[MULTIBOOT2 SPEC PATCH v4 14/27] 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, and
boot_i386.S to use ".balign 8" as well for consistency.

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

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 you cannot detect the wrong header tag
alignment using the "grub-file --is-x86-multiboot2" utility.
It only considers the Multiboot2 header magic fields while
completely ignoring 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..0e8118b2a 100644
--- a/doc/boot_i386.S
+++ b/doc/boot_i386.S
@@ -43,7 +43,7 @@ _start:
        jmp     multiboot_entry
 
        /* Align 64 bits boundary.  */
-       .align  8
+       .balign 8
        
        /* Multiboot header.  */
 multiboot_header:
@@ -56,6 +56,7 @@ multiboot_header:
        /* checksum */
        .long   -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_I386 + 
(multiboot_header_end - multiboot_header))
 #ifndef __ELF__
+       .balign 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:
+       .balign 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__ */
+       .balign 8
 framebuffer_tag_start: 
        .short MULTIBOOT_HEADER_TAG_FRAMEBUFFER
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -85,6 +88,7 @@ framebuffer_tag_start:
        .long 768
        .long 32
 framebuffer_tag_end:
+       .balign 8
        .short MULTIBOOT_HEADER_TAG_END
        .short 0
        .long 8
diff --git a/doc/boot_mips.S b/doc/boot_mips.S
index ed604214d..a8d3fb0e4 100644
--- a/doc/boot_mips.S
+++ b/doc/boot_mips.S
@@ -46,7 +46,7 @@ _start:
         nop
 
        /* Align 64 bits boundary.  */
-       .align  8
+       .balign 8
        
        /* Multiboot header.  */
 multiboot_header:
@@ -59,6 +59,7 @@ multiboot_header:
        /* checksum */
        .long   -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_MIPS32 + 
(multiboot_header_end - multiboot_header))
 #ifndef __ELF__
+       .balign 8
 address_tag_start:     
        .short MULTIBOOT_HEADER_TAG_ADDRESS
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -72,6 +73,7 @@ address_tag_start:
        /* bss_end_addr */
        .long   _end
 address_tag_end:
+       .balign 8
 entry_address_tag_start:       
        .short MULTIBOOT_HEADER_TAG_ENTRY_ADDRESS
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -80,6 +82,7 @@ entry_address_tag_start:
        .long multiboot_entry
 entry_address_tag_end:
 #endif /* __ELF__ */
+       .balign 8
 framebuffer_tag_start: 
        .short MULTIBOOT_HEADER_TAG_FRAMEBUFFER
        .short MULTIBOOT_HEADER_TAG_OPTIONAL
@@ -88,6 +91,7 @@ framebuffer_tag_start:
        .long 768
        .long 32
 framebuffer_tag_end:
+       .balign 8
        .short MULTIBOOT_HEADER_TAG_END
        .short 0
        .long 8
@@ -114,6 +118,6 @@ loop:       nop
 halt_message:
        .asciz  "Halted."
 
-       .align 8
+       .balign 8
        /* Our stack area.  */
        .comm   stack, STACK_SIZE
-- 
2.26.2