[PATCH v2 04/15] efi/libstub: Add a general way to get symbols from the vmlinux into zboot

[Jonathan Cameron]

On Fri,  2 Oct 2026 20:31:41 -0300
Jason Gunthorpe <jgg at nvidia.com> wrote:

> Currently the zboot stub has no access to symbols in the vmlinux. In
> contrast x86 has a voffsets.h header that allows the stub to know whatever
> symbols it wants for the decompressed image. ARM DRTM needs another 4
> symbol locations to work, and it also wants to compile the DRTM libstub
> code once and have it work in both zboot and embedded.
> 
> The current solution in arm is "code_size" from commit 45dd403da851
> ("efi/zboot: arm64: Inject kernel code size symbol into the zboot
> payload"). This is neat, but it makes the zboot and embedded cases
> work differently.

Not a real review as linker fun is not my forte.  So just some
trivial formatting comments.


> 
> The x86 voffsets.h approach is also nice, but it can only work in zboot
> since it relies on 'nm vmlinux' to extract the locations. That's a
> circular dependency for the embedded stub.
> 
> One idea that meets the requirements is to embed a struct of offsets
> inside the vmlinux and then have both stubs know its offset from the start
> of the image.
> 
> First, each arch defines what information it wants the stub to have in a
> struct:
> 
>        struct efi_image_info {
> 	       __le64 code_size;
>        };
> 
> Second, it populates the struct with the linker:
> 
>        EFI_IMAGE_INFO(
> 	       /* code_size */
> 	       EFI_IMAGE_INFO_OFFSET(__inittext_end)
>        )
> 
> This approach with the linker is guaranteed to emit a value and not a
> relocation. I looked at writing the struct initializer in C, but that can
> result in relocations inside the struct. That seemed sketchy so I stuck
> with a linker script approach.
> 
> Finally, any stub code, either in zboot or embedded can access the struct
> through:
> 
>   const struct efi_image_info *efi_get_image_info(unsigned long image_base);
> 
> The makefiles arrange that both links have a "u64 __efi_image_info_offset"
> symbol that contains the value. For vmlinux this links to a symbol created
> by the EFI_IMAGE_INFO() macro, for zboot it uses the same technique as
> code_size through the zboot.lds.
> 
> Since this needs arch changes, protect it under
> CONFIG_EFI_STUB_IMAGE_INFO. The arch should define the struct and use the

Odd line wrap as moving the CONFIG up a line is still shorter than some of
the lines above.

> linker macro before enabling that config.
> 
> Since everything is linked together, and the actual location of the struct
> is not fixed, this is all private within the kernel build and can't become
> ABI to any bootloader.
> 
> As some follow-up work, it seems interesting to standardize on this
> mechanism and remove the PROVIDE() and voffset.h alternatives. This would
> also avoid needing to pass sysfb_primary_display through EFI from zboot,
> for example.
> 
> Signed-off-by: Jason Gunthorpe <jgg at nvidia.com>


> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b2988aa12f6645..4dd52ddb148401 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -83,6 +83,44 @@
>  #define RO_EXCEPTION_TABLE
>  #endif
>  
> +/*
> + * Architecture specific information made available to the EFI stub, see
> + * CONFIG_EFI_STUB_IMAGE_INFO. The architecture describes the data with a
> + * struct efi_image_info in its asm/image.h and emits the matching bytes here
> + * through contents, using linker expressions the compiler cannot compute.
> + *
> + * Place in the same section as INIT_DATA.
> + *
> + * The architecture must also define EFI_IMAGE_INFO_SIZE to sizeof(struct
> + * efi_image_info), and that definition has to be visible to the linker script

I'd wrap this to have the whole sizeof() on one line line.

> + * before this macro is used. Keeping the two in sync is checked by a
> + * static_assert() next to the struct and by the ASSERT() below.
> + *
> + * The struct's offset from _text is emitted as a u64 at
> + * __efi_image_info_offset. The architecture aliases this location to
> + * __efistub_efi_image_info_offset for the embedded stub. The absolute
> + * _efi_image_info_offset_value contains the offset itself; Makefile.zboot
> + * extracts it with nm and zboot.lds emits it for the compressed image.
> + */

>