Syslinux - Feb 2012 - [PATCH] isohybrid: Generate MBR even when in EFI mode

Various EFI systems insist that there be no active flag in the pMBR in
order to parse the GPT. The only way around this is to also generate a
valid MBR - the firmware will then pick that up and use the system
partition provided there. In order to deal with other EFI "sanity"
checks,
the partition type for the non-EFI partitions is set to 0 to skip the
firmware bailing because of overlapping partitions.
---
 utils/isohybrid.c |   54 ++++++++++++++++++++++++++++++++++++++++++++++++----
 1 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/utils/isohybrid.c b/utils/isohybrid.c
index 1dcbaa1..ac04bfd 100644
--- a/utils/isohybrid.c
+++ b/utils/isohybrid.c
@@ -54,7 +54,7 @@ enum { VERBOSE = 1 , EFI = 2 , MAC = 4};
 uint16_t head = 64;             /* 1 <= head <= 256 */
 uint8_t sector = 32;            /* 1 <= sector <= 63  */
 
-uint8_t entry = 1;              /* partition number: 1 <= entry <= 4 */
+uint8_t entry = 0;              /* partition number: 1 <= entry <= 4 */
 uint8_t offset = 0;             /* partition offset: 0 <= offset <= 64 */
 uint16_t type = 0x17;           /* partition type: 0 <= type <= 255 */
 uint32_t id = 0;                /* MBR: 0 <= id <= 0xFFFFFFFF(4294967296)
*/
@@ -300,6 +300,8 @@ check_option(int argc, char *argv[])
             entry = strtoul(optarg, &err, 0);
             if (entry < 1 || entry > 4)
                 errx(1, "invalid entry: `%s', 1 <= entry <=
4", optarg);
+	    if (mode & MAC || mode & EFI)
+		errx(1, "setting an entry is unsupported with EFI or Mac");
             break;
 
         case 'o':
@@ -334,10 +336,14 @@ check_option(int argc, char *argv[])
 
 	case 'u':
 	    mode |= EFI;
+	    if (entry)
+		errx(1, "setting an entry is unsupported with EFI or Mac");
 	    break;
 
 	case 'm':
 	    mode |= MAC;
+	    if (entry)
+		errx(1, "setting an entry is unsupported with EFI or Mac");
 	    break;
 
         case 'v':
@@ -581,6 +587,12 @@ initialise_mbr(uint8_t *mbr)
 	memcpy(mbr, afp_header, sizeof(afp_header));
     }
 
+    if (!entry)
+	entry = 1;
+
+    if (mode & EFI)
+	type = 0;
+
     mbr += MBRSIZE;                                 /* offset 432 */
 
     tmp = lendian_int(de_lba * 4);
@@ -633,6 +645,40 @@ initialise_mbr(uint8_t *mbr)
             tmp = lendian_int(psize);
             memcpy(&mbr[12], &tmp, sizeof(tmp));
         }
+        if (i == 2 && (mode & EFI))
+        {
+            mbr[0] = 0x0;
+            mbr[1] = 0xfe;
+            mbr[2] = 0xff;
+            mbr[3] = 0xff;
+            mbr[4] = 0xef;
+            mbr[5] = 0xfe;
+            mbr[6] = 0xff;
+            mbr[7] = 0xff;
+
+            tmp = lendian_int(efi_lba * 4);
+            memcpy(&mbr[8], &tmp, sizeof(tmp));
+
+            tmp = lendian_int(efi_count);
+            memcpy(&mbr[12], &tmp, sizeof(tmp));
+        }
+        if (i == 3 && (mode & MAC))
+        {
+            mbr[0] = 0x0;
+            mbr[1] = 0xfe;
+            mbr[2] = 0xff;
+            mbr[3] = 0xff;
+            mbr[4] = 0x0;
+            mbr[5] = 0xfe;
+            mbr[6] = 0xff;
+            mbr[7] = 0xff;
+
+            tmp = lendian_int(mac_lba * 4);
+            memcpy(&mbr[8], &tmp, sizeof(tmp));
+
+            tmp = lendian_int(mac_count);
+            memcpy(&mbr[12], &tmp, sizeof(tmp));
+        }
         mbr += 16;
     }
     mbr[0] = 0x55;
@@ -908,8 +954,7 @@ main(int argc, char *argv[])
 	if (!read_efi_section(buf)) {
 	    buf += 32;
 	    if (!read_efi_catalogue(buf, &efi_count, &efi_lba) &&
efi_lba) {
-		offset = 1;
-		type = 0xee;
+		offset = 0;
 	    } else {
 		errx(1, "%s: invalid efi catalogue", argv[0]);
 	    }
@@ -925,8 +970,7 @@ main(int argc, char *argv[])
 	if (!read_efi_section(buf)) {
 	    buf += 32;
 	    if (!read_efi_catalogue(buf, &mac_count, &mac_lba) &&
mac_lba) {
-		offset = 1;
-		type = 0xee;
+		offset = 0;
 	    } else {
 		errx(1, "%s: invalid efi catalogue", argv[0]);
 	    }
-- 
1.7.7.6

