Re: [PATCH v8 01/10] iov_iter: add copy_struct_from_iter()

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Mar 17, 2021 at 11:45:02AM -0700, Omar Sandoval wrote:
> On Wed, Mar 17, 2021 at 06:56:11PM +0100, Christian Brauner wrote:
> > On Tue, Mar 16, 2021 at 12:42:57PM -0700, Omar Sandoval wrote:
> > > From: Omar Sandoval <osandov@fb.com>
> > > 
> > > This is essentially copy_struct_from_user() but for an iov_iter.
> > > 
> > > Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
> > > Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> > > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > > ---
> > >  include/linux/uio.h |  2 ++
> > >  lib/iov_iter.c      | 82 +++++++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 84 insertions(+)
> > > 
> > > diff --git a/include/linux/uio.h b/include/linux/uio.h
> > > index 72d88566694e..f4e6ea85a269 100644
> > > --- a/include/linux/uio.h
> > > +++ b/include/linux/uio.h
> > > @@ -121,6 +121,8 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
> > >  			 struct iov_iter *i);
> > >  size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
> > >  			 struct iov_iter *i);
> > > +int copy_struct_from_iter(void *dst, size_t ksize, struct iov_iter *i,
> > > +			  size_t usize);
> > >  
> > >  size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i);
> > >  size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i);
> > > diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> > > index a21e6a5792c5..f45826ed7528 100644
> > > --- a/lib/iov_iter.c
> > > +++ b/lib/iov_iter.c
> > > @@ -948,6 +948,88 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
> > >  }
> > >  EXPORT_SYMBOL(copy_page_from_iter);
> > >  
> > > +/**
> > > + * copy_struct_from_iter - copy a struct from an iov_iter
> > > + * @dst: Destination buffer.
> > > + * @ksize: Size of @dst struct.
> > > + * @i: Source iterator.
> > > + * @usize: (Alleged) size of struct in @i.
> > > + *
> > > + * Copies a struct from an iov_iter in a way that guarantees
> > > + * backwards-compatibility for struct arguments in an iovec (as long as the
> > > + * rules for copy_struct_from_user() are followed).
> > > + *
> > > + * The recommended usage is that @usize be taken from the current segment:
> > > + *
> > > + *   int do_foo(struct iov_iter *i)
> > > + *   {
> > > + *     size_t usize = iov_iter_single_seg_count(i);
> > > + *     struct foo karg;
> > > + *     int err;
> > > + *
> > > + *     if (usize > PAGE_SIZE)
> > > + *       return -E2BIG;
> > > + *     if (usize < FOO_SIZE_VER0)
> > > + *       return -EINVAL;
> > > + *     err = copy_struct_from_iter(&karg, sizeof(karg), i, usize);
> > > + *     if (err)
> > > + *       return err;
> > > + *
> > > + *     // ...
> > > + *   }
> > > + *
> > > + * Return: 0 on success, -errno on error (see copy_struct_from_user()).
> > > + *
> > > + * On success, the iterator is advanced @usize bytes. On error, the iterator is
> > > + * not advanced.
> > > + */
> > > +int copy_struct_from_iter(void *dst, size_t ksize, struct iov_iter *i,
> > > +			  size_t usize)
> > > +{
> > > +	if (usize <= ksize) {
> > > +		if (!copy_from_iter_full(dst, usize, i))
> > > +			return -EFAULT;
> > > +		memset(dst + usize, 0, ksize - usize);
> > > +	} else {
> > > +		size_t copied = 0, copy;
> > > +		int ret;
> > > +
> > > +		if (WARN_ON(iov_iter_is_pipe(i)) || unlikely(i->count < usize))
> > > +			return -EFAULT;
> > > +		if (iter_is_iovec(i))
> > > +			might_fault();
> > > +		iterate_all_kinds(i, usize, v, ({
> > > +			copy = min(ksize - copied, v.iov_len);
> > > +			if (copy && copyin(dst + copied, v.iov_base, copy))
> > > +				return -EFAULT;
> > > +			copied += copy;
> > > +			ret = check_zeroed_user(v.iov_base + copy,
> > > +						v.iov_len - copy);
> > > +			if (ret <= 0)
> > > +				return ret ?: -E2BIG;
> > > +			0;}), ({
> > > +			char *addr = kmap_atomic(v.bv_page);
> > > +			copy = min_t(size_t, ksize - copied, v.bv_len);
> > > +			memcpy(dst + copied, addr + v.bv_offset, copy);
> > > +			copied += copy;
> > > +			ret = memchr_inv(addr + v.bv_offset + copy, 0,
> > > +					 v.bv_len - copy) ? -E2BIG : 0;
> > > +			kunmap_atomic(addr);
> > > +			if (ret)
> > > +				return ret;
> > > +			}), ({
> > > +			copy = min(ksize - copied, v.iov_len);
> > > +			memcpy(dst + copied, v.iov_base, copy);
> > > +			if (memchr_inv(v.iov_base, 0, v.iov_len))
> > > +				return -E2BIG;
> > > +			})
> > > +		)
> > 
> > 
> > Following the semantics of copy_struct_from_user() is certainly a good
> > idea but can this in any way be rewritten to not look like this; at
> > least not as crammed. It's a bit painful to follow here what's going.
> 
> I think that's just the nature of the iov_iter code :) I'm just
> following the rest of this file, which uses some mind-expanding macros.
> Do you have any suggestions for how to clean this function up?

I think the follow-up discussion this triggered caused an improvement now. :)
Christian