[PATCH RERESEND v9 1/9] iov_iter: add copy_struct_from_iter()

[Omar Sandoval <osandov@osandov.com>]

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 |  1 +
 lib/iov_iter.c      | 91 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 92 insertions(+)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index d3ec87706d75..cbaf6b3bfcbc 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -129,6 +129,7 @@ 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 _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 c701b7a187f2..129f264416ff 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -995,6 +995,97 @@ 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.
+ *
+ * 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 source struct is assumed to be stored in the current segment of the
+ * iov_iter, and its size is the size of the current segment. The iov_iter must
+ * be positioned at the beginning of the current segment.
+ *
+ * The recommended usage is something like the following:
+ *
+ *   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);
+ *     if (err)
+ *       return err;
+ *
+ *     // ...
+ *   }
+ *
+ * Returns 0 on success or one of the following errors:
+ *  * -E2BIG:  (size of current segment > @ksize) and there are non-zero
+ *             trailing bytes in the current segment.
+ *  * -EFAULT: access to userspace failed.
+ *  * -EINVAL: the iterator is not at the beginning of the current segment.
+ *
+ * On success, the iterator is advanced to the next segment. On error, the
+ * iterator is not advanced.
+ */
+int copy_struct_from_iter(void *dst, size_t ksize, struct iov_iter *i)
+{
+	size_t usize;
+	int ret;
+
+	if (i->iov_offset != 0)
+		return -EINVAL;
+	if (iter_is_iovec(i)) {
+		usize = i->iov->iov_len;
+		might_fault();
+		if (copyin(dst, i->iov->iov_base, min(ksize, usize)))
+			return -EFAULT;
+		if (usize > ksize) {
+			ret = check_zeroed_user(i->iov->iov_base + ksize,
+						usize - ksize);
+			if (ret < 0)
+				return ret;
+			else if (ret == 0)
+				return -E2BIG;
+		}
+	} else if (iov_iter_is_kvec(i)) {
+		usize = i->kvec->iov_len;
+		memcpy(dst, i->kvec->iov_base, min(ksize, usize));
+		if (usize > ksize &&
+		    memchr_inv(i->kvec->iov_base + ksize, 0, usize - ksize))
+			return -E2BIG;
+	} else if (iov_iter_is_bvec(i)) {
+		char *p;
+
+		usize = i->bvec->bv_len;
+		p = kmap_local_page(i->bvec->bv_page);
+		memcpy(dst, p + i->bvec->bv_offset, min(ksize, usize));
+		if (usize > ksize &&
+		    memchr_inv(p + i->bvec->bv_offset + ksize, 0,
+			       usize - ksize)) {
+			kunmap_local(p);
+			return -E2BIG;
+		}
+		kunmap_local(p);
+	} else {
+		return -EFAULT;
+	}
+	if (usize < ksize)
+		memset(dst + usize, 0, ksize - usize);
+	iov_iter_advance(i, usize);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(copy_struct_from_iter);
+
 static size_t pipe_zero(size_t bytes, struct iov_iter *i)
 {
 	struct pipe_inode_info *pipe = i->pipe;
-- 
2.31.1