Re: AF_UNIX/zerocopy/pipe/vmsplice/splice vs FOLL_PIN

[Christian Brauner]

On Mon, May 12, 2025 at 03:51:30PM +0100, David Howells wrote:
> I'm looking at how to make sendmsg() handle page pinning - and also working
> towards supporting the page refcount eventually being removed and only being
> available with certain memory types.
> 
> One of the outstanding issues is in sendmsg().  Analogously with DIO writes,
> sendmsg() should be pinning memory (FOLL_PIN/GUP) rather than simply getting
> refs on it before it attaches it to an sk_buff.  Without this, if memory is
> spliced into an AF_UNIX socket and then the process forks, that memory gets
> attached to the child process, and the child can alter the data, probably by
> accident, if the memory is on the stack or in the heap.
> 
> Further, kernel services can use MSG_SPLICE_PAGES to attach memory directly to
> an AF_UNIX pipe (though I'm not sure if anyone actually does this).

I would possible be interested in using this for the coredump af_unix socket.