sysctl: Teach sysctl to attach and run itself in a jail

This allows the parent jail to retrieve or set kernel state when child
does not have sysctl(8) installed (e.g. light weighted OCI containers
or slim jails).

This is especially useful when manipulating jail prison or vnet sysctls.
For example, `sysctl -j foo -Ja` or `sysctl -j foo net.fibs=2`.

Reviewed by:	dfr (previous version), markj
MFC after:	1 week
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D48618

sbin/sysctl/Makefile

@@ -6,6 +6,11 @@ PROG=	sysctl
 WARNS?=	3
 MAN=	sysctl.8
 
+.if ${MK_JAIL} != "no" && !defined(RESCUE)
+CFLAGS+=	-DJAIL
+LIBADD+=	jail
+.endif
+
 HAS_TESTS=
 SUBDIR.${MK_TESTS}+=	tests
 

sbin/sysctl/sysctl.8

@@ -28,20 +28,22 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd January 23, 2025
+.Dd January 31, 2025
 .Dt SYSCTL 8
 .Os
 .Sh NAME
 .Nm sysctl
 .Nd get or set kernel state
 .Sh SYNOPSIS
 .Nm
+.Op Fl j Ar jail
 .Op Fl bdeFhiJlNnoqTtVWx
 .Op Fl B Ar bufsize
 .Op Fl f Ar filename
 .Ar name Ns Op = Ns Ar value Ns Op , Ns Ar value
 .Ar ...
 .Nm
+.Op Fl j Ar jail
 .Op Fl bdeFhJlNnoqTtVWx
 .Op Fl B Ar bufsize
 .Fl a
@@ -103,6 +105,10 @@ Specify a file which contains a pair of name and value in each line.
 .Nm
 reads and processes the specified file first and then processes the name
 and value pairs in the command line argument.
+Note that when the
+.Fl j Ar jail
+option is specified, the file will be opened before attaching to the jail and
+then be processed inside the jail.
 .It Fl h
 Format output for human, rather than machine, readability.
 .It Fl i
@@ -113,6 +119,10 @@ for collecting data from a variety of machines (not all of which
 are necessarily running exactly the same software) easier.
 .It Fl J
 Display only jail prision sysctl variables (CTLFLAG_PRISON).
+.It Fl j Ar jail
+Perform the actions inside the
+.Ar jail
+(by jail id or jail name).
 .It Fl l
 Show the length of variables along with their values.
 This option cannot be combined with the

sbin/sysctl/sysctl.c

@@ -33,6 +33,9 @@
 #include <sys/time.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
+#ifdef JAIL
+#include <sys/jail.h>
+#endif
 #include <sys/sysctl.h>
 #include <sys/vmmeter.h>
 #include <dev/evdev/input.h>
@@ -51,6 +54,9 @@
 #include <err.h>
 #include <errno.h>
 #include <inttypes.h>
+#ifdef JAIL
+#include <jail.h>
+#endif
 #include <locale.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -59,12 +65,16 @@
 #include <sysexits.h>
 #include <unistd.h>
 
+#ifdef JAIL
+static const char *jailname;
+#endif
 static const char *conffile;
 
 static int	aflag, bflag, Bflag, dflag, eflag, hflag, iflag;
 static int	Nflag, nflag, oflag, qflag, tflag, Tflag, Wflag, xflag;
 static bool	Fflag, Jflag, lflag, Vflag;
 
+static void	attach_jail(void);
 static int	oidfmt(int *, int, char *, u_int *);
 static int	parsefile(FILE *);
 static int	parse(const char *, int);
@@ -121,8 +131,8 @@ usage(void)
 {
 
 	(void)fprintf(stderr, "%s\n%s\n",
-	    "usage: sysctl [-bdeFhiJlNnoqTtVWx] [ -B <bufsize> ] [-f filename] name[=value] ...",
-	    "       sysctl [-bdeFhJlNnoqTtVWx] [ -B <bufsize> ] -a");
+	    "usage: sysctl [-j jail] [-bdeFhiJlNnoqTtVWx] [ -B <bufsize> ] [-f filename] name[=value] ...",
+	    "       sysctl [-j jail] [-bdeFhJlNnoqTtVWx] [ -B <bufsize> ] -a");
 	exit(1);
 }
 
@@ -137,7 +147,7 @@ main(int argc, char **argv)
 	setbuf(stdout,0);
 	setbuf(stderr,0);
 
-	while ((ch = getopt(argc, argv, "AaB:bdeFf:hiJlNnoqTtVWwXx")) != -1) {
+	while ((ch = getopt(argc, argv, "AaB:bdeFf:hiJj:lNnoqTtVWwXx")) != -1) {
 		switch (ch) {
 		case 'A':
 			/* compatibility */
@@ -173,6 +183,14 @@ main(int argc, char **argv)
 		case 'J':
 			Jflag = true;
 			break;
+		case 'j':
+#ifdef JAIL
+			if ((jailname = optarg) == NULL)
+				usage();
+#else
+			errx(1, "not built with jail support");
+#endif
+			break;
 		case 'l':
 			lflag = true;
 			break;
@@ -222,15 +240,20 @@ main(int argc, char **argv)
 	/* TODO: few other combinations do not make sense but come back later */
 	if (Nflag && (lflag || nflag))
 		usage();
-	if (aflag && argc == 0)
+	if (aflag && argc == 0) {
+		attach_jail();
 		exit(sysctl_all(NULL, 0));
+	}
 	if (argc == 0 && conffile == NULL)
 		usage();
 
 	if (conffile != NULL) {
 		file = fopen(conffile, "r");
 		if (file == NULL)
 			err(EX_NOINPUT, "%s", conffile);
+	}
+	attach_jail();
+	if (file != NULL) {
 		warncount += parsefile(file);
 		fclose(file);
 	}
@@ -241,6 +264,23 @@ main(int argc, char **argv)
 	return (warncount);
 }
 
+static void
+attach_jail(void)
+{
+#ifdef JAIL
+	int jid;
+
+	if (jailname == NULL)
+		return;
+
+	jid = jail_getid(jailname);
+	if (jid == -1)
+		errx(1, "jail not found");
+	if (jail_attach(jid) != 0)
+		errx(1, "cannot attach to jail");
+#endif
+}
+
 /*
  * Parse a single numeric value, append it to 'newbuf', and update
  * 'newsize'.  Returns true if the value was parsed and false if the