intel: Add a helper for setting backlight without root rights

Once the xserver stops running as root on kms capabable systems, we will need
some other way to access the backlight.

The approach taken in this patch moves most of the heavy lifting to a
helper that runs with root privileges and pipes our requested brightness
into the sysfs backlight interface. Where required, we use pkexec to
launch the helper with the elevated privilege.

v2: Amalgamate much more of the duplicate code.
    Keep the daemon and pipe alive for the lifetime of the backlight interface.
    Provide an alternative for systems without PolicyKit.

Signed-off-by: Hans de Goede <hdegoede@redhat.com> [v1]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

4 files changed, 98 insertions, 4 deletions

diff --git a/tools/.gitignore b/tools/.gitignore
index 72150ff9..36868c69 100644
--- a/tools/.gitignore
+++ b/tools/.gitignore
@@ -1 +1,3 @@
 intel-virtual-output
+xf86-video-intel-backlight-helper
+org.x.xf86-video-intel.backlight-helper.policy
diff --git a/tools/Makefile.am b/tools/Makefile.am
index d294e2ad..a5667f34 100644
--- a/tools/Makefile.am
+++ b/tools/Makefile.am
@@ -20,23 +20,45 @@
 
 AM_CFLAGS = \
 	@CWARNFLAGS@ \
-	$(TOOL_CFLAGS) \
 	@NOWARNFLAGS@ \
 	$(NULL)
 
 drivermandir = $(DRIVER_MAN_DIR)
+backlight_helperdir = $(prefix)/libexec
+policydir = $(datarootdir)/polkit-1/actions
 
+if BUILD_TOOLS
 bin_PROGRAMS = intel-virtual-output
 driverman_DATA = intel-virtual-output.$(DRIVER_MAN_SUFFIX)
+endif
+
+if BUILD_BACKLIGHT_HELPER
+backlight_helper_PROGRAMS = xf86-video-intel-backlight-helper
+nodist_policy_DATA = org.x.xf86-video-intel.backlight-helper.policy
+
+backlight_helper = $(backlight_helperdir)/xf86-video-intel-backlight-helper
+install-data-hook:
+	-chown root $(backlight_helper) && chmod u+s $(backlight_helper)
+endif
 
+intel_virtual_output_CFLAGS = \
+	@CWARNFLAGS@ \
+	$(TOOL_CFLAGS) \
+	@NOWARNFLAGS@ \
+	$(NULL)
 intel_virtual_output_SOURCES = \
 	virtual.c \
 	$(NULL)
+intel_virtual_output_LDADD = \
+	$(TOOL_LIBS) \
+	$(NULL)
 
-intel_virtual_output_LDADD = $(TOOL_LIBS)
+xf86_video_intel_backlight_helper_SOURCES = \
+	backlight_helper.c \
+	$(NULL)
 
-EXTRA_DIST = intel-virtual-output.man
-CLEANFILES = $(driverman_DATA)
+EXTRA_DIST = intel-virtual-output.man org.x.xf86-video-intel.backlight-helper.policy.in
+CLEANFILES = $(driverman_DATA) $(nodist_policy_DATA)
 
 # String replacements in MAN_SUBSTS now come from xorg-macros.m4 via configure
 SUFFIXES = .$(DRIVER_MAN_SUFFIX) .man
diff --git a/tools/backlight_helper.c b/tools/backlight_helper.c
new file mode 100644
index 00000000..fc16fcea
--- /dev/null
+++ b/tools/backlight_helper.c
@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+
+int main(int argc, char *argv[])
+{
+	struct stat st;
+	char buf[1024], *b = buf;
+	int len, fd;
+
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s <iface>\n", argv[0]);
+		return 1;
+	}
+
+	snprintf(buf, sizeof(buf), "/sys/class/backlight/%s/brightness", argv[1]);
+	fd = open(buf, O_RDWR);
+	if (fd < 0 || fstat(fd, &st) || major(st.st_dev)) {
+		fprintf(stderr, "Cannot access backlight interface '%s'\n", argv[1]);
+		return 1;
+	}
+
+	while ((len = read(0, b, sizeof(buf) - (b - buf) - 1)) > 0) {
+		len += b - buf;
+		buf[len] = '\0';
+
+		b = buf;
+		do {
+			char *end = strchr(b, '\n');
+			if (end == NULL)
+				break;
+
+			++end;
+			if (write(fd, b, end - b) != end - b) {
+				fprintf(stderr, "Failed to update backlight interface '%s'\n", argv[1]);
+				return 2;
+			}
+
+			b = end;
+		} while (1);
+
+		memmove(buf, b, len = buf + len - b);
+		b = buf + len;
+	}
+
+	return 0;
+}
diff --git a/tools/org.x.xf86-video-intel.backlight-helper.policy.in b/tools/org.x.xf86-video-intel.backlight-helper.policy.in
new file mode 100644
index 00000000..37e96226
--- /dev/null
+++ b/tools/org.x.xf86-video-intel.backlight-helper.policy.in
@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE policyconfig PUBLIC
+ "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
+ "http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd">
+<policyconfig>
+  <vendor>The X.Org project</vendor>
+  <vendor_url>https://01.org/linuxgraphics/community/xf86-video-intel</vendor_url>
+  <icon_name>brightness</icon_name>
+  <action id="org.x.xf86-video-intel.backlight-helper">
+    <description>Modify lcd panel brightness</description>
+    <message>Authentication is required to modify the lcd panel brightness</message>
+    <defaults>
+      <allow_any>no</allow_any>
+      <allow_inactive>no</allow_inactive>
+      <allow_active>yes</allow_active>
+    </defaults>
+    <annotate key="org.freedesktop.policykit.exec.path">@prefix@/libexec/xf86-video-intel-backlight-helper</annotate>
+  </action>
+</policyconfig>