Add support for R_ARM_THM_PC8 and R_ARM_THM_PC11 (#342)

* Add support for `R_ARM_THM_JUMP8` and `R_ARM_THM_JUMP11`

* Fix `scan_instructions_internal` when reading 2-bytes instruction

Author: macabeus

objdiff-core/src/arch/arm.rs

@@ -224,7 +224,9 @@ impl Arch for ArchArm {
             }
 
             // Check how many bytes we can/should read
-            let num_code_bytes = if data.len() >= 4 {
+            let bytes_until_next_mapping =
+                next_mapping.map(|m| (m.address - address) as usize).unwrap_or(usize::MAX);
+            let num_code_bytes = if data.len() >= 4 && bytes_until_next_mapping >= 4 {
                 if mode == unarm::ParseMode::Data && address & 3 != 0 {
                     // 32-bit .word value should be aligned on a 4-byte boundary, otherwise use .hword
                     2
@@ -366,6 +368,22 @@ impl Arch for ArchArm {
                             (imm22 << 1) << 9 >> 9
                         }
 
+                        // Thumb unconditional branch (B, 11-bit offset)
+                        elf::R_ARM_THM_PC11 => {
+                            let data = section_data[address..address + 2].try_into()?;
+                            let insn = self.endianness.read_u16_bytes(data) as i32;
+                            let imm11 = insn & 0x7ff;
+                            (imm11 << 1) << 20 >> 20
+                        }
+
+                        // Thumb conditional branch (B<cond>, 8-bit offset)
+                        elf::R_ARM_THM_PC9 => {
+                            let data = section_data[address..address + 2].try_into()?;
+                            let insn = self.endianness.read_u16_bytes(data) as i32;
+                            let imm8 = insn & 0xff;
+                            (imm8 << 1) << 23 >> 23
+                        }
+
                         // Data
                         elf::R_ARM_ABS32 => {
                             let data = section_data[address..address + 4].try_into()?;
@@ -399,6 +417,8 @@ impl Arch for ArchArm {
                 elf::R_ARM_PC24 => Some("R_ARM_PC24"),
                 elf::R_ARM_XPC25 => Some("R_ARM_XPC25"),
                 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"),
                 _ => None,
             },
             _ => None,
@@ -418,6 +438,8 @@ impl Arch for ArchArm {
                 elf::R_ARM_PC24 => 4,
                 elf::R_ARM_XPC25 => 4,
                 elf::R_ARM_CALL => 4,
+                elf::R_ARM_THM_PC11 => 2,
+                elf::R_ARM_THM_PC9 => 2,
                 _ => 1,
             },
             _ => 1,
@@ -544,7 +566,9 @@ impl unarm::FormatIns for ArgsFormatter<'_> {
                 | RelocationFlags::Elf(elf::R_ARM_THM_PC22)
                 | RelocationFlags::Elf(elf::R_ARM_PC24)
                 | RelocationFlags::Elf(elf::R_ARM_XPC25)
-                | RelocationFlags::Elf(elf::R_ARM_CALL) => {
+                | RelocationFlags::Elf(elf::R_ARM_CALL)
+                | RelocationFlags::Elf(elf::R_ARM_THM_PC11)
+                | RelocationFlags::Elf(elf::R_ARM_THM_PC9) => {
                     return self.write(InstructionPart::reloc());
                 }
                 _ => {}

objdiff-core/tests/arch_arm.rs

@@ -57,6 +57,31 @@ fn combine_text_sections() {
     insta::assert_snapshot!(output);
 }
 
+#[test]
+#[cfg(feature = "arm")]
+fn thumb_short_data_mapping() {
+    // When a .2byte directive is used in Thumb code, the assembler emits
+    // $d/$t mapping symbols for a 2-byte data region. The disassembler must
+    // not read 4 bytes as a .word when the next mapping symbol limits the
+    // data region to 2 bytes.
+    let diff_config = diff::DiffObjConfig::default();
+    let obj = obj::read::parse(
+        include_object!("data/arm/code_1_vblank.o"),
+        &diff_config,
+        diff::DiffSide::Base,
+    )
+    .unwrap();
+    let symbol_idx = obj.symbols.iter().position(|s| s.name == "VBlankDMA_Level1").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);
+    // .2byte data followed by Thumb code must not be merged into a 4-byte .word
+    assert!(
+        !output.contains(".word"),
+        "2-byte data regions should not be decoded as 4-byte .word values.\n\
+         The disassembler must respect mapping symbol boundaries."
+    );
+}
+
 #[test]
 #[cfg(feature = "arm")]
 fn trim_trailing_hword() {