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.p...@oracle.com