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:

quoted

From: Omar Sandoval <redacted>

This is essentially copy_struct_from_user() but for an iov_iter.

Suggested-by: Aleksa Sarai <redacted>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Omar Sandoval <redacted>
---
 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?