From: ard.biesheuvel@linaro.org (Ard Biesheuvel)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] arm: boot/compressed: fix decompressor header layout for v7-M
Date: Fri, 19 May 2017 18:33:52 +0100 [thread overview]
Message-ID: <2E764C85-57F3-4DC2-ADEB-B6B75A70B898@linaro.org> (raw)
In-Reply-To: <67bd95eb-1f4b-d9d2-9a68-3574d03ac072@arm.com>
> On 19 May 2017, at 18:16, Robin Murphy <robin.murphy@arm.com> wrote:
>
>> On 19/05/17 17:59, Ard Biesheuvel wrote:
>>> On 19 May 2017 at 17:46, Dave Martin <Dave.Martin@arm.com> wrote:
>>>> On Fri, May 19, 2017 at 03:59:59PM +0100, Ard Biesheuvel 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@st.com>
>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>>> ---
>>>> arch/arm/boot/compressed/efi-header.S | 4 +---
>>>> arch/arm/boot/compressed/head.S | 14 +++++++-------
>>>> 2 files changed, 8 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..8c336a6da210 100644
>>>> --- a/arch/arm/boot/compressed/head.S
>>>> +++ b/arch/arm/boot/compressed/head.S
>>>> @@ -130,19 +130,19 @@ start:
>>>> .rept 7
>>>> __nop
>>>> .endr
>>>> - ARM( mov r0, r0 )
>>>> - ARM( b 1f )
>>>> - THUMB( badr r12, 1f )
>>>> - THUMB( bx r12 )
>>>> + ARM( b 1f )
>>>> + AR_CLASS( sub pc, pc, #3 ) @ A/R: switch to Thumb2 mode
>>>> + M_CLASS( nop.w ) @ M: already in Thumb2 mode
>>>> + THUMB( .thumb )
>>>> + THUMB( b.w 1f )
>>>
>>> It's not so easy to see that this always assembles to two words, or that
>>> the "sub pc, pc, #3" is assembled but not executed in an ARM kernel.
>>>
>>> Spelling it out might be more readable:
>>>
>>> ARM( mov r0, r0 )
>>> ARM( mov r0, r0 )
>>>
>>> THUMB( AR_CLASS( sub pc, pc, #3 ))
>>> THUMB( M_CLASS( nop.w ))
>>> THUMB( .thumb )
>>> THUMB( b.w 1f )
>>>
>>> But I guess it works either way.
>>>
>>
>> Indeed. Apart from the error in the second line, this sequence should
>> be equivalent, but the nested macro invocations don't make it clearer
>> imo.
>>
>> I could add some more comments as well if that helps?
>
> Hmm, couldn't it be ISA-agnostic?
>
> ldr pc, 1f
> .ltorg
> THUMB( .thumb )
> ...
>
> given that that should be interworking on anything where THUMB() is
> valid in the first place.
>
The decompressor runs at a a priori unknown offset so literals containing absolute values are not helpful here.
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel at lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>
>
next prev parent reply other threads:[~2017-05-19 17:33 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-19 14:59 [PATCH] arm: boot/compressed: fix decompressor header layout for v7-M Ard Biesheuvel
2017-05-19 15:31 ` Nicolas Pitre
2017-05-19 16:46 ` Dave Martin
2017-05-19 16:59 ` Ard Biesheuvel
2017-05-19 17:08 ` Russell King - ARM Linux
2017-05-19 17:13 ` Ard Biesheuvel
2017-05-19 17:16 ` Robin Murphy
2017-05-19 17:33 ` Ard Biesheuvel [this message]
2017-05-19 17:58 ` Robin Murphy
2017-05-19 18:33 ` Russell King - ARM Linux
2017-05-19 17:34 ` Dave Martin
2017-05-19 17:38 ` Dave Martin
2017-05-19 17:37 ` Russell King - ARM Linux
2017-05-19 17:29 ` Dave Martin
2017-05-19 17:35 ` Russell King - ARM Linux
2017-05-19 17:37 ` Dave Martin
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