Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

[Daniel Colascione]

On Fri, Nov 9, 2018 at 2:37 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> Another, more general fix might be to prevent /proc/pid/fd/N opens
>> from "upgrading" access modes. But that'd be a bigger ABI break.
>
> I think we should fix that, too.  I consider it a bug fix, not an ABI break, personally.

Someone, somewhere is probably relying on it though, and that means
that we probably can't change it unless it's actually causing
problems.

<mumble>spacebar heating</mumble>

>>> That aside: I wonder whether a better API would be something that
>>> allows you to create a new readonly file descriptor, instead of
>>> fiddling with the writability of an existing fd.
>>
>> That doesn't work, unfortunately. The ashmem API we're replacing with
>> memfd requires file descriptor continuity. I also looked into opening
>> a new FD and dup2(2)ing atop the old one, but this approach doesn't
>> work in the case that the old FD has already leaked to some other
>> context (e.g., another dup, SCM_RIGHTS). See
>> https://developer.android.com/ndk/reference/group/memory. We can't
>> break ASharedMemory_setProt.
>
>
> Hmm.  If we fix the general reopen bug, a way to drop write access from an existing struct file would do what Android needs, right?  I donât know if there are general VFS issues with that.

I also proposed that. :-) Maybe it'd work best as a special case of
the perennial revoke(2) that people keep proposing. You'd be able to
selectively revoke all access or just write access.