precise getrandom return type and align_offset arithmetic

Author: RalfJung

src/shims/foreign_items/posix/linux.rs

@@ -140,6 +140,6 @@ fn getrandom<'tcx>(
     let _flags = this.read_scalar(args[2])?.to_i32()?;
 
     this.gen_random(ptr, len)?;
-    this.write_scalar(Scalar::from_uint(len, dest.layout.size), dest)?;
+    this.write_scalar(Scalar::from_machine_usize(len, this), dest)?;
     Ok(())
 }

src/shims/mod.rs

@@ -75,7 +75,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let ptr_scalar = this.read_scalar(ptr_op)?.not_undef()?;
 
         // Default: no result.
-        let mut result = this.truncate(u128::MAX, dest.layout);
+        let mut result = this.usize_max();
         if let Ok(ptr) = this.force_ptr(ptr_scalar) {
             // Only do anything if we can identify the allocation this goes to.
             let cur_align =
@@ -84,15 +84,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 // If the allocation alignment is at least the required alignment we use the
                 // libcore implementation.
                 // FIXME: is this correct in case of truncation?
-                result = u128::try_from(
+                result = u64::try_from(
                     (this.force_bits(ptr_scalar, this.pointer_size())? as *const i8)
                         .align_offset(usize::try_from(req_align).unwrap())
                 ).unwrap();
             }
         }
 
         // Return result, and jump to caller.
-        this.write_scalar(Scalar::from_uint(result, this.pointer_size()), dest)?;
+        this.write_scalar(Scalar::from_machine_usize(result, this), dest)?;
         this.go_to_block(ret);
         Ok(())
     }