[PATCH] sunrpc: account for xdr->page_base in xdr_alloc_bvec

[Jeff Layton]

I've been seeing a regression in mainline (v6.5-rc) kernels where
unaligned reads were returning corrupt data.

9d96acbc7f37 added a routine to allocate and populate a bvec array that
can be used to back an iov_iter. When it does this, it always sets the
offset in the first bvec to zero, even when the xdr->page_base is
non-zero.

The old code in svc_tcp_sendmsg used to account for this, as it was
sending the pages one at a time anyway, but now that we just hand the
iov to the network layer, we need to ensure that the bvecs are properly
initialized.

Fix xdr_alloc_bvec to set the offset in the first bvec to the offset
indicated by xdr->page_base, and then 0 in all subsequent bvecs.

Fixes: 9d96acbc7f37 ("SUNRPC: Add a bvec array to struct xdr_buf for use with iovec_iter()")
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
NB: This is only lightly tested so far, but it seems to fix the pynfs
regressions I've been seeing.
---
 net/sunrpc/xdr.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index 2a22e78af116..d0f5fc8605b8 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -144,6 +144,7 @@ int
 xdr_alloc_bvec(struct xdr_buf *buf, gfp_t gfp)
 {
 	size_t i, n = xdr_buf_pagecount(buf);
+	unsigned int offset = offset_in_page(buf->page_base);
 
 	if (n != 0 && buf->bvec == NULL) {
 		buf->bvec = kmalloc_array(n, sizeof(buf->bvec[0]), gfp);
@@ -151,7 +152,8 @@ xdr_alloc_bvec(struct xdr_buf *buf, gfp_t gfp)
 			return -ENOMEM;
 		for (i = 0; i < n; i++) {
 			bvec_set_page(&buf->bvec[i], buf->pages[i], PAGE_SIZE,
-				      0);
+				      offset);
+			offset = 0;
 		}
 	}
 	return 0;

---
base-commit: 2ccdd1b13c591d306f0401d98dedc4bdcd02b421
change-id: 20230814-sendpage-b04874eed249

Best regards,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>