Re: linux-next: build failure after merge of the devsec-tsm tree

[Dan Williams]

Stephen Rothwell wrote:
> Hi all,
> 
> After merging the devsec-tsm tree, today's linux-next build (x86_64
> allmodconfig) failed like this:
> 
> drivers/virt/coco/guest/tsm-mr.c: In function 'tsm_mr_create_attribute_group':
> drivers/virt/coco/guest/tsm-mr.c:228:29: error: assignment to 'const struct bin_attribute * const*' from incompatible pointer type 'struct bin_attribute **' [-Wincompatible-pointer-types]
>   228 |         ctx->agrp.bin_attrs = no_free_ptr(bas);
>       |                             ^
> 
> Caused by commit
> 
>   29b07a7b8f41 ("tsm-mr: Add TVM Measurement Register support")
> 
> interacting with commit
> 
>   9bec944506fa ("sysfs: constify attribute_group::bin_attrs")
> 
> from the driver-core tree.
> 
> I have applied the following merge resolution for today (there must be
> a better solution).

Indeed.

So it looks like while there are plenty of dynamic binary attribute
creation users (see sysfs_bin_attr_init() callers). There are zero that
attempt to assign dynamically allocated attributes to be registered by a
static @groups.

The @groups publishing model is preferable because the lifetime rules
are all handled by the driver core at device add/del time.

So, while there is still casting involved, I think a better solution is
to make the allocation const and then cast for init ala incremental
patch below. Cedric, if this looks ok to you I'll send out another
partial-reroll to get this fixed up so the build breakage stays out of
bisection runs.

-- 8< --
diff --git a/drivers/virt/coco/tsm-mr.c b/drivers/virt/coco/tsm-mr.c
index d75b08548292..7fe90fae2738 100644
--- a/drivers/virt/coco/tsm-mr.c
+++ b/drivers/virt/coco/tsm-mr.c
@@ -169,24 +169,27 @@ tsm_mr_create_attribute_group(const struct tsm_measurements *tm)
 	}
 
 	/*
-	 * @bas and the MR name strings are combined into a single allocation
+	 * @attrs and the MR name strings are combined into a single allocation
 	 * so that we don't have to free MR names one-by-one in
 	 * tsm_mr_free_attribute_group()
 	 */
-	struct bin_attribute **bas __free(kfree) =
-		kzalloc(sizeof(*bas) * (tm->nr_mrs + 1) + nlen, GFP_KERNEL);
+	const struct bin_attribute * const *attrs __free(kfree) =
+		kzalloc(sizeof(*attrs) * (tm->nr_mrs + 1) + nlen, GFP_KERNEL);
 	struct tm_context *ctx __free(kfree) =
 		kzalloc(struct_size(ctx, mrs, tm->nr_mrs), GFP_KERNEL);
 	char *name, *end;
 
-	if (!ctx || !bas)
+	if (!ctx || !attrs)
 		return ERR_PTR(-ENOMEM);
 
-	/* @bas is followed immediately by MR name strings */
-	name = (char *)&bas[tm->nr_mrs + 1];
+	/* @attrs is followed immediately by MR name strings */
+	name = (char *)&attrs[tm->nr_mrs + 1];
 	end = name + nlen;
 
 	for (size_t i = 0; i < tm->nr_mrs; ++i) {
+		/* break const for init */
+		struct bin_attribute **bas = (struct bin_attribute **)attrs;
+
 		bas[i] = &ctx->mrs[i];
 		sysfs_bin_attr_init(bas[i]);
 
@@ -225,7 +228,7 @@ tsm_mr_create_attribute_group(const struct tsm_measurements *tm)
 
 	init_rwsem(&ctx->rwsem);
 	ctx->agrp.name = "measurements";
-	ctx->agrp.bin_attrs = no_free_ptr(bas);
+	ctx->agrp.bin_attrs = no_free_ptr(attrs);
 	ctx->tm = tm;
 	return &no_free_ptr(ctx)->agrp;
 }