Re: [PATCH][next] nfsd: Replace one-element array with flexible-array member

From: Gustavo A. R. Silva
Date: Wed May 24 2023 - 00:39:54 EST

On 5/23/23 19:31, Chuck Lever wrote:
On Tue, May 23, 2023 at 07:11:37PM -0600, Gustavo A. R. Silva wrote:

On 5/23/23 19:01, Chuck Lever wrote:
On Tue, May 23, 2023 at 06:44:23PM -0600, Gustavo A. R. Silva wrote:
One-element arrays are deprecated, and we are replacing them with
flexible array members instead. So, replace a one-element array
with a flexible-arrayº member in struct vbi_anc_data and refactor

I don't know what "struct vbi_anc_data" is. Is the patch description

Oops, copy/paste error. I'll fix it up. :)

the rest of the code, accordingly.

This results in no differences in binary output.

Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

fs/nfsd/nfs4callback.c | 2 +-
fs/nfsd/xdr4.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index 4039ffcf90ba..2c688d51135d 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -353,7 +353,7 @@ encode_cb_recallany4args(struct xdr_stream *xdr,
encode_nfs_cb_opnum4(xdr, OP_CB_RECALL_ANY);
encode_uint32(xdr, ra->ra_keep);
- encode_bitmap4(xdr, ra->ra_bmval, ARRAY_SIZE(ra->ra_bmval));
+ encode_bitmap4(xdr, ra->ra_bmval, 1);

I find the new code less self-documenting.

diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h
index 510978e602da..68072170eac8 100644
--- a/fs/nfsd/xdr4.h
+++ b/fs/nfsd/xdr4.h
@@ -899,7 +899,7 @@ struct nfsd4_operation {
struct nfsd4_cb_recall_any {
struct nfsd4_callback ra_cb;
u32 ra_keep;
- u32 ra_bmval[1];
+ u32 ra_bmval[];

This is not a placeholder for "1 or more elements". We actually want
just a single u32 element in this array. Doesn't this change the
sizeof(struct nfsd4_cb_recall_any) ?

I see. Yes, it does change the size. Can we replace it with a simple
object of type u32? or do you actually need this to stay an array?

It's not impossible to make it a scalar u32, however...

In this area of code, *_bmval is always a bitmap -- an array of u32s.
Helpers like encode_bitmap4() assume an array. I think it would be
less confusing overall to human readers if it remained an array.

In this case, it is a single element array because CB_RECALL_ANY
doesn't happen to use bits above the first 32-bit word of the

I see. If this is never going to be treated as a flexible array, then
it can stay as is.

-fstrict-flex-arrays=3 should not warn about this because the array
will never ever be tried to be accessed beyond element 1. :)

Thanks for the feedback!

We could make it a 2-element array, I think, without harm. Send a
patch for that, and Dai can test it to make sure there are no
unexpected interoperability consequences.

I hope that would avoid suspicious-looking array definitions.