[MULTIBOOT2 SPEC PATCH v3 4/4] multiboot2: fix example kernel header tag alignment

[Hans Ulrich Niedermann]

Properly align all Multiboot2 header tags to 8 byte
boundaries as per the Multiboot2 specification.

Note that the assembler directive ".align 8" is machine
dependent: On i386, it means "align to 8 byte boundary".
On mips, it means that the lower 8 bits of address must
be zero, i.e. ".align 8" aligns to a 256 byte boundary!

Therefore, this changes boot_mips.S to use ".balign 8"
where it had mistakenly used ".align 8" before.

The boot_i386.S file keeps using ".align 8".

Note also that the Multiboot2 header termination tag
(unfortunately unlabeled in the source) actually needs
that extra alignment to an 8 byte boundary, as the
preceding tag (framebuffer_tag_start) is 20 bytes long
which is not a multiple of 8.

You can add "-Wa,-adhlns=$(@:.o=.lst)" to kernel_CCASFLAGS
to generate a listing file which shows the offsets of the
labels in the boot_*.S files to verify the alignment issues.

Note also that the "grub-file --is-x86-multiboot2" utility
does not recognize the wrong alignment of the Multiboot2
header tags, as it only considers the beginning of the
Multiboot2 header and completely ignores the Multiboot2
header tags.

Signed-off-by: Hans Ulrich Niedermann <address@hidden>

diff --git a/doc/boot_i386.S b/doc/boot_i386.S
index 9ab016612..1418afe1a 100644
--- a/doc/boot_i386.S
+++ b/doc/boot_i386.S
@@ -56,6 +56,7 @@ multiboot_header:
        /* checksum */
        .long   -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_I386 + 
(multiboot_header_end - multiboot_header))
 #ifndef __ELF__
+       .align  8
 address_tag_start:     
        .short MULTIBOOT_HEADER_TAG_ADDRESS
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -69,6 +70,7 @@ address_tag_start:
        /* bss_end_addr */
        .long   _end
 address_tag_end:
+       .align  8
 entry_address_tag_start:       
        .short MULTIBOOT_HEADER_TAG_ENTRY_ADDRESS
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -77,6 +79,7 @@ entry_address_tag_start:
        .long multiboot_entry
 entry_address_tag_end:
 #endif /* __ELF__ */
+       .align  8
 framebuffer_tag_start: 
        .short MULTIBOOT_HEADER_TAG_FRAMEBUFFER
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -85,10 +88,12 @@ framebuffer_tag_start:
        .long 768
        .long 32
 framebuffer_tag_end:
+       .align  8
        .short MULTIBOOT_HEADER_TAG_END
        .short 0
        .long 8
 multiboot_header_end:
+
 multiboot_entry:
        /* Initialize the stack pointer.  */
        movl    $(stack + STACK_SIZE), %esp
diff --git a/doc/boot_mips.S b/doc/boot_mips.S
index ed604214d..e7bb7df9f 100644
--- a/doc/boot_mips.S
+++ b/doc/boot_mips.S
@@ -46,8 +46,8 @@ _start:
         nop
 
        /* Align 64 bits boundary.  */
-       .align  8
-       
+       .balign 8
+
        /* Multiboot header.  */
 multiboot_header:
        /* magic */
@@ -59,7 +59,8 @@ multiboot_header:
        /* checksum */
        .long   -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_MIPS32 + 
(multiboot_header_end - multiboot_header))
 #ifndef __ELF__
-address_tag_start:     
+       .balign 8
+address_tag_start:
        .short MULTIBOOT_HEADER_TAG_ADDRESS
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
        .long address_tag_end - address_tag_start
@@ -72,7 +73,8 @@ address_tag_start:
        /* bss_end_addr */
        .long   _end
 address_tag_end:
-entry_address_tag_start:       
+       .balign 8
+entry_address_tag_start:
        .short MULTIBOOT_HEADER_TAG_ENTRY_ADDRESS
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
        .long entry_address_tag_end - entry_address_tag_start
@@ -80,7 +82,8 @@ entry_address_tag_start:
        .long multiboot_entry
 entry_address_tag_end:
 #endif /* __ELF__ */
-framebuffer_tag_start: 
+       .balign 8
+framebuffer_tag_start:
        .short MULTIBOOT_HEADER_TAG_FRAMEBUFFER
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
        .long framebuffer_tag_end - framebuffer_tag_start
@@ -88,10 +91,12 @@ framebuffer_tag_start:
        .long 768
        .long 32
 framebuffer_tag_end:
+       .balign 8
        .short MULTIBOOT_HEADER_TAG_END
        .short 0
        .long 8
 multiboot_header_end:
+
 multiboot_entry:
        /* Initialize the stack pointer.  */
        lui     $sp, %hi (stack + STACK_SIZE)
@@ -114,6 +119,6 @@ loop:       nop
 halt_message:
        .asciz  "Halted."
 
-       .align 8
+       .balign 8
        /* Our stack area.  */
        .comm   stack, STACK_SIZE
-- 
2.26.2