[REGRESSION] mm: process_vm_readv testcase no longer works after compat_prcoess_vm_readv removed

[Al Viro]

On Mon, Oct 26, 2020 at 05:56:11PM -0600, Jens Axboe wrote:
> On 10/26/20 4:55 PM, Kyle Huey wrote:
> > A test program from the rr[0] test suite, vm_readv_writev[1], no
> > longer works on 5.10-rc1 when compiled as a 32 bit binary and executed
> > on a 64 bit kernel. The first process_vm_readv call (on line 35) now
> > fails with EFAULT. I have bisected this to
> > c3973b401ef2b0b8005f8074a10e96e3ea093823.
> > 
> > It should be fairly straightforward to extract the test case from our
> > repository into a standalone program.
> 
> Can you check with this applied?
> 
> diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
> index fd12da80b6f2..05676722d9cd 100644
> --- a/mm/process_vm_access.c
> +++ b/mm/process_vm_access.c
> @@ -273,7 +273,8 @@ static ssize_t process_vm_rw(pid_t pid,
>  		return rc;
>  	if (!iov_iter_count(&iter))
>  		goto free_iov_l;
> -	iov_r = iovec_from_user(rvec, riovcnt, UIO_FASTIOV, iovstack_r, false);
> +	iov_r = iovec_from_user(rvec, riovcnt, UIO_FASTIOV, iovstack_r,
> +				in_compat_syscall());

_ouch_

There's a bug, all right, but I'm not sure that this is all there is to it.
For now it's probably the right fix, but...  Consider the fun trying to
use that from 32bit process to access the memory of 64bit one.  IOW, we
might want to add an explicit flag for "force 64bit addresses/sizes
in rvec".