seccomp: allow x32 guests on amd64 hosts.

Without this patch, x32 guests (and no others) worked "natively" with x32
host lxc, but not on regular amd64 hosts.  That was especially problematic
as a number of ioctls such as those needed by netfilter don't work in such
scenarios, thus you want to run amd64 on the host.

With the patch, you can use all three ABIs: i386 x32 amd64 on amd64 hosts.

Despite x32 being little used, there's no reason to deny it by default:
the admin needs to compile their own kernel with CONFIG_X86_X32=y or (on
Debian) boot with syscall.x32=y.  If they've done so, it is a reasonable
assumption they want x32 guests.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>

src/lxc/seccomp.c

@@ -119,6 +119,7 @@ enum lxc_hostarch_t {
 	lxc_seccomp_arch_all = 0,
 	lxc_seccomp_arch_native,
 	lxc_seccomp_arch_i386,
+	lxc_seccomp_arch_x32,
 	lxc_seccomp_arch_amd64,
 	lxc_seccomp_arch_arm,
 	lxc_seccomp_arch_arm64,
@@ -152,6 +153,7 @@ int get_hostarch(void)
 	}
 	if (strcmp(uts.machine, "i686") == 0)
 		return lxc_seccomp_arch_i386;
+	// no x32 kernels
 	else if (strcmp(uts.machine, "x86_64") == 0)
 		return lxc_seccomp_arch_amd64;
 	else if (strncmp(uts.machine, "armv7", 5) == 0)
@@ -181,6 +183,7 @@ scmp_filter_ctx get_new_ctx(enum lxc_hostarch_t n_arch, uint32_t default_policy_
 
 	switch(n_arch) {
 	case lxc_seccomp_arch_i386: arch = SCMP_ARCH_X86; break;
+	case lxc_seccomp_arch_x32: arch = SCMP_ARCH_X32; break;
 	case lxc_seccomp_arch_amd64: arch = SCMP_ARCH_X86_64; break;
 	case lxc_seccomp_arch_arm: arch = SCMP_ARCH_ARM; break;
 #ifdef SCMP_ARCH_AARCH64
@@ -336,7 +339,10 @@ static int parse_config_v2(FILE *f, char *line, struct lxc_conf *conf)
 		compat_arch[0] = SCMP_ARCH_X86;
 		compat_ctx[0] = get_new_ctx(lxc_seccomp_arch_i386,
 				default_policy_action);
-		if (!compat_ctx[0])
+		compat_arch[1] = SCMP_ARCH_X32;
+		compat_ctx[1] = get_new_ctx(lxc_seccomp_arch_x32,
+				default_policy_action);
+		if (!compat_ctx[0] || !compat_ctx[1])
 			goto bad;
 #ifdef SCMP_ARCH_PPC
 	} else if (native_arch == lxc_seccomp_arch_ppc64) {
@@ -410,6 +416,13 @@ static int parse_config_v2(FILE *f, char *line, struct lxc_conf *conf)
 					continue;
 				}
 				cur_rule_arch = lxc_seccomp_arch_i386;
+			} else if (strcmp(line, "[x32]") == 0 ||
+				   strcmp(line, "[X32]") == 0) {
+				if (native_arch != lxc_seccomp_arch_amd64) {
+					cur_rule_arch = lxc_seccomp_arch_unknown;
+					continue;
+				}
+				cur_rule_arch = lxc_seccomp_arch_x32;
 			} else if (strcmp(line, "[X86_64]") == 0 ||
 				   strcmp(line, "[x86_64]") == 0) {
 				if (native_arch != lxc_seccomp_arch_amd64) {