Re: [PATCH v2 net-next 1/2] tcp: add TCP_ZEROCOPY_RECEIVE support for zerocopy receive

[Ka-Cheong Poon]

On 04/26/2018 05:43 AM, Eric Dumazet wrote:
> When adding tcp mmap() implementation, I forgot that socket lock
> had to be taken before current->mm->mmap_sem. syzbot eventually caught
> the bug.
>
> Since we can not lock the socket in tcp mmap() handler we have to
> split the operation in two phases.
>
> 1) mmap() on a tcp socket simply reserves VMA space, and nothing else.
>    This operation does not involve any TCP locking.
>
> 2) setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) implements
>   the transfert of pages from skbs to one VMA.
>    This operation only uses down_read(&current->mm->mmap_sem) after
>    holding TCP lock, thus solving the lockdep issue.


A quick question.  Is it a normal practice to return a result
in setsockopt() given that the optval parameter is supposed to
be a const void *?




--
K. Poon
ka-cheong.poon@xxxxxxxxxx