nvmet-fc: Remove __counted_by from nvmet_fc_tgt_queue.fod[]

Work for __counted_by on generic pointers in structures (not just
flexible array members) has started landing in Clang 19 (current tip of
tree). During the development of this feature, a restriction was added
to __counted_by to prevent the flexible array member's element type from
including a flexible array member itself such as:

  struct foo {
    int count;
    char buf[];
  };

  struct bar {
    int count;
    struct foo data[] __counted_by(count);
  };

because the size of data cannot be calculated with the standard array
size formula:

  sizeof(struct foo) * count

This restriction was downgraded to a warning but due to CONFIG_WERROR,
it can still break the build. The application of __counted_by on the fod
member of 'struct nvmet_fc_tgt_queue' triggers this restriction,
resulting in:

  drivers/nvme/target/fc.c:151:2: error: 'counted_by' should not be applied to an array with element of unknown size because 'struct nvmet_fc_fcp_iod' is a struct type with a flexible array member. This will be an error in a future compiler version [-Werror,-Wbounds-safety-counted-by-elt-type-unknown-size]
    151 |         struct nvmet_fc_fcp_iod         fod[] __counted_by(sqsize);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

Remove this use of __counted_by to fix the warning/error. However,
rather than remove it altogether, leave it commented, as it may be
possible to support this in future compiler releases.

Cc: [email protected]
Closes: ClangBuiltLinux/linux#2027
Fixes: ccd3129 ("nvmet-fc: Annotate struct nvmet_fc_tgt_queue with __counted_by")
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Keith Busch <[email protected]>

Author: nathanchance

drivers/nvme/target/fc.c

@@ -148,7 +148,7 @@ struct nvmet_fc_tgt_queue {
 	struct workqueue_struct		*work_q;
 	struct kref			ref;
 	/* array of fcp_iods */
-	struct nvmet_fc_fcp_iod		fod[] __counted_by(sqsize);
+	struct nvmet_fc_fcp_iod		fod[] /* __counted_by(sqsize) */;
 } __aligned(sizeof(unsigned long long));
 
 struct nvmet_fc_hostport {