bzip2/lzma: centralize format detection

Centralize the compression format detection to a common routine in the
lib directory, and use it for both initramfs and initrd.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

lib/decompress.c

@@ -0,0 +1,50 @@
+/*
+ * decompress.c
+ *
+ * Detect the decompression method based on magic number
+ */
+
+#include <linux/decompress/generic.h>
+
+#include <linux/decompress/bunzip2.h>
+#include <linux/decompress/unlzma.h>
+#include <linux/decompress/inflate.h>
+
+#include <linux/types.h>
+#include <linux/string.h>
+
+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 }
+};
+
+decompress_fn decompress_method(const unsigned char *inbuf, int len,
+				const char **name)
+{
+	const struct compress_format *cf;
+
+	if (len < 2)
+		return NULL;	/* Need at least this much... */
+
+	for (cf = compressed_formats; cf->decompressor; cf++) {
+		if (!memcmp(inbuf, cf->magic, 2))
+			break;
+
+	}
+	if (name)
+		*name = cf->name;
+	return cf->decompressor;
+}