Place unwind info directly after the text section, even when debug info is enabled

When debug info was enabled, we would put the debug info sections in between the
text section and the unwind info section. But the unwind info is encoded in a
position-independent manner (so that we don't need relocs for it) that relies on
it directly following the text section. The result of the misplacement was some
crashes inside the unwinder.

crates/cranelift/src/compiler.rs

@@ -271,6 +271,8 @@ impl wasmtime_environ::Compiler for Compiler {
             trampolines.push(builder.trampoline(*i, &func));
         }
 
+        builder.unwind_info();
+
         if emit_dwarf && funcs.len() > 0 {
             let ofs = VMOffsets::new(
                 self.isa
@@ -330,6 +332,7 @@ impl wasmtime_environ::Compiler for Compiler {
         let mut builder = ObjectBuilder::new(obj, &module, &*self.isa);
         let a = builder.trampoline(SignatureIndex::new(0), &host_to_wasm);
         let b = builder.trampoline(SignatureIndex::new(1), &wasm_to_host);
+        builder.unwind_info();
         builder.finish()?;
         Ok((a, b))
     }

crates/cranelift/src/obj.rs

@@ -114,10 +114,14 @@ pub struct ObjectBuilder<'a> {
     /// a relocation against a libcall.
     libcalls: HashMap<LibCall, SymbolId>,
 
+    windows_unwind_info_id: Option<SectionId>,
+
     /// Packed form of windows unwind tables which, if present, will get emitted
     /// to a windows-specific unwind info section.
     windows_unwind_info: Vec<RUNTIME_FUNCTION>,
 
+    systemv_unwind_info_id: Option<SectionId>,
+
     /// Pending unwinding information for DWARF-based platforms. This is used to
     /// build a `.eh_frame` lookalike at the very end of object building.
     systemv_unwind_info: Vec<(u64, &'a systemv::UnwindInfo)>,
@@ -139,6 +143,11 @@ pub struct ObjectBuilder<'a> {
     /// In-progress text section that we're using cranelift's `MachBuffer` to
     /// build to resolve relocations (calls) between functions.
     pub text: Box<dyn TextSectionBuilder>,
+
+    /// The unwind info _must_ come directly after the text section. Our FDE's
+    /// instructions are encoded to rely on this placement. We use this `bool`
+    /// for debug assertions to ensure that we get the ordering correct.
+    added_unwind_info: bool,
 }
 
 // This is a mirror of `RUNTIME_FUNCTION` in the Windows API, but defined here
@@ -190,7 +199,9 @@ impl<'a> ObjectBuilder<'a> {
             text_section,
             func_symbols,
             libcalls,
+            windows_unwind_info_id: None,
             windows_unwind_info: Vec::new(),
+            systemv_unwind_info_id: None,
             systemv_unwind_info: Vec::new(),
             relocations: Vec::new(),
             text: match isa.get_mach_backend() {
@@ -199,6 +210,7 @@ impl<'a> ObjectBuilder<'a> {
                 ),
                 None => Box::new(DummyBuilder::default()),
             },
+            added_unwind_info: false,
         }
     }
 
@@ -332,6 +344,7 @@ impl<'a> ObjectBuilder<'a> {
     ///
     /// This is expected to be called in-order for ascending `index` values.
     pub fn func(&mut self, index: DefinedFuncIndex, func: &'a CompiledFunction) -> Range<u64> {
+        assert!(!self.added_unwind_info);
         let index = self.module.func_index(index);
         let name = obj::func_symbol_name(index);
         let (symbol_id, range) = self.append_func(true, name.into_bytes(), func);
@@ -340,6 +353,7 @@ impl<'a> ObjectBuilder<'a> {
     }
 
     pub fn trampoline(&mut self, sig: SignatureIndex, func: &'a CompiledFunction) -> Trampoline {
+        assert!(!self.added_unwind_info);
         let name = obj::trampoline_symbol_name(sig);
         let (_, range) = self.append_func(false, name.into_bytes(), func);
         Trampoline {
@@ -350,6 +364,11 @@ impl<'a> ObjectBuilder<'a> {
     }
 
