Add support for R_ARM_V4BX

Author: macabeus

objdiff-core/src/arch/arm.rs

@@ -343,6 +343,15 @@ impl Arch for ArchArm {
             // Handle ELF implicit relocations
             object::RelocationFlags::Elf { r_type } => {
                 if relocation.has_implicit_addend() {
+                    // R_ARM_V4BX marks a `bx` instruction so the linker can optionally
+                    // rewrite it for ARMv4 interworking. Drops the addend since it's meaningless.
+                    if r_type == elf::R_ARM_V4BX {
+                        return Ok(Some(RelocationOverride {
+                            target: RelocationOverrideTarget::Skip,
+                            addend: 0,
+                        }));
+                    }
+
                     let section_data = section.data()?;
                     let address = address as usize;
                     let addend = match r_type {
@@ -419,6 +428,7 @@ impl Arch for ArchArm {
                 elf::R_ARM_CALL => Some("R_ARM_CALL"),
                 elf::R_ARM_THM_PC11 => Some("R_ARM_THM_PC11"),
                 elf::R_ARM_THM_PC9 => Some("R_ARM_THM_PC9"),
+                elf::R_ARM_V4BX => Some("R_ARM_V4BX"),
                 _ => None,
             },
             _ => None,

objdiff-core/tests/arch_arm.rs

@@ -82,6 +82,22 @@ fn thumb_short_data_mapping() {
     );
 }
 
+#[test]
+#[cfg(feature = "arm")]
+fn read_arm_v4bx() {
+    // R_ARM_V4BX relocations mark `bx` instructions so the linker can optionally
+    // rewrite them for ARMv4 interworking.
+    let diff_config = diff::DiffObjConfig::default();
+    let obj =
+        obj::read::parse(include_object!("data/arm/v4bx.o"), &diff_config, diff::DiffSide::Base)
+            .unwrap();
+    let symbol_idx = obj.symbols.iter().position(|s| s.name == "v4bx_func").unwrap();
+    let diff = diff::code::no_diff_code(&obj, symbol_idx, &diff_config).unwrap();
+    let output = common::display_diff(&obj, &diff, symbol_idx, &diff_config);
+    // The `bx lr` still disassembles normally; the V4BX hint is simply dropped.
+    assert!(output.contains("bx"), "expected a `bx` instruction, got:\n{output}");
+}
+
 #[test]
 #[cfg(feature = "arm")]
 fn trim_trailing_hword() {