Re: [PATCH v2 2/2] arm64: don't load 32-bit binaries if platform has no aarch32_el0

[Ard Biesheuvel]

On 2 September 2015 at 16:00, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> 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 <ynorov@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/cpufeature.h |  1 +
>  arch/arm64/include/asm/cputype.h    |  8 ++++++++
>  arch/arm64/include/asm/elf.h        |  6 ++++--
>  arch/arm64/kernel/cpuinfo.c         | 12 ++++++++++++
>  4 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index f0e4017..35f2654 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -83,6 +83,7 @@ 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/cputype.h b/arch/arm64/include/asm/cputype.h
> index a84ec60..a24e42c 100644
> --- a/arch/arm64/include/asm/cputype.h
> +++ b/arch/arm64/include/asm/cputype.h
> @@ -81,6 +81,9 @@
>  #define ID_AA64MMFR0_BIGEND(mmfr0)     \
>         (((mmfr0) & ID_AA64MMFR0_BIGEND_MASK) >> ID_AA64MMFR0_BIGEND_SHIFT)
>
> +#define ID_AA64PFR0_EL0_64     1
> +#define ID_AA64PFR0_EL0_6432   2
> +
>  #define SCTLR_EL1_CP15BEN      (0x1 << 5)
>  #define SCTLR_EL1_SED          (0x1 << 8)
>
> @@ -116,6 +119,11 @@ static inline u32 __attribute_const__ read_cpuid_cachetype(void)
>         return read_cpuid(CTR_EL0);
>  }
>
> +static inline bool id_aa64pfr0_aarch32_el0(u64 pfr0)
> +{
> +       return pfr0 == ID_AA64PFR0_EL0_6432;
> +}
> +

Shouldn't you be masking some bits in pfr0 before doing the comparison?
ID_AA64PFR0_EL1.EL0 occupies bits [3:0] only.

>  static inline bool id_aa64mmfr0_mixed_endian_el0(u64 mmfr0)
>  {
>         return (ID_AA64MMFR0_BIGEND(mmfr0) == 0x1) ||
> 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..4a6ae31 100644
> --- a/arch/arm64/kernel/cpuinfo.c
> +++ b/arch/arm64/kernel/cpuinfo.c
> @@ -36,6 +36,7 @@
>  DEFINE_PER_CPU(struct cpuinfo_arm64, cpu_data);
>  static struct cpuinfo_arm64 boot_cpu_data;
>  static bool mixed_endian_el0 = true;
> +static bool aarch32_el0 = true;
>
>  static char *icache_policy_str[] = {
>         [ICACHE_POLICY_RESERVED] = "RESERVED/UNKNOWN",
> @@ -79,13 +80,24 @@ bool system_supports_mixed_endian_el0(void)
>         return mixed_endian_el0;
>  }
>
> +bool system_supports_aarch32_el0(void)
> +{
> +       return aarch32_el0;
> +}
> +
>  static void update_mixed_endian_el0_support(struct cpuinfo_arm64 *info)
>  {
>         mixed_endian_el0 &= id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0);
>  }
>
> +static void update_aarch32_el0_support(struct cpuinfo_arm64 *info)
> +{
> +       aarch32_el0 &= id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0);
> +}
> +
>  static void update_cpu_features(struct cpuinfo_arm64 *info)
>  {
> +       update_aarch32_el0_support(info);
>         update_mixed_endian_el0_support(info);
>  }
>
> --
> 2.1.4
>
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