bzip2/lzma: comprehensible error messages for missing decompressor

Instead of failing to identify a compressed image with a decompressor
that we don't have compiled in, identify it and fail with a
comprehensible panic message.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>

init/do_mounts_rd.c

@@ -79,9 +79,12 @@ identify_ramdisk_image(int fd, int start_block, decompress_fn *decompressor)
 	sys_read(fd, buf, size);
 
 	*decompressor = decompress_method(buf, size, &compress_name);
-	if (*decompressor) {
+	if (compress_name) {
 		printk(KERN_NOTICE "RAMDISK: %s image found at block %d\n",
 		       compress_name, start_block);
+		if (!*decompressor)
+			printk(KERN_CRIT "RAMDISK: %s decompressor not configured!\n",
+			       compress_name);
 		nblocks = 0;
 		goto done;
 	}

init/initramfs.c

@@ -421,6 +421,8 @@ static char * __init unpack_to_rootfs(char *buf, unsigned len, int check_only)
 {
 	int written;
 	decompress_fn decompress;
+	const char *compress_name;
+	static __initdata char msg_buf[64];
 
 	dry_run = check_only;
 	header_buf = kmalloc(110, GFP_KERNEL);
@@ -449,10 +451,18 @@ static char * __init unpack_to_rootfs(char *buf, unsigned len, int check_only)
 			continue;
 		}
 		this_header = 0;
-		decompress = decompress_method(buf, len, NULL);
+		decompress = decompress_method(buf, len, &compress_name);
 		if (decompress)
 			decompress(buf, len, NULL, flush_buffer, NULL,
 				   &my_inptr, error);
+		else if (compress_name) {
+			if (!message) {
+				snprintf(msg_buf, sizeof msg_buf,
+					 "compression method %s not configured",
+					 compress_name);
+				message = msg_buf;
+			}
+		}
 		if (state != Reset)
 			error("junk in compressed archive");
 		this_header = saved_offset + my_inptr;

lib/decompress.c

@@ -13,21 +13,25 @@
 #include <linux/types.h>
 #include <linux/string.h>
 
+#ifndef CONFIG_DECOMPRESS_GZIP
+# define gunzip NULL
+#endif
+#ifndef CONFIG_DECOMPRESS_BZIP2
+# define bunzip2 NULL
+#endif
+#ifndef CONFIG_DECOMPRESS_LZMA
+# define unlzma NULL
+#endif
+
 static const struct compress_format {
 	unsigned char magic[2];
 	const char *name;
 	decompress_fn decompressor;
 } compressed_formats[] = {
-#ifdef CONFIG_DECOMPRESS_GZIP
 	{ {037, 0213}, "gzip", gunzip },
 	{ {037, 0236}, "gzip", gunzip },
-#endif
-#ifdef CONFIG_DECOMPRESS_BZIP2
 	{ {0x42, 0x5a}, "bzip2", bunzip2 },
-#endif
-#ifdef CONFIG_DECOMPRESS_LZMA
 	{ {0x5d, 0x00}, "lzma", unlzma },
-#endif
 	{ {0, 0}, NULL, NULL }
 };