sysctl(9): Ease exporting struct sizes; Discourage doing that

Introduce two helpers, the more general SYSCTL_SIZEOF() and
a struct-specific one SYSCTL_SIZEOF_STRUCT() which prepends 'struct' in
the description and in the use of sizeof() but uses the raw structure
name as the knob's name.  The size of the object/structure is exported
under 'debug.sizeof'.

Existing knobs under 'debug.sizeof' were all converted to use the
helpers.

Add a note before the helpers discouraging the introduction of new
leaves for ad-hoc reasons.  List alternative means for developers to
obtain the size of arbitrary kernel structures easily (thanks to markj@
for providing these).

No functional change (intended).

Reviewed by:    kib, markj
MFC after:      3 days
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D50121

sys/contrib/openzfs/module/os/freebsd/zfs/zfs_znode_os.c

@@ -67,8 +67,12 @@
 #include "zfs_comutil.h"
 
 /* Used by fstat(1). */
+#ifdef SYSCTL_SIZEOF
+SYSCTL_SIZEOF(znode, znode_t);
+#else
 SYSCTL_INT(_debug_sizeof, OID_AUTO, znode, CTLFLAG_RD,
 	SYSCTL_NULL_INT_PTR, sizeof (znode_t), "sizeof(znode_t)");
+#endif
 
 /*
  * Define ZNODE_STATS to turn on statistic gathering. By default, it is only

sys/fs/devfs/devfs_devs.c

@@ -121,11 +121,8 @@ SYSCTL_PROC(_kern, OID_AUTO, devname,
     CTLTYPE_OPAQUE|CTLFLAG_RW|CTLFLAG_ANYBODY|CTLFLAG_MPSAFE,
     NULL, 0, sysctl_devname, "", "devname(3) handler");
 
-SYSCTL_INT(_debug_sizeof, OID_AUTO, cdev, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct cdev), "sizeof(struct cdev)");
-
-SYSCTL_INT(_debug_sizeof, OID_AUTO, cdev_priv, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct cdev_priv), "sizeof(struct cdev_priv)");
+SYSCTL_SIZEOF_STRUCT(cdev);
+SYSCTL_SIZEOF_STRUCT(cdev_priv);
 
 struct cdev *
 devfs_alloc(int flags)

sys/geom/geom_kern.c

@@ -229,13 +229,8 @@ SYSCTL_INT(_kern_geom, OID_AUTO, collectstats, CTLFLAG_RW,
 	&g_collectstats, 0,
 	"Control statistics collection on GEOM providers and consumers");
 
-SYSCTL_INT(_debug_sizeof, OID_AUTO, g_class, CTLFLAG_RD,
-	SYSCTL_NULL_INT_PTR, sizeof(struct g_class), "sizeof(struct g_class)");
-SYSCTL_INT(_debug_sizeof, OID_AUTO, g_geom, CTLFLAG_RD,
-	SYSCTL_NULL_INT_PTR, sizeof(struct g_geom), "sizeof(struct g_geom)");
-SYSCTL_INT(_debug_sizeof, OID_AUTO, g_provider, CTLFLAG_RD,
-	SYSCTL_NULL_INT_PTR, sizeof(struct g_provider), "sizeof(struct g_provider)");
-SYSCTL_INT(_debug_sizeof, OID_AUTO, g_consumer, CTLFLAG_RD,
-	SYSCTL_NULL_INT_PTR, sizeof(struct g_consumer), "sizeof(struct g_consumer)");
-SYSCTL_INT(_debug_sizeof, OID_AUTO, g_bioq, CTLFLAG_RD,
-	SYSCTL_NULL_INT_PTR, sizeof(struct g_bioq), "sizeof(struct g_bioq)");
+SYSCTL_SIZEOF_STRUCT(g_class);
+SYSCTL_SIZEOF_STRUCT(g_geom);
+SYSCTL_SIZEOF_STRUCT(g_provider);
+SYSCTL_SIZEOF_STRUCT(g_consumer);
+SYSCTL_SIZEOF_STRUCT(g_bioq);

sys/kern/kern_mib.c

@@ -730,11 +730,9 @@ SYSCTL_STRING(_user, USER_LOCALBASE, localbase, CTLFLAG_RWTUN,
     localbase, sizeof(localbase), "Prefix used to install and locate add-on packages");
 
