SO_TIMESTAMP (sizeof(time_t) == sizeof(__kernel_long_t) ? \
>              SO_TIMESTAMP_OLD : SO_TIMESTAMP_NEW)
> ...
>
> to ensure that user space picks an interface that matches the
> user space definition of 'struct timeval'.
>
> Your change looks correct otherwise, but I think you should first
> add the new interface for 64-bit timeouts, since that likely changes
> the code in a way that makes your current patch no longer the
> best way to write it.
>
>        Arnd

Ah, yes, it will still be broken if libc is configured with 64bit
timeval only.

-- 
Thank you,
Richard.