     pub fn dwarf_sections(&mut self, sections: &[DwarfSection]) -> Result<()> {
+        assert!(
+            self.added_unwind_info,
+            "can't add dwarf yet; unwind info must directly follow the text section"
+        );
+
         // If we have DWARF data, write it in the object file.
         let (debug_bodies, debug_relocs): (Vec<_>, Vec<_>) = sections
             .iter()
@@ -394,6 +413,29 @@ impl<'a> ObjectBuilder<'a> {
         Ok(())
     }
 
+    pub fn unwind_info(&mut self) {
+        assert!(!self.added_unwind_info);
+
+        if self.windows_unwind_info.len() > 0 {
+            let segment = self.obj.segment_name(StandardSegment::Data).to_vec();
+            self.windows_unwind_info_id = Some(self.obj.add_section(
+                segment,
+                b"_wasmtime_winx64_unwind".to_vec(),
+                SectionKind::ReadOnlyData,
+            ));
+        }
+        if self.systemv_unwind_info.len() > 0 {
+            let segment = self.obj.segment_name(StandardSegment::Data).to_vec();
+            self.systemv_unwind_info_id = Some(self.obj.add_section(
+                segment,
+                b".eh_frame".to_vec(),
+                SectionKind::ReadOnlyData,
+            ));
+        }
+
+        self.added_unwind_info = true;
+    }
+
     pub fn finish(&mut self) -> Result<()> {
         // Now that all function symbols are available register all final
         // relocations between functions.
@@ -438,6 +480,7 @@ impl<'a> ObjectBuilder<'a> {
         if self.systemv_unwind_info.len() > 0 {
             self.append_systemv_unwind_info();
         }
+
         Ok(())
     }
 
@@ -454,17 +497,13 @@ impl<'a> ObjectBuilder<'a> {
         // This may need updates for other platforms.
         assert_eq!(self.obj.architecture(), Architecture::X86_64);
 
+        let section_id = self.windows_unwind_info_id.unwrap();
+
         // Page-align the text section so the unwind info can reside on a
         // separate page that doesn't need executable permissions.
         self.obj
             .append_section_data(self.text_section, &[], self.isa.code_section_alignment());
 
-        let segment = self.obj.segment_name(StandardSegment::Data).to_vec();
-        let section_id = self.obj.add_section(
-            segment,
-            b"_wasmtime_winx64_unwind".to_vec(),
-            SectionKind::ReadOnlyData,
-        );
         let mut unwind_info = Vec::with_capacity(self.windows_unwind_info.len() * 3 * 4);
         for info in self.windows_unwind_info.iter() {
             unwind_info.extend_from_slice(&info.begin.to_le_bytes());
@@ -513,12 +552,7 @@ impl<'a> ObjectBuilder<'a> {
     /// such as being purely read-only instead of read/execute like the code
     /// bits.
     fn append_systemv_unwind_info(&mut self) {
-        let segment = self.obj.segment_name(StandardSegment::Data).to_vec();
-        let section_id = self.obj.add_section(
-            segment,
-            b".eh_frame".to_vec(),
-            SectionKind::ReadOnlyData,
-        );
+        let section_id = self.systemv_unwind_info_id.unwrap();
         let mut cie = self
             .isa
             .create_systemv_cie()

crates/jit/src/unwind/systemv.rs

@@ -25,6 +25,12 @@ impl UnwindRegistration {
         unwind_info: *mut u8,
         unwind_len: usize,
     ) -> Result<UnwindRegistration> {
+        debug_assert_eq!(
+            unwind_info as usize % region::page::size(),
+            0,
+            "The unwind info must always be aligned to a page"
+        );
+
         let mut registrations = Vec::new();
         if cfg!(any(
             all(target_os = "linux", target_env = "gnu"),