Use pcrel pointers in .eh_frame

Fixes #1055

Author: bjorn3

src/backend.rs

@@ -68,8 +68,17 @@ impl WriteDebugInfo for ObjectProduct {
         .into_bytes();
 
         let segment = self.object.segment_name(StandardSegment::Debug).to_vec();
-        let section_id = self.object.add_section(segment, name, SectionKind::Debug);
-        self.object.section_mut(section_id).set_data(data, 1);
+        // FIXME use SHT_X86_64_UNWIND for .eh_frame
+        let section_id = self.object.add_section(segment, name.clone(), if id == SectionId::EhFrame {
+            SectionKind::ReadOnlyData
+        } else {
+            SectionKind::Debug
+        });
+        self.object.section_mut(section_id).set_data(data, if id == SectionId::EhFrame {
+            8
+        } else {
+            1
+        });
         let symbol_id = self.object.section_symbol(section_id);
         (section_id, symbol_id)
     }
@@ -95,7 +104,7 @@ impl WriteDebugInfo for ObjectProduct {
                 Relocation {
                     offset: u64::from(reloc.offset),
                     symbol,
-                    kind: RelocationKind::Absolute,
+                    kind: reloc.kind,
                     encoding: RelocationEncoding::Generic,
                     size: reloc.size * 8,
                     addend: i64::try_from(symbol_offset).unwrap() + reloc.addend,

src/debuginfo/emit.rs

@@ -46,6 +46,7 @@ pub(crate) struct DebugReloc {
     pub(crate) size: u8,
     pub(crate) name: DebugRelocName,
     pub(crate) addend: i64,
+    pub(crate) kind: object::RelocationKind,
 }
 
 #[derive(Clone)]
@@ -122,21 +123,21 @@ impl Writer for WriterRelocate {
                     size,
                     name: DebugRelocName::Symbol(symbol),
                     addend: addend as i64,
+                    kind: object::RelocationKind::Absolute,
                 });
                 self.write_udata(0, size)
             }
         }
     }
 
-    // TODO: implement write_eh_pointer
-
     fn write_offset(&mut self, val: usize, section: SectionId, size: u8) -> Result<()> {
         let offset = self.len() as u32;
         self.relocs.push(DebugReloc {
             offset,
             size,
             name: DebugRelocName::Section(section),
             addend: val as i64,
+            kind: object::RelocationKind::Absolute,
         });
         self.write_udata(0, size)
     }
@@ -153,7 +154,55 @@ impl Writer for WriterRelocate {
             size,
             name: DebugRelocName::Section(section),
             addend: val as i64,
+            kind: object::RelocationKind::Absolute,
         });
         self.write_udata_at(offset, 0, size)
     }
+
+    fn write_eh_pointer(
+        &mut self,
+        address: Address,
+        eh_pe: gimli::DwEhPe,
+        size: u8,
+    ) -> Result<()> {
+        match address {
+            // Address::Constant arm copied from gimli
+            Address::Constant(val) => {
+                // Indirect doesn't matter here.
+                let val = match eh_pe.application() {
+                    gimli::DW_EH_PE_absptr => val,
+                    gimli::DW_EH_PE_pcrel => {
+                        // TODO: better handling of sign
+                        let offset = self.len() as u64;
+                        offset.wrapping_sub(val)
+                    }
+                    _ => {
+                        return Err(gimli::write::Error::UnsupportedPointerEncoding(eh_pe));
+                    }
+                };
+                self.write_eh_pointer_data(val, eh_pe.format(), size)
+            }
+            Address::Symbol { symbol, addend } => {
+                match eh_pe.application() {
+                    gimli::DW_EH_PE_pcrel => {
+                        let size = match eh_pe.format() {
+                            gimli::DW_EH_PE_sdata4 => 4,
+                            _ => return Err(gimli::write::Error::UnsupportedPointerEncoding(eh_pe)),
+                        };
+                        self.relocs.push(DebugReloc {
+                            offset: self.len() as u32,
+                            size,
+                            name: DebugRelocName::Symbol(symbol),
+                            addend,
+                            kind: object::RelocationKind::Relative,
+                        });
+                        self.write_udata(0, size)
+                    }
+                    _ => {
+                        return Err(gimli::write::Error::UnsupportedPointerEncoding(eh_pe));
+                    }
+                }
+            }
+        }
+    }
 }

src/debuginfo/unwind.rs

@@ -16,7 +16,10 @@ impl<'tcx> UnwindContext<'tcx> {
     pub(crate) fn new(tcx: TyCtxt<'tcx>, isa: &dyn TargetIsa) -> Self {
         let mut frame_table = FrameTable::default();
 
-        let cie_id = if let Some(cie) = isa.create_systemv_cie() {
+        let cie_id = if let Some(mut cie) = isa.create_systemv_cie() {
+            if isa.flags().is_pic() {
+                cie.fde_address_encoding = gimli::DwEhPe(gimli::DW_EH_PE_pcrel.0 | gimli::DW_EH_PE_sdata4.0);
+            }
             Some(frame_table.add_cie(cie))
         } else {
             None