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

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Nov 09, 2018 at 03:14:02PM -0800, Andy Lutomirski wrote:
> >>>> 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 don’t
> >> know if there are general VFS issues with that.
> > 

I don't think there is a way to fix this in /proc/pid/fd. At the proc
level, the /proc/pid/fd/N files are just soft symlinks that follow through to
the actual file. The open is actually done on that inode/file. I think
changing it the way being discussed here means changing the way symlinks work
in Linux.

I think the right way to fix this is at the memfd inode level. I am working
on a follow up patch on top of this patch, and will send that out in a few
days (along with the man page updates).

thanks!

 - Joel