kvm tools: Fail if passed initrd is not really an initrd

We recently changed the meaning of "-i" from disk image to initrd. This has
confused many users because kvm just reports:

  Fatal: mmap() failed.

if a disk image is passed as initrd. This patch fixes that by checking for the
first two ID bytes in initrd:

  $ ./kvm run -i ~/images/linux-0.2.qcow
    # kvm run -k ../../arch/x86/boot/bzImage -m 256 -c 1
    Fatal: /home/penberg/images/linux-0.2.qcow is not an initrd

Reported-by: Thomas Heil <heil@terminal-consulting.de>
Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

kvm.c

@@ -1,10 +1,11 @@
 #include "kvm/kvm.h"
 
-#include "kvm/cpufeature.h"
-#include "kvm/interrupt.h"
 #include "kvm/boot-protocol.h"
-#include "kvm/util.h"
+#include "kvm/cpufeature.h"
+#include "kvm/read-write.h"
+#include "kvm/interrupt.h"
 #include "kvm/mptable.h"
+#include "kvm/util.h"
 
 #include <linux/kvm.h>
 
@@ -422,6 +423,23 @@ static bool load_bzimage(struct kvm *kvm, int fd_kernel,
 	return true;
 }
 
+/* RFC 1952 */
+#define GZIP_ID1		0x1f
+#define GZIP_ID2		0x8b
+
+static bool initrd_check(int fd)
+{
+	unsigned char id[2];
+
+	if (read_in_full(fd, id, ARRAY_SIZE(id)) < 0)
+		return false;
+
+	if (lseek(fd, 0, SEEK_SET) < 0)
+		die_perror("lseek");
+
+	return id[0] == GZIP_ID1 && id[1] == GZIP_ID2;
+}
+
 bool kvm__load_kernel(struct kvm *kvm, const char *kernel_filename,
 		const char *initrd_filename, const char *kernel_cmdline)
 {
@@ -436,6 +454,9 @@ bool kvm__load_kernel(struct kvm *kvm, const char *kernel_filename,
 		fd_initrd = open(initrd_filename, O_RDONLY);
 		if (fd_initrd < 0)
 			die("Unable to open initrd %s", initrd_filename);
+
+		if (!initrd_check(fd_initrd))
+			die("%s is not an initrd", initrd_filename);
 	}
 
 	ret = load_bzimage(kvm, fd_kernel, fd_initrd, kernel_cmdline);