[LLD][ARM] Allow R_ARM_SBREL32 relocations in debug info (#116956)

The R_ARM_SBREL32 relocation is used in debug info for ARM RWPI
(read-write position independent) code. Compiler-generated DWARF info
will use an expression to add the relocated value to the actual value of
the static base (held in r9) at run-time, so it should be relocated as
if the static base is at address 0.

Author: ostannard

lld/ELF/InputSection.cpp

@@ -1094,7 +1094,7 @@ void InputSection::relocateNonAlloc(Ctx &ctx, uint8_t *buf,
     // R_ABS/R_DTPREL and some other relocations can be used from non-SHF_ALLOC
     // sections.
     if (LLVM_LIKELY(expr == R_ABS) || expr == R_DTPREL || expr == R_GOTPLTREL ||
-        expr == R_RISCV_ADD) {
+        expr == R_RISCV_ADD || expr == R_ARM_SBREL) {
       target.relocateNoSym(bufLoc, type,
                            SignExtend64<bits>(sym.getVA(ctx, addend)));
       continue;

lld/test/ELF/arm-rwpi-debug-relocs.s

@@ -0,0 +1,54 @@
+/// Test that R_ARM_SBREL32 relocations in debug info are relocated as if the
+/// static base register (r9) is zero. Real DWARF info will use an expression to
+/// add this to the real value of the static base at runtime.
+
+// REQUIRES: arm
+// RUN: rm -rf %t && split-file %s %t && cd %t
+
+// RUN: llvm-mc -filetype=obj -triple=armv7a asm.s -o obj.o
+// RUN: ld.lld -T lds.ld obj.o -o exe.elf 2>&1 | FileCheck %s --implicit-check-not=warning: --allow-empty
+// RUN: llvm-objdump -D exe.elf | FileCheck --check-prefix=DISASM %s
+
+// DISASM-LABEL: <rw>:
+// DISASM-NEXT:      1000: 0000002a
+
+// DISASM-LABEL: <rw2>:
+// DISASM-NEXT:      2000: 000004d2
+
+// DISASM-LABEL: <.debug_something>:
+// DISASM-NEXT:         0: 00001000
+// DISASM-NEXT:        ...
+// DISASM-NEXT:       104: 00002000
+
+//--- lds.ld
+SECTIONS {
+  data1 0x1000 : { *(data1) }
+  data2 0x2000 : { *(data2) }
+}
+
+//--- asm.s
+  .text
+  .type _start,%function
+  .globl  _start
+_start:
+  bx lr
+  .size _start, .-_start
+
+  .section data1, "aw", %progbits
+  .type rw,%object
+  .globl  rw
+rw:
+  .long 42
+  .size rw, 4
+
+  .section data2, "aw", %progbits
+  .type rw2,%object
+  .globl  rw2
+rw2:
+  .long 1234
+  .size rw2, 4
+
+  .section  .debug_something, "", %progbits
+  .long rw(sbrel)
+  .space 0x100
+  .long rw2(sbrel)