Subject: [PATCH 2/2] arm64: don't load 32-bit binaries if platform has no aarch32_el0

From: [email-masked] (Suzuki K. Poulose)
Date: Tue, 01 Sep 2015 18:41:29 +0100

On 01/09/15 17:04, Mark Rutland wrote:
> On Tue, Sep 01, 2015 at 03:41:12PM +0100, Yury Norov wrote:
>> Kernel option COMPAT defines the ability of executing aarch32 binaries.
>> Some platforms does not support aarch32 mode, and so cannot execute that
>> binaries. But we cannot just disable COMPAT for them because the same
>> kernel binary may be used by multiple platforms.
>>
>> In this patch, system_supports_aarch32_el0() is introduced to detect
>> aarch32 support at run-time.
>>
>> Signed-off-by: Yury Norov <[email-masked]>
>> ---
>>   arch/arm64/include/asm/cpufeature.h | 1 +
>>   arch/arm64/include/asm/elf.h        | 6 ++++--
>>   arch/arm64/kernel/cpuinfo.c         | 9 +++++++++
>>   3 files changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>> index 20cdc26..d24ea15 100644
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -81,6 +81,7 @@ void check_local_cpu_errata(void);
>>   void check_local_cpu_features(void);
>>   bool cpu_supports_mixed_endian_el0(void);
>>   bool system_supports_mixed_endian_el0(void);
>> +bool system_supports_aarch32_el0(void);
>>
>>   #endif /* __ASSEMBLY__ */
>>
>> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
>> index faad6df..461897b 100644
>> --- a/arch/arm64/include/asm/elf.h
>> +++ b/arch/arm64/include/asm/elf.h
>> @@ -21,6 +21,7 @@
>>   /*
>>    * ELF register definitions..
>>    */
>> +#include <asm/cpufeature.h>
>>   #include <asm/ptrace.h>
>>   #include <asm/user.h>
>>
>> @@ -173,8 +174,9 @@ typedef compat_elf_greg_t		compat_elf_gregset_t[COMPAT_ELF_NGREG];
>>
>>   /* AArch32 EABI. */
>>   #define EF_ARM_EABI_MASK		0xff000000
>> -#define compat_elf_check_arch(x)	(((x)->e_machine == EM_ARM) && \
>> -					 ((x)->e_flags & EF_ARM_EABI_MASK))
>> +#define compat_elf_check_arch(x)	(system_supports_aarch32_el0()	\
>> +					&& ((x)->e_machine == EM_ARM)	\
>> +					&& ((x)->e_flags & EF_ARM_EABI_MASK))
>>
>>   #define compat_start_thread		compat_start_thread
>>   #define COMPAT_SET_PERSONALITY(ex)	set_thread_flag(TIF_32BIT);
>> diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
>> index 75d5a86..95d953f 100644
>> --- a/arch/arm64/kernel/cpuinfo.c
>> +++ b/arch/arm64/kernel/cpuinfo.c
>> @@ -79,6 +79,15 @@ bool system_supports_mixed_endian_el0(void)
>>   	return mixed_endian_el0;
>>   }
>>
>> +#define AARCH64		1
>> +#define AARCH32_64	2
>
> These should be better namespaced. Perhaps ID_AA64PFR0_EL1_EL0_64 and
> ID_AA64PFR0_EL1_EL0_6432 ?
>
>> +bool system_supports_aarch32_el0(void)
>> +{
>> +	struct cpuinfo_arm64 *info = this_cpu_ptr(&cpu_data);
>> +	u64 arm64_el0 = info->reg_id_aa64pfr0 & 0xf;
>> +	return arm64_el0 == AARCH32_64;
>> +}
>
> We should handle this the same way as we do for endianness support and
> check that all CPUs support AArch32, and set a global flag, rather than
> assuming that all CPUs are symmetric. Likewise for any other feature we
> have to dynamically detect.

Correct. With my series, which I will post after the merge window, we should be able to
define this as a CPU capability and check that bit instead, which would give us a system
wide value.

Suzuki

> Thanks,
> Mark.