[PATCH v2] arm: boot/compressed: fix decompressor header layout for v7-M

[Ard Biesheuvel]

On 21 May 2017 at 00:50, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> As reported by Patrice, the header layout of the decompressor is
> incorrect when building for v7-M. In this case, the __nop macro
> resolves to 'mov r0, r0', which is emitted as a narrow encoding,
> resulting in the header data fields to end up at lower offsets than
> required.
>
> Given the variety of targets we need to support with the same code,
> the startup sequence is a bit of a jumble, and uses instructions
> and macros whose encoding width cannot be specified (badr), or only
> exists in a narrow encoding (bx)
>
> So force the use of a wide encoding in __nop, and replace the start
> sequence with a simple jump to the label marking the start of code,
> preceded by a Thumb2 mode switch if required (using explicit wide
> encodings where appropriate). The label itself can be moved to the
> start of code [where it belongs] due to the larger range of branch
> instructions as compared to adr instructions.
>
> Reported-by: Patrice CHOTARD <patrice.chotard at st.com>
> Acked-by: Nicolas Pitre <nico at linaro.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
> v2: use #ifdefs to simplify the code
>     add Nico's ack
>
>  arch/arm/boot/compressed/efi-header.S |  4 +---
>  arch/arm/boot/compressed/head.S       | 17 ++++++++++-------
>  2 files changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/boot/compressed/efi-header.S b/arch/arm/boot/compressed/efi-header.S
> index 9d5dc4fda3c1..3f7d1b74c5e0 100644
> --- a/arch/arm/boot/compressed/efi-header.S
> +++ b/arch/arm/boot/compressed/efi-header.S
> @@ -17,14 +17,12 @@
>                 @ there.
>                 .inst   'M' | ('Z' << 8) | (0x1310 << 16)   @ tstne r0, #0x4d000
>  #else
> -               mov     r0, r0
> +               W(mov)  r0, r0
>  #endif
>                 .endm
>
>                 .macro  __EFI_HEADER
>  #ifdef CONFIG_EFI_STUB
> -               b       __efi_start
> -
>                 .set    start_offset, __efi_start - start
>                 .org    start + 0x3c
>                 @
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index 7c711ba61417..8a756870c238 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -130,19 +130,22 @@ start:
>                 .rept   7
>                 __nop
>                 .endr
> -   ARM(                mov     r0, r0          )
> -   ARM(                b       1f              )
> - THUMB(                badr    r12, 1f         )
> - THUMB(                bx      r12             )
> +#ifndef CONFIG_THUMB2_KERNEL
> +               mov     r0, r0
> +#else
> + AR_CLASS(     sub     pc, pc, #3      )       @ A/R: switch to Thumb2 mode
> +  M_CLASS(     nop.w                   )       @ M: already in Thumb2 mode
> +               .thumb
> +#endif
> +               W(b)    1f
>
>                 .word   _magic_sig      @ Magic numbers to help the loader
>                 .word   _magic_start    @ absolute load/run zImage address
>                 .word   _magic_end      @ zImage end address
>                 .word   0x04030201      @ endianness flag
>
> - THUMB(                .thumb                  )
> -1:             __EFI_HEADER
> -
> +               __EFI_HEADER
> +1:
>   ARM_BE8(      setend  be              )       @ go BE8 if compiled for BE8
>   AR_CLASS(     mrs     r9, cpsr        )
>  #ifdef CONFIG_ARM_VIRT_EXT
> --
> 2.7.4
>

I have dropped this into the patch system as 8677/1

I will leave it up to the maintainer to decide whether this should go
to -stable or not, so I have submitted the patch without a Cc:stable
tag

-- 
Ard.