images: Add protobuf definition for pidfd

We only use the last pid from the list in NSpid entry (from
/proc/<pid>/fdinfo/<pidfd>) while restoring pidfds.
The last pid refers to the pid of the process in the most deeply nested
pid namespace. Since CRIU does not currently support nested pid
namespaces, this entry is the one we want.

After Linux 6.9, inode numbers can be used to compare pidfds. pidfds
referring to the same process will have the same inode numbers. We use
inode numbers to restore pidfds that point to dead processes.

Signed-off-by: Bhavik Sachdev <[email protected]>

images/Makefile

@@ -73,6 +73,7 @@ proto-obj-y	+= bpfmap-file.o
 proto-obj-y	+= bpfmap-data.o
 proto-obj-y	+= apparmor.o
 proto-obj-y	+= rseq.o
+proto-obj-y	+= pidfd.o
 
 CFLAGS		+= -iquote $(obj)/
 

images/fdinfo.proto

@@ -17,6 +17,7 @@ import "ext-file.proto";
 import "sk-unix.proto";
 import "fifo.proto";
 import "pipe.proto";
+import "pidfd.proto";
 import "tty.proto";
 import "memfd.proto";
 import "bpfmap-file.proto";
@@ -42,6 +43,7 @@ enum fd_types {
 	TIMERFD		= 17;
 	MEMFD		= 18;
 	BPFMAP		= 19;
+	PIDFD           = 20;
 
 	/* Any number above the real used. Not stored to image */
 	CTL_TTY		= 65534;
@@ -78,4 +80,5 @@ message file_entry {
 	optional tty_file_entry		tty	= 19;
 	optional memfd_file_entry	memfd	= 20;
 	optional bpfmap_file_entry	bpf	= 21;
+	optional pidfd_entry		pidfd   = 22;
 }

images/pidfd.proto

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: MIT
+
+syntax = "proto2";
+
+import "fown.proto";
+
+message pidfd_entry {
+	required uint32		id		= 1;
+	required uint32		ino		= 2;
+	required uint32		flags		= 3;
+	required int32		nspid		= 4;
+	required fown_entry	fown		= 5;
+}