Modified error handling

Simplified the error handler in mlb, removing the reboot on keypress
and instead just looping a hlt. Rebooting a failed bootloader probably
wouldn't help much, anyway.

Added single-letter error codes to mlb to explain what went wrong.

Moved the version check to the installer. There is no need to check this
at boot time unless the user is doing something very silly.

The above changes have netted a nice 40 b savings in the mlb binary size.

Updated the Readme to reflect the changes.

README.md

@@ -52,16 +52,27 @@ where:
  - `<kernel>` is the kernel file to boot
  - `<command line>` is the command line to pass to the kernel - don't forget to
    pass `root=` as `mlbinstall` will not calculate the root device. Currently
-   the command line cannot be longer than 40 characters (more just won't fit in
-   the MBR).
+   the command line cannot be longer than 80 bytes (more just won't fit in the
+   MBR).
  - `[-vbr]` will make `mlbinstall` ignore the space for the partition table,
-   giving you more space for the command line. You can safely use it if you're
-   installing to a Volume Boot Record (partition) and not the Master Boot
-   Record, e.g. for using with another bootloader in multi-OS setups.
+   giving you 144 bytes for the command line. You can safely use it if you're
+   installing to a Volume Boot Record (partition), e.g. for using with another
+   bootloader in multi-OS setups.
 
 `mlbinstall` is quite noisy if it detects any problems. Restore your MBR from
 backup, fix the problems, and try again. If it succeeds, it won't say anything.
 
+Error codes
+-----------
+
+If MLB fails to boot your kernel, it will print a one-letter error code.
+Currently, there are two error codes:
+
+    Error What it means
+    ----- ----------------------------------------
+      R   Failed reading data from disk
+      M   Failed moving data to its final location
+
 Contact
 -------
 

mlb.asm

@@ -68,13 +68,6 @@ read_kernel_setup:
 	mov	cx, 0x1000
 	call	read_from_hdd
 
-check_version:
-
-	cmp	word [es:0x206], 0x204		; we need protocol version >= 2.04
-	jb	error
-	test	byte [es:0x211], 1
-	jz	error
-
 set_header_fields:
 
 	mov	byte [es:0x210], 0xe1		; set type_of_loader
@@ -149,6 +142,7 @@ read_from_hdd:
 	mov	si, dap
 	mov	dl, 0x80			; first hard disk
 	int	0x13
+	mov	al, 'R'
 	jc	error
 	pop	edx
 	ret
@@ -162,31 +156,20 @@ do_move:
 	mov	ah, 0x87
 	mov	si, gdt
 	int	0x15
+	mov	al, 'M'
 	jc	error
 	pop	es
 	pop	edx
 	ret
 
 error:
 
-	mov	si, error_msg
-
-msg_loop:
-
-	lodsb
-	and	al, al
-	jz	reboot
 	mov	ah, 0xe
-	mov	bx, 7
+	xor	bh, bh
 	int	0x10
-	jmp	short msg_loop
-
-reboot:
-
-	xor	ax, ax
-	int	0x16
-	int	0x19
-	jmp	0xf000:0xfff0			; BIOS reset code
+.hlt:
+	hlt
+	jmp	error.hlt
 
 ; Global Descriptor Table
 
@@ -223,6 +206,5 @@ dap:
 	dd	0				; low bytes of LBA address
 	dd	0				; high bytes of LBA address
 
-error_msg	db	'err', 0
 current_lba	dd	0xefbeadde		; mlbinstall replaces this with the kernel LBA
 cmd_line	db	0

mlbinstall.c

@@ -19,25 +19,57 @@
  * along with mlbinstall. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#define _BSD_SOURCE
 #define _POSIX_C_SOURCE 200809L
 #define _XOPEN_SOURCE 500
 
 #include <stdint.h>
-#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
 #include <err.h>
+#include <endian.h>
+#include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <linux/fiemap.h>
 #include <linux/fs.h>
 #include <sys/ioctl.h>
-#include <unistd.h>
 #include <stdbool.h>
 #include <stdio.h>
 
 #include "mlb_bin.h"
 
+/* Checks if the kernel boot protocol is supported */
+void check_version(const char *kernel)
+{
+	int fd = open(kernel, O_RDONLY);
+	if (fd == -1)
+		err(1, "Failed opening %s", kernel);
+
+	long ps = sysconf(_SC_PAGESIZE);
+	size_t mlength = (512 / ps + 1) * ps;
+	uint8_t *m = mmap(NULL, mlength, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (m == MAP_FAILED)
+		err(1, "Failed mapping %s", kernel);
+
+	uint32_t header = *(uint32_t *)(m + 0x202);
+	uint16_t version = be16toh(*(uint16_t *)(m + 0x206));
+	uint8_t loadflags = m[0x211];
+
+	if (header != 0x53726448)
+		errx(1, "%s is missing a Linux kernel header", kernel);
+	if (version < 0x204)
+		errx(1, "Kernel too old, boot protocol version >= 0x204/\
+kernel version >= 2.6.14 required, but %s is 0x%x", kernel, version);
+	if (!(loadflags & 0x01))
+		errx(1, "Kernel needs to be loaded high");
+
+	munmap(m, mlength);
+	close(fd);
+}
+
 /* Returns the length of cmdline, including the terminating null. */
 uint16_t cmdlen(const char *cmdline, size_t mlblen, size_t mbrlen)
 {
@@ -171,6 +203,8 @@ argument to not reserve space for a partition table and gain an extra\n\
 	const char *kernel = argv[2];
 	const char *cmdline = argv[3];
 
+	check_version(kernel);
+
 	size_t mbr_len = vbr ? 510 : 446;
 	uint16_t cmdline_len = cmdlen(cmdline, mlb_bin_len, mbr_len);
 	uint32_t kernel_lba = lba(kernel);