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

[David Hildenbrand <david@redhat.com>]

On 03.06.25 20:29, Matthew Wilcox 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:
> 
> : 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.
> 
> It's always been my understanding that you really, really shouldn't call
> fork() from a multithreaded process.

I have the same recollection, but rather because of concurrent O_DIRECT 
and locking (pthread_atfork ...).

Using the allocator above example: what makes sure that no other thread 
is halfway through modifying allocator state? You really have to sync 
somehow before calling fork() -- e.g., grabbing allocator locks in 
pthread_atfork().


For Linux we document in the man page

"After  a  fork() in a multithreaded program, the child can safely call 
only async-signal-safe functions (see signal-safety(7)) until such time 
as it calls execve(2)."

-- 
Cheers,

David / dhildenb