fcaps.eclass: leave permissions alone by default

Removing the read bit from suid binaries has questionable security
benefit, and may cause problems for some software.

Instead of clobbering the entire file mode, just toggle the suid bit if
needed. In most cases this will result in a world-readable file.

Introduce the FCAPS_DENY_WORLD_READ setting for users who insist on
having their suid binaries unreadable.

Skip calling chown/chmod if the owner/mode is empty. This may be used by
ebuild authors in certain use cases.

Bug: https://bugs.gentoo.org/938164
Signed-off-by: Mike Gilbert <floppym@gentoo.org>

eclass/fcaps.eclass

@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: fcaps.eclass
@@ -66,6 +66,12 @@ esac
 #
 # Note: If you override pkg_postinst, you must call fcaps_pkg_postinst yourself.
 
+# @ECLASS_VARIABLE: FCAPS_DENY_WORLD_READ
+# @USER_VARIABLE
+# @DEFAULT_UNSET
+# @DESCRIPTION:
+# When set, deny read access on files updated by the fcaps function.
+
 # @FUNCTION: fcaps
 # @USAGE: [-o <owner>] [-g <group>] [-m <mode>] [-M <caps mode>] <capabilities> <file[s]>
 # @DESCRIPTION:
@@ -96,8 +102,13 @@ fcaps() {
 	# Process the user options first.
 	local owner='0'
 	local group='0'
-	local mode='4711'
-	local caps_mode='711'
+	local mode=u+s
+	local caps_mode=
+
+	if [[ -n ${FCAPS_DENY_WORLD_READ} ]]; then
+		mode=u+s,go-r
+		caps_mode=go-r
+	fi
 
 	while [[ $# -gt 0 ]] ; do
 		case $1 in
@@ -137,9 +148,10 @@ fcaps() {
 			# fs doesn't support it, but abort on all others.
 			debug-print "${FUNCNAME}: setting caps '${caps}' on '${file}'"
 
-			# If everything goes well, we don't want the file to be readable
-			# by people.
-			chmod ${caps_mode} "${file}" || die
+			# Remove the read bits if requested.
+			if [[ -n ${caps_mode} ]]; then
+				chmod ${caps_mode} "${file}" || die
+			fi
 
 			if ! out=$(LC_ALL=C setcap "${caps}" "${file}" 2>&1) ; then
 				case ${out} in
@@ -170,9 +182,14 @@ fcaps() {
 		fi
 
 		# If we're still here, setcaps failed.
-		debug-print "${FUNCNAME}: setting owner/mode on '${file}'"
-		chown "${owner}:${group}" "${file}" || die
-		chmod ${mode} "${file}" || die
+		if [[ -n ${owner} || -n ${group} ]]; then
+			debug-print "${FUNCNAME}: setting owner on '${file}'"
+			chown "${owner}:${group}" "${file}" || die
+		fi
+		if [[ -n ${mode} ]]; then
+			debug-print "${FUNCNAME}: setting mode on '${file}'"
+			chmod ${mode} "${file}" || die
+		fi
 	done
 }