diff mbox series

[v3,3/5] xen/x86: add some addresses to the Multiboot header

Message ID 0b05bedc32833a2022d2698d4c116cb867a9119c.1611273359.git.bobbyeshleman@gmail.com (mailing list archive)
State New, archived
Headers show
Series Support Secure Boot for multiboot2 Xen | expand

Commit Message

Bobby Eshleman Jan. 22, 2021, 12:51 a.m. UTC 
From: Daniel Kiper <daniel.kiper@oracle.com>

In comparison to ELF the PE format is not supported by the Multiboot
protocol. So, if we wish to load xen.mb.efi using this protocol we
have to put header_addr, load_addr, load_end_addr, bss_end_addr and
entry_addr data into Multiboot header.

The Multiboot protocol spec can be found at
  https://www.gnu.org/software/grub/manual/multiboot/

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Bobby Eshleman <bobbyeshleman@gmail.com>
---
 xen/arch/x86/boot/head.S | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

Comments

Jan Beulich March 15, 2021, 3:05 p.m. UTC | #1 
On 22.01.2021 01:51, Bobby Eshleman wrote:
> From: Daniel Kiper <daniel.kiper@oracle.com>
> 
> In comparison to ELF the PE format is not supported by the Multiboot
> protocol. So, if we wish to load xen.mb.efi using this protocol we
> have to put header_addr, load_addr, load_end_addr, bss_end_addr and
> entry_addr data into Multiboot header.
> 
> The Multiboot protocol spec can be found at
>   https://www.gnu.org/software/grub/manual/multiboot/

And because of this spec saying "the boot loader should use them
instead of the fields in the actual executable header to calculate
where to load the OS image" this change will affect the ELF image
as well. For example ...

> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -50,13 +50,24 @@ ENTRY(start)
>          .balign 4
>  multiboot1_header:             /*** MULTIBOOT1 HEADER ****/
>  #define MULTIBOOT_HEADER_FLAGS (MULTIBOOT_HEADER_MODS_ALIGNED | \
> -                                MULTIBOOT_HEADER_WANT_MEMORY)
> +                                MULTIBOOT_HEADER_WANT_MEMORY | \
> +                                MULTIBOOT_HEADER_HAS_ADDR)
>          /* Magic number indicating a Multiboot header. */
>          .long   MULTIBOOT_HEADER_MAGIC
>          /* Flags to bootloader (see Multiboot spec). */
>          .long   MULTIBOOT_HEADER_FLAGS
>          /* Checksum: must be the negated sum of the first two fields. */
>          .long   -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
> +        /* header_addr */
> +        .long   sym_offs(multiboot1_header)
> +        /* load_addr */
> +        .long   sym_offs(start)
> +        /* load_end_addr */
> +        .long   sym_offs(__bss_start)
> +        /* bss_end_addr */
> +        .long   sym_offs(__2M_rwdata_end)

... the ELF image end at _end, not at __2M_rwdata_end. I realize
that with 2M alignment in use, this may actually be a problem, as
one of the modules (the initrd in particular) could be placed
overlapping the (_end, __2M_rwdata_end) range. Nevertheless I
think you want to specify _end (or __bss_end) here.

As to the initial point made - would it be possible to leave the
flag unset in the EFL image and force it set only in xen.mb.efi?
Yes, this may require yet another post-processing step.

Jan

> +        /* entry_addr */
> +        .long   sym_offs(__start)
>  
>          .size multiboot1_header, . - multiboot1_header
>          .type multiboot1_header, @object
>
diff mbox series

Patch

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 2987b4f03a..189d91a872 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -50,13 +50,24 @@  ENTRY(start)
         .balign 4
 multiboot1_header:             /*** MULTIBOOT1 HEADER ****/
 #define MULTIBOOT_HEADER_FLAGS (MULTIBOOT_HEADER_MODS_ALIGNED | \
-                                MULTIBOOT_HEADER_WANT_MEMORY)
+                                MULTIBOOT_HEADER_WANT_MEMORY | \
+                                MULTIBOOT_HEADER_HAS_ADDR)
         /* Magic number indicating a Multiboot header. */
         .long   MULTIBOOT_HEADER_MAGIC
         /* Flags to bootloader (see Multiboot spec). */
         .long   MULTIBOOT_HEADER_FLAGS
         /* Checksum: must be the negated sum of the first two fields. */
         .long   -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
+        /* header_addr */
+        .long   sym_offs(multiboot1_header)
+        /* load_addr */
+        .long   sym_offs(start)
+        /* load_end_addr */
+        .long   sym_offs(__bss_start)
+        /* bss_end_addr */
+        .long   sym_offs(__2M_rwdata_end)
+        /* entry_addr */
+        .long   sym_offs(__start)
 
         .size multiboot1_header, . - multiboot1_header
         .type multiboot1_header, @object