[PATCH][next] nfs41: pnfs: filelayout: Replace one-element array with flexible-array member

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code a bit according to the use of a flexible-array member
in struct nfs4_file_layout_dsaddr instead of a one-element array, and
use the struct_size() helper.

This helps with the ongoing efforts to globally enable -Warray-bounds
and get us closer to being able to tighten the FORTIFY_SOURCE routines
on memcpy().

This issue was found with the help of Coccinelle and audited and fixed,
manually.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 fs/nfs/filelayout/filelayout.h    | 2 +-
 fs/nfs/filelayout/filelayoutdev.c | 4 +---
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/nfs/filelayout/filelayout.h b/fs/nfs/filelayout/filelayout.h
index 79323b5dab0c..aed0748fd6ec 100644
--- a/fs/nfs/filelayout/filelayout.h
+++ b/fs/nfs/filelayout/filelayout.h
@@ -51,7 +51,7 @@ struct nfs4_file_layout_dsaddr {
 	u32				stripe_count;
 	u8				*stripe_indices;
 	u32				ds_num;
-	struct nfs4_pnfs_ds		*ds_list[1];
+	struct nfs4_pnfs_ds		*ds_list[];
 };
 
 struct nfs4_filelayout_segment {
diff --git a/fs/nfs/filelayout/filelayoutdev.c b/fs/nfs/filelayout/filelayoutdev.c
index 86c3f7e69ec4..acf4b88889dc 100644
--- a/fs/nfs/filelayout/filelayoutdev.c
+++ b/fs/nfs/filelayout/filelayoutdev.c
@@ -136,9 +136,7 @@ nfs4_fl_alloc_deviceid_node(struct nfs_server *server, struct pnfs_device *pdev,
 		goto out_err_free_stripe_indices;
 	}
 
-	dsaddr = kzalloc(sizeof(*dsaddr) +
-			(sizeof(struct nfs4_pnfs_ds *) * (num - 1)),
-			gfp_flags);
+	dsaddr = kzalloc(struct_size(dsaddr, ds_list, num), gfp_flags);
 	if (!dsaddr)
 		goto out_err_free_stripe_indices;
 
-- 
2.27.0