[PATCH] optionrom: Remove build-id section

[PATCH] optionrom: Remove build-id section

[Fabiano Rosas]

Our linker script for optionroms specifies only the placement of the
.text section, leaving the linker free to place the remaining sections
at arbitrary places in the file.

Since at least binutils 2.39, the .note.gnu.build-id section is now
being placed at the start of the file, which causes label addresses to
be shifted. For linuxboot_dma.bin that means that the PnP header
(among others) will not be found when determining the type of ROM at
optionrom_setup():

(0x1c is the label _pnph, where the magic "PnP" is)

$ xxd /usr/share/qemu/linuxboot_dma.bin | grep "PnP"
00000010: 0000 0000 0000 0000 0000 1c00 2450 6e50  ............$PnP

$ xxd pc-bios/optionrom/linuxboot_dma.bin | grep "PnP"
00000010: 0000 0000 0000 0000 0000 4c00 2450 6e50  ............$PnP
                                   ^bad

Using a freshly built linuxboot_dma.bin ROM results in a broken boot:

  SeaBIOS (version rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org)
  Booting from Hard Disk...
  Boot failed: could not read the boot disk

  Booting from Floppy...
  Boot failed: could not read the boot disk

  No bootable device.

We're not using the build-id section, so pass the --build-id=none
option to the linker to remove it entirely.

Note: In theory, this same issue could happen with any other
section. The ideal solution would be to have all unused sections
discarded in the linker script. However that would be a larger change,
specially for the pvh rom which uses the .bss and COMMON sections so
I'm addressing only the immediate issue here.

Reported-by: Vasiliy Ulyanov <vulyanov@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 pc-bios/optionrom/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pc-bios/optionrom/Makefile b/pc-bios/optionrom/Makefile
index b1fff0ba6c..30d07026c7 100644
--- a/pc-bios/optionrom/Makefile
+++ b/pc-bios/optionrom/Makefile
@@ -36,7 +36,7 @@ config-cc.mak: Makefile
 	    $(call cc-option,-Wno-array-bounds)) 3> config-cc.mak
 -include config-cc.mak
 
-override LDFLAGS = -nostdlib -Wl,-T,$(SRC_DIR)/flat.lds
+override LDFLAGS = -nostdlib -Wl,--build-id=none,-T,$(SRC_DIR)/flat.lds
 
 pvh.img: pvh.o pvh_main.o
 
-- 
2.35.3

Re: [PATCH] optionrom: Remove build-id section

[Thomas Huth]

On 26/09/2023 21.25, Fabiano Rosas wrote:
> Our linker script for optionroms specifies only the placement of the
> .text section, leaving the linker free to place the remaining sections
> at arbitrary places in the file.
> 
> Since at least binutils 2.39, the .note.gnu.build-id section is now
> being placed at the start of the file, which causes label addresses to
> be shifted. For linuxboot_dma.bin that means that the PnP header
> (among others) will not be found when determining the type of ROM at
> optionrom_setup():
> 
> (0x1c is the label _pnph, where the magic "PnP" is)
> 
> $ xxd /usr/share/qemu/linuxboot_dma.bin | grep "PnP"
> 00000010: 0000 0000 0000 0000 0000 1c00 2450 6e50  ............$PnP
> 
> $ xxd pc-bios/optionrom/linuxboot_dma.bin | grep "PnP"
> 00000010: 0000 0000 0000 0000 0000 4c00 2450 6e50  ............$PnP
>                                     ^bad
> 
> Using a freshly built linuxboot_dma.bin ROM results in a broken boot:
> 
>    SeaBIOS (version rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org)
>    Booting from Hard Disk...
>    Boot failed: could not read the boot disk
> 
>    Booting from Floppy...
>    Boot failed: could not read the boot disk
> 
>    No bootable device.
> 
> We're not using the build-id section, so pass the --build-id=none
> option to the linker to remove it entirely.
> 
> Note: In theory, this same issue could happen with any other
> section. The ideal solution would be to have all unused sections
> discarded in the linker script. However that would be a larger change,
> specially for the pvh rom which uses the .bss and COMMON sections so
> I'm addressing only the immediate issue here.
> 
> Reported-by: Vasiliy Ulyanov <vulyanov@suse.de>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>   pc-bios/optionrom/Makefile | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/pc-bios/optionrom/Makefile b/pc-bios/optionrom/Makefile
> index b1fff0ba6c..30d07026c7 100644
> --- a/pc-bios/optionrom/Makefile
> +++ b/pc-bios/optionrom/Makefile
> @@ -36,7 +36,7 @@ config-cc.mak: Makefile
>   	    $(call cc-option,-Wno-array-bounds)) 3> config-cc.mak
>   -include config-cc.mak
>   
> -override LDFLAGS = -nostdlib -Wl,-T,$(SRC_DIR)/flat.lds
> +override LDFLAGS = -nostdlib -Wl,--build-id=none,-T,$(SRC_DIR)/flat.lds
>   
>   pvh.img: pvh.o pvh_main.o

I remember that we had to do the same in other projects that use their own 
linker script (kvm-unit-tests?) ... so this looks fine to me.

Reviewed-by: Thomas Huth <thuth@redhat.com>

Re: [PATCH] optionrom: Remove build-id section

[Paolo Bonzini]

Queued, thanks.

Paolo