git: 719e307f80c7 - main - bhyve: Cast away const when fetching a config nvlist

The branch main has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=719e307f80c724a39814557e35ac890c75dcd402

commit 719e307f80c724a39814557e35ac890c75dcd402
Author:     Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2022-11-11 15:02:42 +0000
Commit:     Mark Johnston <markj@FreeBSD.org>
CommitDate: 2022-11-11 15:02:42 +0000

    bhyve: Cast away const when fetching a config nvlist
    
    Silence a warning from the compiler about "const" being discarded.  The
    warning is correct: nvlist values are supposed to be immutable.
    However, fixing this properly will require some contortions on behalf of
    consumers who look up a subtree of the config and modify it.  Per a
    discussion on freebsd-virtualization@, the solution will probably be to
    outright replace the use of nvlists for VM configuration, but until that
    happens let's document the problem and silence the warning.
    
    No functional change intended.
    
    MFC after:      2 weeks
    Reviewed by:    corvink, jhb
    Differential Revision:  https://reviews.freebsd.org/D37293
---
 usr.sbin/bhyve/config.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/usr.sbin/bhyve/config.c b/usr.sbin/bhyve/config.c
index de138795f4d1..6339ddb9f2ee 100644
--- a/usr.sbin/bhyve/config.c
+++ b/usr.sbin/bhyve/config.c
@@ -65,7 +65,17 @@ _lookup_config_node(nvlist_t *parent, const char *path, bool create)
 			break;
 		}
 		if (nvlist_exists_nvlist(nvl, name))
-			nvl = (nvlist_t *)nvlist_get_nvlist(nvl, name);
+			/*
+			 * XXX-MJ it is incorrect to cast away the const
+			 * qualifier like this since the contract with nvlist
+			 * says that values are immuatable, and some consumers
+			 * will indeed add nodes to the returned nvlist.  In
+			 * practice, however, it appears to be harmless with the
+			 * current nvlist implementation, so we just live with
+			 * it until the implementation is reworked.
+			 */
+			nvl = __DECONST(nvlist_t *,
+			    nvlist_get_nvlist(nvl, name));
 		else if (nvlist_exists(nvl, name)) {
 			for (copy = tofree; copy < name; copy++)
 				if (*copy == '\0')
@@ -76,6 +86,10 @@ _lookup_config_node(nvlist_t *parent, const char *path, bool create)
 			nvl = NULL;
 			break;
 		} else if (create) {
+			/*
+			 * XXX-MJ as with the case above, "new_nvl" shouldn't be
+			 * mutated after its ownership is given to "nvl".
+			 */
 			new_nvl = nvlist_create(0);
 			if (new_nvl == NULL)
 				errx(4, "Failed to allocate memory");