On 2020-11-24 15:50, Will Deacon wrote:
When confronted with a mixture of CPUs, some of which support 32-bit
applications and others which don't, we quite sensibly treat the system
as 64-bit only for userspace and prevent execve() of 32-bit binaries.
Unfortunately, some crazy folks have decided to build systems like this
with the intention of running 32-bit applications, so relax our
sanitisation logic to continue to advertise 32-bit support to userspace
on these systems and track the real 32-bit capable cores in a cpumask
instead. For now, the default behaviour remains but will be tied to
a command-line option in a later patch.
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
arch/arm64/include/asm/cpucaps.h | 2 +-
arch/arm64/include/asm/cpufeature.h | 8 ++-
arch/arm64/kernel/cpufeature.c | 106 ++++++++++++++++++++++++++--
3 files changed, 107 insertions(+), 9 deletions(-)
diff --git a/arch/arm64/include/asm/cpucaps.h
b/arch/arm64/include/asm/cpucaps.h
index e7d98997c09c..e6f0eb4643a0 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -20,7 +20,7 @@
#define ARM64_ALT_PAN_NOT_UAO 10
#define ARM64_HAS_VIRT_HOST_EXTN 11
#define ARM64_WORKAROUND_CAVIUM_27456 12
-#define ARM64_HAS_32BIT_EL0 13
+#define ARM64_HAS_32BIT_EL0_DO_NOT_USE 13
#define ARM64_HARDEN_EL2_VECTORS 14
#define ARM64_HAS_CNP 15
#define ARM64_HAS_NO_FPSIMD 16
diff --git a/arch/arm64/include/asm/cpufeature.h
b/arch/arm64/include/asm/cpufeature.h
index 97244d4feca9..f447d313a9c5 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -604,9 +604,15 @@ static inline bool
cpu_supports_mixed_endian_el0(void)
return id_aa64mmfr0_mixed_endian_el0(read_cpuid(ID_AA64MMFR0_EL1));
}
+const struct cpumask *system_32bit_el0_cpumask(void);
+DECLARE_STATIC_KEY_FALSE(arm64_mismatched_32bit_el0);
+
static inline bool system_supports_32bit_el0(void)
{
- return cpus_have_const_cap(ARM64_HAS_32BIT_EL0);
+ u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
+
+ return id_aa64pfr0_32bit_el0(pfr0) ||
+ static_branch_unlikely(&arm64_mismatched_32bit_el0);
nit: swapping the two sides of this expression has the potential
for slightly better code, resulting in better performance on
these asymmetric systems. Not a big real, but since this lands
on the fast path on vcpu exit, I'll take every bit of optimisation.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
