elfctl: add knowledge about the la57 bit

(cherry picked from commit b989c1dd1e)

usr.bin/elfctl/elfctl.1

@@ -95,6 +95,20 @@ Features may also be specified as a single combined value:
 .Bd -literal -offset indent
 elfctl -e =0x5 file
 .Ed
+.Sh NOTES
+On amd64, on a machine with LA57 (5-level paging) mode supported and
+enabled, if both
+.Va la57
+and
+.Va la48
+feature flags are specified, the
+.Va la57
+feature has priority over
+.Va la48 .
+The
+.Va vm.pmap.prefer_uva_la48
+sysctl MIB defines the default user address space size for binaries
+which do not set either of these flags.
 .Sh SEE ALSO
 .Xr mitigations 7
 .Sh HISTORY

usr.bin/elfctl/elfctl.c

@@ -67,7 +67,10 @@ static struct ControlFeatures featurelist[] = {
 	    "Disable implicit PROT_MAX" },
 	{ "nostackgap",	NT_FREEBSD_FCTL_STKGAP_DISABLE, "Disable stack gap" },
 	{ "wxneeded",	NT_FREEBSD_FCTL_WXNEEDED, "Requires W+X mappings" },
-	{ "la48",	NT_FREEBSD_FCTL_LA48, "amd64: Limit user VA to 48bit" },
+	{ "la48",	NT_FREEBSD_FCTL_LA48,
+	    "amd64: Limit user virtual addresses to 48 bits" },
+	{ "la57",	NT_FREEBSD_FCTL_LA57,
+	    "amd64: Allow the use of 57-bit virtual addresses when available" },
 };
 
 static struct option long_opts[] = {