arch/aarch64: replace offset__of with a safe macro

src/arch/src/aarch64/linux/regs.rs

@@ -5,11 +5,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the THIRD-PARTY file.
 
-use std::{mem, num::TryFromIntError, result};
+use std::{mem, mem::offset_of, num::TryFromIntError, result};
 
 use super::super::get_fdt_addr;
 use kvm_bindings::{
-    user_pt_regs, KVM_REG_ARM64, KVM_REG_ARM64_SYSREG, KVM_REG_ARM64_SYSREG_CRM_MASK,
+    kvm_regs, user_pt_regs, KVM_REG_ARM64, KVM_REG_ARM64_SYSREG, KVM_REG_ARM64_SYSREG_CRM_MASK,
     KVM_REG_ARM64_SYSREG_CRM_SHIFT, KVM_REG_ARM64_SYSREG_CRN_MASK, KVM_REG_ARM64_SYSREG_CRN_SHIFT,
     KVM_REG_ARM64_SYSREG_OP0_MASK, KVM_REG_ARM64_SYSREG_OP0_SHIFT, KVM_REG_ARM64_SYSREG_OP1_MASK,
     KVM_REG_ARM64_SYSREG_OP1_SHIFT, KVM_REG_ARM64_SYSREG_OP2_MASK, KVM_REG_ARM64_SYSREG_OP2_SHIFT,
@@ -42,24 +42,9 @@ const PSR_D_BIT: u64 = 0x0000_0200;
 // Taken from arch/arm64/kvm/inject_fault.c.
 const PSTATE_FAULT_BITS_64: u64 = PSR_MODE_EL1h | PSR_A_BIT | PSR_F_BIT | PSR_I_BIT | PSR_D_BIT;
 
-// Following are macros that help with getting the ID of a aarch64 core register.
+// This is a macro that helps with getting the ID of a aarch64 core register.
 // The core register are represented by the user_pt_regs structure. Look for it in
 // arch/arm64/include/uapi/asm/ptrace.h.
-
-// This macro gets the offset of a structure (i.e `str`) member (i.e `field`) without having
-// an instance of that structure.
-// It uses a null pointer to retrieve the offset to the field.
-// Inspired by C solution: `#define offsetof(str, f) ((size_t)(&((str *)0)->f))`.
-// Doing `offset__of!(user_pt_regs, pstate)` in our rust code will trigger the following:
-// unsafe { &(*(0 as *const user_pt_regs)).pstate as *const _ as usize }
-// The dereference expression produces an lvalue, but that lvalue is not actually read from,
-// we're just doing pointer math on it, so in theory, it should safe.
-macro_rules! offset__of {
-    ($str:ty, $field:ident) => {
-        unsafe { &(*(std::ptr::null::<user_pt_regs>())).$field as *const _ as usize }
-    };
-}
-
 macro_rules! arm64_core_reg {
     ($reg: tt) => {
         // As per `kvm_arm_copy_reg_indices`, the id of a core register can be obtained like this:
@@ -87,7 +72,7 @@ macro_rules! arm64_core_reg {
         KVM_REG_ARM64 as u64
             | KVM_REG_SIZE_U64 as u64
             | u64::from(KVM_REG_ARM_CORE)
-            | ((offset__of!(user_pt_regs, $reg) / mem::size_of::<u32>()) as u64)
+            | (((offset_of!(kvm_regs, regs) + offset_of!(user_pt_regs, $reg)) / mem::size_of::<u32>()) as u64)
     };
 }
 
@@ -126,22 +111,19 @@ arm64_sys_reg!(MPIDR_EL1, 3, 0, 0, 0, 5);
 /// * `mem` - Reserved DRAM for current VM.
 pub fn setup_regs(vcpu: &VcpuFd, cpu_id: u8, boot_ip: u64, mem: &GuestMemoryMmap) -> Result<()> {
     // Get the register index of the PSTATE (Processor State) register.
-    #[allow(deref_nullptr)]
     vcpu.set_one_reg(arm64_core_reg!(pstate), &PSTATE_FAULT_BITS_64.to_le_bytes())
         .map_err(Error::SetCoreRegister)?;
 
     // Other vCPUs are powered off initially awaiting PSCI wakeup.
     if cpu_id == 0 {
         // Setting the PC (Processor Counter) to the current program address (kernel address).
-        #[allow(deref_nullptr)]
         vcpu.set_one_reg(arm64_core_reg!(pc), &boot_ip.to_le_bytes())
             .map_err(Error::SetCoreRegister)?;
 
         // Last mandatory thing to set -> the address pointing to the FDT (also called DTB).
         // "The device tree blob (dtb) must be placed on an 8-byte boundary and must
         // not exceed 2 megabytes in size." -> https://www.kernel.org/doc/Documentation/arm64/booting.txt.
         // We are choosing to place it the end of DRAM. See `get_fdt_addr`.
-        #[allow(deref_nullptr)]
         vcpu.set_one_reg(arm64_core_reg!(regs), &get_fdt_addr(mem).to_le_bytes())
             .map_err(Error::SetCoreRegister)?;
     }