[PATCH] arm64: errata: add module build workaround for erratum #843419

[Ard Biesheuvel]

On 6 October 2015 at 22:44, Dann Frazier <dann.frazier at canonical.com> wrote:
> On Wed, Sep 16, 2015 at 5:16 AM, Will Deacon <will.deacon at arm.com> wrote:
>> Cortex-A53 processors <= r0p4 are affected by erratum #843419 which can
>> lead to a memory access using an incorrect address in certain sequences
>> headed by an ADRP instruction.
>
> Just a heads up that we're seeing a regression in the Ubuntu 4.2 kernel on
> X-Gene after this patch is applied and the CONFIG enabled.
>
> Modules loads fail with messages like:
>
> [ 2.192721] module gpio_xgene_sb: unsupported RELA relocation: 275
> [ 2.193609] module xgene_enet: unsupported RELA relocation: 275
> [ 2.249402] module libahci: unsupported RELA relocation: 275
> [ 2.249628] module xgene_enet: unsupported RELA relocation: 275
> [ 2.359451] module xgene_enet: unsupported RELA relocation: 275
> [ 2.389444] module xgene_enet: unsupported RELA relocation: 275
> [ 3.473766] module linear: unsupported RELA relocation: 275
> [ 3.543252] module multipath: unsupported RELA relocation: 275
> [ 3.593268] module raid0: unsupported RELA relocation: 275
> [ 3.663695] module raid1: unsupported RELA relocation: 275
> [ 3.713964] module raid6_pq: unsupported RELA relocation: 275
> [ 3.763983] module raid6_pq: unsupported RELA relocation: 275
> [ 3.803975] module raid6_pq: unsupported RELA relocation: 275
> [ 3.853881] module raid10: unsupported RELA relocation: 275
> [ 3.924962] module raid6_pq: unsupported RELA relocation: 275
>

RELA #275 is the relocation against ADRP instructions, which GCC
should not emit anymore when -mcmodel=large is in effect.

Can you confirm that the modules have been rebuilt with this config as
well? Can you double check the GCC command line (with V=1) when doing
'make modules' to ensure that '-mcmodel=large' is being passed? Can
you check with 'readelf -r' which objects still contain
R_AARCH64_ADR_PREL_PG_HI21 relocations?



> I haven't attempted to reproduce with a pure upstream kernel yet.
>
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1502946
>
>    -dann
>
>> There is a linker fix to generate veneers for ADRP instructions, but
>> this doesn't work for kernel modules which are built as unlinked ELF
>> objects.
>>
>> This patch adds a new config option for the erratum which, when enabled,
>> builds kernel modules with the mcmodel=large flag. This uses absolute
>> addressing for all kernel symbols, thereby removing the use of ADRP as
>> a PC-relative form of addressing. The ADRP relocs are removed from the
>> module loader so that we fail to load any potentially affected modules.
>>
>> Signed-off-by: Will Deacon <will.deacon at arm.com>
>> ---
>>  arch/arm64/Kconfig         | 16 ++++++++++++++++
>>  arch/arm64/Makefile        |  4 ++++
>>  arch/arm64/kernel/module.c |  2 ++
>>  3 files changed, 22 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 7d95663c0160..11ff4d57c92a 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -331,6 +331,22 @@ config ARM64_ERRATUM_845719
>>
>>           If unsure, say Y.
>>
>> +config ARM64_ERRATUM_843419
>> +       bool "Cortex-A53: 843419: A load or store might access an incorrect address"
>> +       depends on MODULES
>> +       default y
>> +       help
>> +         This option builds kernel modules using the large memory model in
>> +         order to avoid the use of the ADRP instruction, which can cause
>> +         a subsequent memory access to use an incorrect address on Cortex-A53
>> +         parts up to r0p4.
>> +
>> +         Note that the kernel itself must be linked with a version of ld
>> +         which fixes potentially affected ADRP instructions through the
>> +         use of veneers.
>> +
>> +         If unsure, say Y.
>> +
>>  endmenu
>>
>>
>> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
>> index 15ff5b4156fd..f9914d7c1bb0 100644
>> --- a/arch/arm64/Makefile
>> +++ b/arch/arm64/Makefile
>> @@ -41,6 +41,10 @@ endif
>>
>>  CHECKFLAGS     += -D__aarch64__
>>
>> +ifeq ($(CONFIG_ARM64_ERRATUM_843419), y)
>> +CFLAGS_MODULE  += -mcmodel=large
>> +endif
>> +
>>  # Default value
>>  head-y         := arch/arm64/kernel/head.o
>>
>> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
>> index 67bf4107f6ef..876eb8df50bf 100644
>> --- a/arch/arm64/kernel/module.c
>> +++ b/arch/arm64/kernel/module.c
>> @@ -332,12 +332,14 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
>>                         ovf = reloc_insn_imm(RELOC_OP_PREL, loc, val, 0, 21,
>>                                              AARCH64_INSN_IMM_ADR);
>>                         break;
>> +#ifndef CONFIG_ARM64_ERRATUM_843419
>>                 case R_AARCH64_ADR_PREL_PG_HI21_NC:
>>                         overflow_check = false;
>>                 case R_AARCH64_ADR_PREL_PG_HI21:
>>                         ovf = reloc_insn_imm(RELOC_OP_PAGE, loc, val, 12, 21,
>>                                              AARCH64_INSN_IMM_ADR);
>>                         break;
>> +#endif
>>                 case R_AARCH64_ADD_ABS_LO12_NC:
>>                 case R_AARCH64_LDST8_ABS_LO12_NC:
>>                         overflow_check = false;
>> --
>> 2.1.4
>>
>>
>> _______________________________________________
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>
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