 #include <sys/vnode.h>
-SYSCTL_INT(_debug_sizeof, OID_AUTO, vnode, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct vnode), "sizeof(struct vnode)");
+SYSCTL_SIZEOF_STRUCT(vnode);
 
-SYSCTL_INT(_debug_sizeof, OID_AUTO, proc, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct proc), "sizeof(struct proc)");
+SYSCTL_SIZEOF_STRUCT(proc);
 
 static int
 sysctl_kern_pid_max(SYSCTL_HANDLER_ARGS)
@@ -770,19 +768,15 @@ SYSCTL_INT(_kern, OID_AUTO, pid_max_limit, CTLFLAG_RD,
 
 #include <sys/bio.h>
 #include <sys/buf.h>
-SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct bio), "sizeof(struct bio)");
-SYSCTL_INT(_debug_sizeof, OID_AUTO, buf, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct buf), "sizeof(struct buf)");
+SYSCTL_SIZEOF_STRUCT(bio);
+SYSCTL_SIZEOF_STRUCT(buf);
 
 #include <sys/user.h>
-SYSCTL_INT(_debug_sizeof, OID_AUTO, kinfo_proc, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct kinfo_proc), "sizeof(struct kinfo_proc)");
+SYSCTL_SIZEOF_STRUCT(kinfo_proc);
 
 /* Used by kernel debuggers. */
 const int pcb_size = sizeof(struct pcb);
-SYSCTL_INT(_debug_sizeof, OID_AUTO, pcb, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct pcb), "sizeof(struct pcb)");
+SYSCTL_SIZEOF_STRUCT(pcb);
 
 /* XXX compatibility, remove for 6.0 */
 #include <sys/imgact.h>

sys/kern/subr_devstat.c

@@ -599,5 +599,4 @@ devstat_free(struct devstat *dsp)
 	}
 }
 
-SYSCTL_INT(_debug_sizeof, OID_AUTO, devstat, CTLFLAG_RD,
-    SYSCTL_NULL_INT_PTR, sizeof(struct devstat), "sizeof(struct devstat)");
+SYSCTL_SIZEOF_STRUCT(devstat);

sys/kern/vfs_cache.c

@@ -627,8 +627,7 @@ SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
 #endif
 
 /* Export size information to userland */
-SYSCTL_INT(_debug_sizeof, OID_AUTO, namecache, CTLFLAG_RD, SYSCTL_NULL_INT_PTR,
-    sizeof(struct namecache), "sizeof(struct namecache)");
+SYSCTL_SIZEOF_STRUCT(namecache);
 
 /*
  * The new name cache statistics

sys/sys/sysctl.h

@@ -36,6 +36,7 @@
 #define	_SYS_SYSCTL_H_
 
 #ifdef _KERNEL
+#include <sys/cdefs.h>
 #include <sys/queue.h>
 #include <sys/tree.h>
 #endif
@@ -932,6 +933,26 @@ TAILQ_HEAD(sysctl_ctx_list, sysctl_ctx_entry);
 	    CTLFLAG_RD | CTLFLAG_CAPRD | CTLTYPE_INT | CTLFLAG_MPSAFE,	\
 	    NULL, 1, sysctl_handle_int, "I", desc, "feature");
 
+/*
+ * Adding new leaves to the 'debug.sizeof' MIB tree for ad-hoc reasons is
+ * discouraged, and in particular for reporting to developers the size of some
+ * kernel structures, which can be obtained by the following alternative means:
+ * 1. In GDB, load a full kernel image and use 'print(sizeof(struct XXX))'.
+ *    Alternatively, use 'ptype/o struct XXX' to additionally get the offsets
+ *    and size of all structure's fields.
+ * 2. If the structure is allocated from UMA, then 'vmstat -z' reports its size
+ *    (the mapping between structure types and zones is usually
+ *    straightforward).
+ */
+/* Generates a read-only sysctl reporting the size of an object/structure. */
+#define SYSCTL_SIZEOF(name, expr)					\
+	SYSCTL_INT(_debug_sizeof, OID_AUTO, name, CTLFLAG_RD,		\
+	    SYSCTL_NULL_INT_PTR, sizeof(expr),				\
+	    "sizeof(" __STRING(expr) ")");
+/* Same, specialized for structures. */
+#define SYSCTL_SIZEOF_STRUCT(struct_name)				\
+	SYSCTL_SIZEOF(struct_name, struct struct_name)
+
 #endif /* _KERNEL */
 
 /*