[PATCH 1/1] iov: iov tail helpers

[David Gibson <david@gibson.dropbear.id.au>]

In the vhost-user code we have a number of places where we need to locate
a particular header within the guest-supplied IO vector.  We need to work
out which buffer the header is in, and verify that it's contiguous and
aligned as we need.  At the moment this is open-coded, but introduce a
helper to make this more straightforward.

We add a new datatype 'struct iov_tail' representing an IO vector from
which we've logically consumed some number of headers.  The IOV_PULL_HEADER
macro consumes a new header from the vector, returning a pointer and
updating the iov_tail.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 iov.c | 83 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 iov.h | 24 +++++++++++++++++
 2 files changed, 107 insertions(+)

diff --git a/iov.c b/iov.c
index 3f9e229..3d384ae 100644
--- a/iov.c
+++ b/iov.c
@@ -156,3 +156,86 @@ size_t iov_size(const struct iovec *iov, size_t iov_cnt)
 
 	return len;
 }
+
+/**
+ * iov_tail_shorten() - Remove any buffers from an IOV tail that are wholly consumed
+ * @tail:	IO vector tail (modified)
+ *
+ * Return:	true if the tail still contains any bytes, otherwise false
+ */
+bool iov_tail_shorten(struct iov_tail *tail)
+{
+	size_t i;
+
+	i = iov_skip_bytes(tail->iov, tail->cnt, tail->off, &tail->off);
+	tail->iov += i;
+	tail->cnt -= i;
+
+	return !!tail->cnt;
+}
+
+/**
+ * iov_tail_size - Calculate the total size of an IO vector tail
+ * @tail:	IO vector tail
+ *
+ * Returns:    The total size in bytes.
+ */
+/* cppcheck-suppress unusedFunction */
+size_t iov_tail_size(struct iov_tail *tail)
+{
+	iov_tail_shorten(tail);
+	return iov_size(tail->iov, tail->cnt) - tail->off;
+}
+
+/**
+ * iov_peek_header_() - Get pointer to header from an IOV tail
+ * @tail:	IO vector tail to get header from
+ * @len:	Length of header to remove in bytes
+ * @align:	Required alignment of header in bytes
+ *
+ * @tail may be modified, but will be semantically equivalent.
+ *
+ * Returns:	Pointer to the removed header, NULL if it overruns the IO
+ *		vector, is not contiguous or is misaligned.
+ */
+void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align)
+{
+	char *p;
+
+	if (!iov_tail_shorten(tail))
+		return NULL; /* Nothing left */
+
+	if (tail->off + len < tail->off)
+		return NULL; /* Overflow */
+
+	if (tail->off + len > tail->iov[0].iov_len)
+		return NULL; /* Not contiguous */
+
+	p = (char *)tail->iov[0].iov_base + tail->off;
+	if ((uintptr_t)p % align)
+		return NULL; /* not aligned */
+
+	return p;
+}
+/**
+ * iov_pull_header_() - Remove a header from an IOV tail
+ * @tail:	IO vector tail to remove header from (modified)
+ * @len:	Length of header to remove in bytes
+ * @align:	Required alignment of header in bytes
+ *
+ * @tail is updated so that it no longer includes the extracted header
+ *
+ * Returns:	Pointer to the removed header, NULL if it overruns the IO
+ *		vector, is not contiguous or is misaligned.
+ */
+/* cppcheck-suppress unusedFunction */
+void *iov_pull_header_(struct iov_tail *tail, size_t len, size_t align)
+{
+	char *p = iov_peek_header_(tail, len, align);
+
+	if (!p)
+		return NULL;
+
+	tail->off = tail->off + len;
+	return p;
+}
diff --git a/iov.h b/iov.h
index a9e1722..a2f449c 100644
--- a/iov.h
+++ b/iov.h
@@ -28,4 +28,28 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
                   size_t offset, void *buf, size_t bytes);
 size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+
+/**
+ * struct iov_tail - Represents the fail portion of an IO vector
+ * @iov:	IO vector
+ * @cnt:	Number of entries in @iov
+ * @off:	Current offset in @iov
+ */
+struct iov_tail {
+	const struct iovec *iov;
+	size_t cnt, off;
+};
+
+#define IOV_TAIL(iov_, cnt_, off_) \
+	(struct iov_tail){ .iov = (iov_), .cnt = (cnt_), .off = (off_) }
+
+bool iov_tail_shorten(struct iov_tail *tail);
+size_t iov_tail_size(struct iov_tail *tail);
+void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align);
+#define IOV_PEEK_HEADER(tail_, ty_) \
+	((ty_ *)(iov_peek_header_((tail_), sizeof(ty_), __alignof__(ty_))))
+void *iov_pull_header_(struct iov_tail *tail, size_t len, size_t align);
+#define IOV_PULL_HEADER(tail_, ty_) \
+	((ty_ *)(iov_pull_header_((tail_), sizeof(ty_), __alignof__(ty_))))
+
 #endif /* IOVEC_H */
-- 
@@ -28,4 +28,28 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
                   size_t offset, void *buf, size_t bytes);
 size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+
+/**
+ * struct iov_tail - Represents the fail portion of an IO vector
+ * @iov:	IO vector
+ * @cnt:	Number of entries in @iov
+ * @off:	Current offset in @iov
+ */
+struct iov_tail {
+	const struct iovec *iov;
+	size_t cnt, off;
+};
+
+#define IOV_TAIL(iov_, cnt_, off_) \
+	(struct iov_tail){ .iov = (iov_), .cnt = (cnt_), .off = (off_) }
+
+bool iov_tail_shorten(struct iov_tail *tail);
+size_t iov_tail_size(struct iov_tail *tail);
+void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align);
+#define IOV_PEEK_HEADER(tail_, ty_) \
+	((ty_ *)(iov_peek_header_((tail_), sizeof(ty_), __alignof__(ty_))))
+void *iov_pull_header_(struct iov_tail *tail, size_t len, size_t align);
+#define IOV_PULL_HEADER(tail_, ty_) \
+	((ty_ *)(iov_pull_header_((tail_), sizeof(ty_), __alignof__(ty_))))
+
 #endif /* IOVEC_H */
-- 
2.47.0