Full quote, no new text, just a reminder

On Wed, Feb 15, 2012 at 12:12:17PM -0500, Matthew Garrett
wrote:
> Various EFI systems insist that there be no active flag in the pMBR in
> order to parse the GPT. The only way around this is to also generate a
> valid MBR - the firmware will then pick that up and use the system
> partition provided there. In order to deal with other EFI
"sanity" checks,
> the partition type for the non-EFI partitions is set to 0 to skip the
> firmware bailing because of overlapping partitions.
> ---
>  utils/isohybrid.c |   54
++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 files changed, 49 insertions(+), 5 deletions(-)
> 
> diff --git a/utils/isohybrid.c b/utils/isohybrid.c
> index 1dcbaa1..ac04bfd 100644
> --- a/utils/isohybrid.c
> +++ b/utils/isohybrid.c
> @@ -54,7 +54,7 @@ enum { VERBOSE = 1 , EFI = 2 , MAC = 4};
>  uint16_t head = 64;             /* 1 <= head <= 256 */
>  uint8_t sector = 32;            /* 1 <= sector <= 63  */
>  
> -uint8_t entry = 1;              /* partition number: 1 <= entry <= 4
*/
> +uint8_t entry = 0;              /* partition number: 1 <= entry <= 4
*/
>  uint8_t offset = 0;             /* partition offset: 0 <= offset <=
64 */
>  uint16_t type = 0x17;           /* partition type: 0 <= type <= 255
*/
>  uint32_t id = 0;                /* MBR: 0 <= id <=
0xFFFFFFFF(4294967296) */
> @@ -300,6 +300,8 @@ check_option(int argc, char *argv[])
>              entry = strtoul(optarg, &err, 0);
>              if (entry < 1 || entry > 4)
>                  errx(1, "invalid entry: `%s', 1 <= entry <=
4", optarg);
> +	    if (mode & MAC || mode & EFI)
> +		errx(1, "setting an entry is unsupported with EFI or Mac");
>              break;
>  
>          case 'o':
> @@ -334,10 +336,14 @@ check_option(int argc, char *argv[])
>  
>  	case 'u':
>  	    mode |= EFI;
> +	    if (entry)
> +		errx(1, "setting an entry is unsupported with EFI or Mac");
>  	    break;
>  
>  	case 'm':
>  	    mode |= MAC;
> +	    if (entry)
> +		errx(1, "setting an entry is unsupported with EFI or Mac");
>  	    break;
>  
>          case 'v':
> @@ -581,6 +587,12 @@ initialise_mbr(uint8_t *mbr)
>  	memcpy(mbr, afp_header, sizeof(afp_header));
>      }
>  
> +    if (!entry)
> +	entry = 1;
> +
> +    if (mode & EFI)
> +	type = 0;
> +
>      mbr += MBRSIZE;                                 /* offset 432 */
>  
>      tmp = lendian_int(de_lba * 4);
> @@ -633,6 +645,40 @@ initialise_mbr(uint8_t *mbr)
>              tmp = lendian_int(psize);
>              memcpy(&mbr[12], &tmp, sizeof(tmp));
>          }
> +        if (i == 2 && (mode & EFI))
> +        {
> +            mbr[0] = 0x0;
> +            mbr[1] = 0xfe;
> +            mbr[2] = 0xff;
> +            mbr[3] = 0xff;
> +            mbr[4] = 0xef;
> +            mbr[5] = 0xfe;
> +            mbr[6] = 0xff;
> +            mbr[7] = 0xff;
> +
> +            tmp = lendian_int(efi_lba * 4);
> +            memcpy(&mbr[8], &tmp, sizeof(tmp));
> +
> +            tmp = lendian_int(efi_count);
> +            memcpy(&mbr[12], &tmp, sizeof(tmp));
> +        }
> +        if (i == 3 && (mode & MAC))
> +        {
> +            mbr[0] = 0x0;
> +            mbr[1] = 0xfe;
> +            mbr[2] = 0xff;
> +            mbr[3] = 0xff;
> +            mbr[4] = 0x0;
> +            mbr[5] = 0xfe;
> +            mbr[6] = 0xff;
> +            mbr[7] = 0xff;
> +
> +            tmp = lendian_int(mac_lba * 4);
> +            memcpy(&mbr[8], &tmp, sizeof(tmp));
> +
> +            tmp = lendian_int(mac_count);
> +            memcpy(&mbr[12], &tmp, sizeof(tmp));
> +        }
>          mbr += 16;
>      }
>      mbr[0] = 0x55;
> @@ -908,8 +954,7 @@ main(int argc, char *argv[])
>  	if (!read_efi_section(buf)) {
>  	    buf += 32;
>  	    if (!read_efi_catalogue(buf, &efi_count, &efi_lba) &&
efi_lba) {
> -		offset = 1;
> -		type = 0xee;
> +		offset = 0;
>  	    } else {
>  		errx(1, "%s: invalid efi catalogue", argv[0]);
>  	    }
> @@ -925,8 +970,7 @@ main(int argc, char *argv[])
>  	if (!read_efi_section(buf)) {
>  	    buf += 32;
>  	    if (!read_efi_catalogue(buf, &mac_count, &mac_lba) &&
mac_lba) {
> -		offset = 1;
> -		type = 0xee;
> +		offset = 0;
>  	    } else {
>  		errx(1, "%s: invalid efi catalogue", argv[0]);
>  	    }
> -- 
> 1.7.7.6
> 
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-- 
Groeten
Geert Stappers
-- 
> And is there a policy on top-posting vs. bottom-posting?
Yes.

On 02/15/2012 09:12 AM, Matthew Garrett wrote:
> Various EFI systems insist that there be no active flag in the pMBR in
> order to parse the GPT. The only way around this is to also generate a
> valid MBR - the firmware will then pick that up and use the system
> partition provided there. In order to deal with other EFI
"sanity" checks,
> the partition type for the non-EFI partitions is set to 0 to skip the
> firmware bailing because of overlapping partitions.
> ---
>  utils/isohybrid.c |   54
++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 files changed, 49 insertions(+), 5 deletions(-)
Hmmm... this patch doesn't apply to current master.  What baseline is
this from?

	-hpa