1 //! Exception-throw logic for Wasm exceptions.
2 
3 use super::{VMContext, VMStore};
4 use crate::{store::AutoAssertNoGc, vm::Instance};
5 use core::ptr::NonNull;
6 use wasmtime_environ::TagIndex;
7 use wasmtime_unwinder::{Frame, Handler};
8 
9 /// Compute the target of the pending exception on the store.
10 ///
11 /// # Safety
12 ///
13 /// The stored last-exit state in `store` either must be valid, or
14 /// must have a zeroed exit FP if no Wasm is on the stack.
15 pub unsafe fn compute_handler(store: &mut dyn VMStore) -> Option<Handler> {
16     let mut nogc = AutoAssertNoGc::new(store.store_opaque_mut());
17 
18     // Get the tag identity relative to the store.
19 
20     // Temporarily take, to avoid borrowing issues.
21     let exnref = nogc
22         .take_pending_exception()
23         .expect("Only invoked when an exception is pending");
24     let (throwing_tag_instance_id, throwing_tag_defined_tag_index) =
25         exnref.tag(&mut nogc).expect("cannot read tag");
26     nogc.set_pending_exception(exnref);
27     log::trace!(
28         "throwing: tag defined in instance {throwing_tag_instance_id:?} defined-tag {throwing_tag_defined_tag_index:?}"
29     );
30 
31     // Get the state needed for a stack walk.
32     let (exit_pc, exit_fp, entry_fp) = unsafe {
33         (
34             *nogc.vm_store_context().last_wasm_exit_pc.get(),
35             nogc.vm_store_context().last_wasm_exit_fp(),
36             *nogc.vm_store_context().last_wasm_entry_fp.get(),
37         )
38     };
39 
40     // Early out: if there is no exit FP -- which can happen if a host
41     // func, wrapped up as a `Func`, is called directly via
42     // `Func::call` -- then the only possible action we can take is
43     // `None` (i.e., no handler, unwind to entry from host).
44     if exit_fp == 0 {
45         return None;
46     }
47 
48     // Walk the stack, looking up the module with each PC, and using
49     // that module to resolve local tag indices into (instance, tag)
50     // tuples.
51     let handler_lookup = |frame: &Frame| -> Option<(usize, usize)> {
52         log::trace!(
53             "exception-throw stack walk: frame at FP={:x} PC={:x}",
54             frame.fp(),
55             frame.pc()
56         );
57         let (module, rel_pc) = nogc.modules().module_and_code_by_pc(frame.pc())?;
58         let et = module.module().exception_table();
59         let (frame_offset, handlers) = et.lookup_pc(u32::try_from(rel_pc).unwrap());
60         let fp_to_sp = frame_offset.map(|frame_offset| -isize::try_from(frame_offset).unwrap());
61         for handler in handlers {
62             log::trace!("-> checking handler: {handler:?}");
63             let is_match = match handler.tag {
64                 // Catch-all/default handler. Always come last in sequence.
65                 None => true,
66                 Some(module_local_tag_index) => {
67                     let fp_to_sp =
68                         fp_to_sp.expect("frame offset is necessary for exception unwind");
69                     let fp_offset = fp_to_sp
70                         + isize::try_from(
71                             handler
72                                 .context_sp_offset
73                                 .expect("dynamic context not present for handler record"),
74                         )
75                         .unwrap();
76                     let frame_vmctx = unsafe { frame.read_slot_from_fp(fp_offset) };
77                     log::trace!("-> read vmctx from frame: {frame_vmctx:x}");
78                     let frame_vmctx =
79                         NonNull::new(frame_vmctx as *mut VMContext).expect("null vmctx in frame");
80 
81                     // SAFETY: we use `Instance::from_vmctx` to get a
82                     // `NonNull<Instance>` from a raw vmctx we read off the
83                     // stack frame. That method's safety requirements are that
84                     // the `vmctx` is a valid vmctx allocation which should be
85                     // true of all frames on the stack.
86                     //
87                     // Next the `.as_ref()` call enables reading this pointer,
88                     // and the validity of this relies on the fact that all wasm
89                     // frames for this activation belong to the same store and
90                     // there's no other active instance borrows at this time.
91                     // This function takes `&mut dyn VMStore` representing no
92                     // other active borrows, and internally the borrow is scoped
93                     // to this one block.
94                     let (handler_tag_instance, handler_tag_index) = unsafe {
95                         let store_id = nogc.id();
96                         let instance = Instance::from_vmctx(frame_vmctx);
97                         let tag = instance
98                             .as_ref()
99                             .get_exported_tag(store_id, TagIndex::from_u32(module_local_tag_index));
100                         tag.to_raw_indices()
101                     };
102                     log::trace!(
103                         "-> handler's tag {module_local_tag_index:?} resolves to instance {handler_tag_instance:?} defined-tag {handler_tag_index:?}"
104                     );
105 
106                     handler_tag_instance == throwing_tag_instance_id
107                         && handler_tag_index == throwing_tag_defined_tag_index
108                 }
109             };
110             if is_match {
111                 let fp_to_sp = fp_to_sp.expect("frame offset must be known if we found a handler");
112                 return Some((
113                     (module.store_code().text_range().start
114                         + usize::try_from(handler.handler_offset)
115                             .expect("Module larger than usize"))
116                     .raw(),
117                     frame.fp().wrapping_add_signed(fp_to_sp),
118                 ));
119             }
120         }
121         None
122     };
123     let unwinder = nogc.unwinder();
124     let action = unsafe { Handler::find(unwinder, handler_lookup, exit_pc, exit_fp, entry_fp) };
125     log::trace!("throw action: {action:?}");
126     action
127 }
128