procd: setup /dev/stdin, /dev/stdout and /dev/stderr symlinks

Extend the hotplug.json ruleset to setup the common /dev/std{in,out,err}
symbolic links which are needed by some applications, e.g. nftables when
applying rulesets from stdin.

Signed-off-by: Jo-Philipp Wich <[email protected]>

package/system/procd/files/hotplug.json

@@ -7,9 +7,19 @@
 					[ "has", "MINOR" ]
 				],
 				[
+					[ "if",
+						[ "eq", "DEVNAME", "null" ],
+						[
+							[ "makedev", "/dev/%DEVNAME%", "0666" ],
+							[ "exec", "/bin/ln", "-s", "/proc/self/fd/0", "/dev/stdin" ],
+							[ "exec", "/bin/ln", "-s", "/proc/self/fd/1", "/dev/stdout" ],
+							[ "exec", "/bin/ln", "-s", "/proc/self/fd/2", "/dev/stderr" ],
+							[ "return" ]
+						]
+					],
 					[ "if",
 						[ "eq", "DEVNAME",
-							[ "null", "full", "ptmx", "zero", "tty", "net", "random", "urandom" ]
+							[ "full", "ptmx", "zero", "tty", "net", "random", "urandom" ]
 						],
 						[
 							[ "makedev", "/dev/%DEVNAME%", "0666" ],