Re: [RFC PATCH 1/4] splice: Fix corruption of spliced data after splice() returns

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 19 Jul 2023 at 19:59, Matt Whitlock <kernel@mattwhitlock.name> wrote:
>
> On Wednesday, 19 July 2023 06:17:51 EDT, Miklos Szeredi wrote:
> > On Thu, 29 Jun 2023 at 17:56, David Howells <dhowells@redhat.com> wrote:
> >>
> >> Splicing data from, say, a file into a pipe currently leaves the source
> >> pages in the pipe after splice() returns - but this means that those pages
> >> can be subsequently modified by shared-writable mmap(), write(),
> >> fallocate(), etc. before they're consumed.
> >
> > What is this trying to fix?   The above behavior is well known, so
> > it's not likely to be a problem.
>
> Respectfully, it's not well-known, as it's not documented. If the splice(2)
> man page had mentioned that pages can be mutated after they're already
> ostensibly at rest in the output pipe buffer, then my nightly backups
> wouldn't have been incurring corruption silently for many months.

splice(2):

       Though we talk of copying, actual copies are generally avoided.
The kernel does this by implementing a pipe buffer as a set  of
refer‐
       ence-counted  pointers  to  pages  of kernel memory.  The
kernel creates "copies" of pages in a buffer by creating new pointers
(for the
       output buffer) referring to the pages, and increasing the
reference counts for the pages: only pointers are copied, not the
pages of the
       buffer.

While not explicitly stating that the contents of the pages can change
after being spliced, this can easily be inferred from the above
semantics.

Thanks,
Miklos