gen_init_cpio: support -o <output_file> parameter

This is another preparatory change to allow for reflink-optimized
cpio archives with file data written / cloned via copy_file_range().
The output file is truncated prior to write, so that it maps to
usr/gen_initramfs.sh usage. It may make sense to offer an append option
in future, for easier archive concatenation.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-3-ddiss@suse.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: ddiss

usr/gen_init_cpio.c

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -110,7 +111,7 @@ static int cpio_trailer(void)
 	    push_pad(padlen(offset, 512)) < 0)
 		return -1;
 
-	return 0;
+	return fsync(outfd);
 }
 
 static int cpio_mkslink(const char *name, const char *target,
@@ -532,7 +533,7 @@ static int cpio_mkfile_line(const char *line)
 static void usage(const char *prog)
 {
 	fprintf(stderr, "Usage:\n"
-		"\t%s [-t <timestamp>] [-c] <cpio_list>\n"
+		"\t%s [-t <timestamp>] [-c] [-o <output_file>] <cpio_list>\n"
 		"\n"
 		"<cpio_list> is a file containing newline separated entries that\n"
 		"describe the files to be included in the initramfs archive:\n"
@@ -569,7 +570,8 @@ static void usage(const char *prog)
 		"as mtime for symlinks, directories, regular and special files.\n"
 		"The default is to use the current time for all files, but\n"
 		"preserve modification time for regular files.\n"
-		"-c: calculate and store 32-bit checksums for file data.\n",
+		"-c: calculate and store 32-bit checksums for file data.\n"
+		"<output_file>: write cpio to this file instead of stdout\n",
 		prog);
 }
 
@@ -611,7 +613,7 @@ int main (int argc, char *argv[])
 
 	default_mtime = time(NULL);
 	while (1) {
-		int opt = getopt(argc, argv, "t:ch");
+		int opt = getopt(argc, argv, "t:cho:");
 		char *invalid;
 
 		if (opt == -1)
@@ -630,6 +632,16 @@ int main (int argc, char *argv[])
 		case 'c':
 			do_csum = true;
 			break;
+		case 'o':
+			outfd = open(optarg,
+				     O_WRONLY | O_CREAT | O_LARGEFILE | O_TRUNC,
+				     0600);
+			if (outfd < 0) {
+				fprintf(stderr, "failed to open %s\n", optarg);
+				usage(argv[0]);
+				exit(1);
+			}
+			break;
 		case 'h':
 		case '?':
 			usage(argv[0]);