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

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On Tue, Mar 16, 2021 at 12:42:57PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@xxxxxx>
> 
> This is essentially copy_struct_from_user() but for an iov_iter.
> 
> Suggested-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: Omar Sandoval <osandov@xxxxxx>
> ---
>  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.



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