Re: [PATCH 1/2] mm/memory: ensure fork child sees coherent memory snapshot

[Jann Horn <jannh@google.com>]

On Tue, Jun 3, 2025 at 8:29 PM Matthew Wilcox <willy@infradead.org> wrote:
> On Tue, Jun 03, 2025 at 08:21:02PM +0200, Jann Horn wrote:
> > When fork() encounters possibly-pinned pages, those pages are immediately
> > copied instead of just marking PTEs to make CoW happen later. If the parent
> > is multithreaded, this can cause the child to see memory contents that are
> > inconsistent in multiple ways:
> >
> > 1. We are copying the contents of a page with a memcpy() while userspace
> >    may be writing to it. This can cause the resulting data in the child to
> >    be inconsistent.
> > 2. After we've copied this page, future writes to other pages may
> >    continue to be visible to the child while future writes to this page are
> >    no longer visible to the child.
> >
> > This means the child could theoretically see incoherent states where
> > allocator freelists point to objects that are actually in use or stuff like
> > that. A mitigating factor is that, unless userspace already has a deadlock
> > bug, userspace can pretty much only observe such issues when fancy lockless
> > data structures are used (because if another thread was in the middle of
> > mutating data during fork() and the post-fork child tried to take the mutex
> > protecting that data, it might wait forever).
>
> Um, OK, but isn't that expected behaviour?  POSIX says:

I don't think it is expected behavior that locklessly-updated data
structures in application code could break.

> : A process shall be created with a single thread. If a multi-threaded
> : process calls fork(), the new process shall contain a replica of the
> : calling thread and its entire address space, possibly including the
> : states of mutexes and other resources. Consequently, the application
> : shall ensure that the child process only executes async-signal-safe
> : operations until such time as one of the exec functions is successful.

I think that is only talking about ways in which you can interact with
libc, and does not mean that application code couldn't access its own
lockless data structures or such.

Though admittedly that is a fairly theoretical point, since in
practice the most likely place where you'd encounter this kind of
issue would be in an allocator implementation or such.

> It's always been my understanding that you really, really shouldn't call
> fork() from a multithreaded process.