On Sun, Sep 20, 2020 at 11:07 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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
