3 files changed, 21 insertions, 4 deletions
diff --git a/lib/libc/gen/getvfsbyname.3 b/lib/libc/gen/getvfsbyname.3
index 23036429b27e..61fd48624fbd 100644
--- a/lib/libc/gen/getvfsbyname.3
+++ b/lib/libc/gen/getvfsbyname.3
@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd August 16, 2018
+.Dd October 28, 2025
 .Dt GETVFSBYNAME 3
 .Os
 .Sh NAME
@@ -102,6 +102,7 @@ argument
 specifies a file system that is unknown or not configured in the kernel.
 .El
 .Sh SEE ALSO
+.Xr lsvfs 1 ,
 .Xr jail 2 ,
 .Xr mount 2 ,
 .Xr sysctl 3 ,
diff --git a/lib/libc/gen/sysconf.3 b/lib/libc/gen/sysconf.3
index e38357b898a7..290ef0dc158c 100644
--- a/lib/libc/gen/sysconf.3
+++ b/lib/libc/gen/sysconf.3
@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 26, 2013
+.Dd August 30, 2025
 .Dt SYSCONF 3
 .Os
 .Sh NAME
@@ -77,7 +77,9 @@ The maximum number of supplemental groups.
 .It Li _SC_NPROCESSORS_CONF
 The number of processors configured.
 .It Li _SC_NPROCESSORS_ONLN
-The number of processors currently online.
+The number of processors currently online, taking into account current jail
+restrictions to report only the number of processors that are usable to the
+process.
 .It Li _SC_OPEN_MAX
 One more than the maximum value the system may assign to a new file descriptor.
 .It Li _SC_PAGESIZE
diff --git a/lib/libc/gen/sysconf.c b/lib/libc/gen/sysconf.c
index b5b732eed05d..87aedc07c110 100644
--- a/lib/libc/gen/sysconf.c
+++ b/lib/libc/gen/sysconf.c
@@ -72,6 +72,7 @@ long
 sysconf(int name)
 {
 	struct rlimit rl;
+	cpuset_t cpus;
 	size_t len;
 	int mib[2], sverrno, value;
 	long lvalue, defaultresult;
@@ -581,8 +582,21 @@ yesno:
 		return (_POSIX_IPV6);
 #endif
 
-	case _SC_NPROCESSORS_CONF:
 	case _SC_NPROCESSORS_ONLN:
+		/*
+		 * Consult our root set first, because our CPU availability
+		 * may not match the total number of CPUs available on the
+		 * system and we may have a non-uniform layout even within
+		 * userland.  In particular, each jail has a root set that can
+		 * be constrained by its parent and processes within the jail
+		 * cannot widen beyond those constraints, so to those processes
+		 * it makes sense to claim the more limited count.
+		 */
+		if (cpuset_getaffinity(CPU_LEVEL_ROOT, CPU_WHICH_PID, -1,
+		    sizeof(cpus), &cpus) == 0)
+			return (CPU_COUNT(&cpus));
+		/* FALLTHROUGH */
+	case _SC_NPROCESSORS_CONF:
 		if (_elf_aux_info(AT_NCPUS, &value, sizeof(value)) == 0)
 			return ((long)value);
 		mib[0] = CTL_HW;