[PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

[Andy Lutomirski]

On Sun, Sep 20, 2020 at 11:07 AM Al Viro <viro at zeniv.linux.org.uk> wrote:
>
> On Sun, Sep 20, 2020 at 04:15:10PM +0100, Matthew Wilcox wrote:
> > On Fri, Sep 18, 2020 at 02:45:25PM +0200, Christoph Hellwig wrote:
> > > Add a flag to force processing a syscall as a compat syscall.  This is
> > > required so that in_compat_syscall() works for I/O submitted by io_uring
> > > helper threads on behalf of compat syscalls.
> >
> > Al doesn't like this much, but my suggestion is to introduce two new
> > opcodes -- IORING_OP_READV32 and IORING_OP_WRITEV32.  The compat code
> > can translate IORING_OP_READV to IORING_OP_READV32 and then the core
> > code can know what that user pointer is pointing to.
>
> Let's separate two issues:
>         1) compat syscalls want 32bit iovecs.  Nothing to do with the
> drivers, dealt with just fine.
>         2) a few drivers are really fucked in head.  They use different
> *DATA* layouts for reads/writes, depending upon the calling process.
> IOW, if you fork/exec a 32bit binary and your stdin is one of those,
> reads from stdin in parent and child will yield different data layouts.
> On the same struct file.
>         That's what Christoph worries about (/dev/sg he'd mentioned is
> one of those).
>
>         IMO we should simply have that dozen or so of pathological files
> marked with FMODE_SHITTY_ABI; it's not about how they'd been opened -
> it describes the userland ABI provided by those.  And it's cast in stone.
>

I wonder if this is really quite cast in stone.  We could also have
FMODE_SHITTY_COMPAT and set that when a file like this is *opened* in
compat mode.  Then that particular struct file would be read and
written using the compat data format.  The change would be
user-visible, but the user that would see it would be very strange
indeed.

I don't have a strong opinion as to whether that is better or worse
than denying io_uring access to these things, but at least it moves
the special case out of io_uring.

--Andy