bpf_loader: fix non-portable cross-VM transmutation of Instruction

The `sol_invoke_signed_rust` syscall handler initializes takes a reference to the `Instruction` type backed by VM memory in the `translate_instruction` function. The `Instruction` type contains an std `Vec<AccountMeta>`.
Because the host and VM have different memory maps the resulting `&Vec` object is corrupted in a way that breaks Rust's safety semantics. For example, trying to access any Vec element (a safe operation) will result in a segfault, as the host would try to resolve a VM pointer without translating it.

The transmutation of the `Vec<_>` type in an SBFv2 Rust environment to the host (x86_64 or arm64) Rust environment is also UB. Rust has no ABI stability guarantees, neither across different compiler versions, nor across different architectures. For this reason, the solana-bpf-loader-program would not be able to execute any Rust programs that use CPIs on 32-bit hosts.

This code does not cause a security vulnerability but this construct should be improved to bring it more in line with the [Rust Unsafe Code Guidelines Reference](https://rust-lang.github.io/unsafe-code-guidelines/).

- Introduces a new stable `RustVMVec<T>` type that describes `Vec<T>` on SBFv2.
- Introduces a new stable `RustVMInstruction` type that describes `Instruction` on SBFv2.
- Replaces non-portable `translate_instruction` code with a cross-arch/cross-rustc portable variant.

programs/bpf_loader/src/syscalls.rs

@@ -46,6 +46,7 @@ use {
     std::{
         alloc::Layout,
         cell::{Ref, RefCell, RefMut},
+        marker::PhantomData,
         mem::{align_of, size_of},
         rc::Rc,
         slice::from_raw_parts_mut,
@@ -2027,6 +2028,62 @@ struct CallerAccount<'a> {
 }
 type TranslatedAccounts<'a> = Vec<(usize, Option<CallerAccount<'a>>)>;
 
+/// Stable representation of a `Vec<T>` in the SBFv2 Rust runtime.
+/// Only supports the global allocator.
+#[repr(C)]
+struct RustVMVec<T: Sized> {
+    ptr: u64,
+    raw_cap: u64,
+    raw_len: u64,
+    _phantom: PhantomData<T>,
+}
+
+impl<T: Sized> RustVMVec<T> {
+    fn len(&self) -> usize {
+        // An u64 to usize conversion can only fail in the case of an overflow.
+        // Realistically, this only happens on 32-bit architectures.
+        // Assuming available memory in a VM will never exceed 4 GiB,
+        // "saturating" to UINT32_MAX safely reaches the "slice too large" error path
+        // that a comparable larger value would also trigger on a 64-bit architecture.
+        self.raw_len.try_into().unwrap_or(usize::MAX)
+    }
+
+    /// Returns a slice to the elements pointed to by the Vec object.
+    fn try_as_slice(
+        &self,
+        memory_mapping: &MemoryMapping,
+        invoke_context: &InvokeContext,
+    ) -> Result<&[T], EbpfError<BpfError>> {
+        translate_slice(
+            memory_mapping,
+            self.ptr,
+            self.raw_len,
+            invoke_context.get_check_aligned(),
+            invoke_context.get_check_size(),
+        )
+    }
+}
+
+/// Stable representation of a `solana_program::instruction::Instruction`
+/// in the SBFv2 Rust runtime.
+#[repr(C)]
+struct RustVMInstruction {
+    program_id: Pubkey,
+    accounts: RustVMVec<AccountMeta>,
+    data: RustVMVec<u8>,
+}
+
+impl RustVMInstruction {
+    /// Returns a reference to an Instruction object at the given VM memory address.
+    fn from_vm_memory<'a>(
+        memory_mapping: &MemoryMapping<'a>,
+        vm_addr: u64,
+        invoke_context: &InvokeContext<'a>,
+    ) -> Result<&'a Self, EbpfError<BpfError>> {
+        translate_type(memory_mapping, vm_addr, invoke_context.get_check_aligned())
+    }
+}
+
 /// Implemented by language specific data structure translators
 trait SyscallInvokeSigned<'a, 'b> {
     fn get_context_mut(&self) -> Result<RefMut<&'a mut InvokeContext<'b>>, EbpfError<BpfError>>;
@@ -2093,30 +2150,18 @@ impl<'a, 'b> SyscallInvokeSigned<'a, 'b> for SyscallInvokeSignedRust<'a, 'b> {
         memory_mapping: &mut MemoryMapping,
         invoke_context: &mut InvokeContext,
     ) -> Result<Instruction, EbpfError<BpfError>> {
-        let ix = translate_type::<Instruction>(
-            memory_mapping,
-            addr,
-            invoke_context.get_check_aligned(),
-        )?;
+        let ix = RustVMInstruction::from_vm_memory(memory_mapping, addr, invoke_context)?;
 
         check_instruction_size(ix.accounts.len(), ix.data.len(), invoke_context)?;
 
-        let accounts = translate_slice::<AccountMeta>(
-            memory_mapping,
-            ix.accounts.as_ptr() as u64,
-            ix.accounts.len() as u64,
-            invoke_context.get_check_aligned(),
-            invoke_context.get_check_size(),
-        )?
-        .to_vec();
-        let data = translate_slice::<u8>(
-            memory_mapping,
-            ix.data.as_ptr() as u64,
-            ix.data.len() as u64,
-            invoke_context.get_check_aligned(),
-            invoke_context.get_check_size(),
-        )?
-        .to_vec();
+        let accounts = ix
+            .accounts
+            .try_as_slice(memory_mapping, invoke_context)?
+            .to_vec();
+        let data = ix
+            .data
+            .try_as_slice(memory_mapping, invoke_context)?
+            .to_vec();
         Ok(Instruction {
             program_id: ix.program_id,
             accounts,