[LLD][ELF] Include offset when adding Thunk symbols (#144995)

Include the offset of a thunk in the ThunkSection when adding symbols.

At Thunk creation time the offset is set to 0 as we don't know where in
the ThunkSection the Thunk will end up. The symbol values are updated by
the setOffset() call in assignOffsets().

When we transform a thunk from a short to a long, we sometimes add a
mapping symbol. At this point the offset of the thunk is non zero and we
need to account for that when defining the symbol, as the setOffset()
call subtracts the offset before adding the new one back in.

To test; added a second thunk that is converted to a long thunk to
aarch64-thunk-bit-multipass. This second thunk is given a non zero
offset from the start of the Thunk Section so we can observe the mapping
symbol being put in the wrong place without accounting for the offset.

fixes: #142326

Author: smithp35

lld/ELF/Thunks.cpp

@@ -601,7 +601,8 @@ class PPC64PDLongBranchThunk final : public PPC64LongBranchThunk {
 
 Defined *Thunk::addSymbol(StringRef name, uint8_t type, uint64_t value,
                           InputSectionBase &section) {
-  Defined *d = addSyntheticLocal(ctx, name, type, value, /*size=*/0, section);
+  Defined *d =
+      addSyntheticLocal(ctx, name, type, value + offset, /*size=*/0, section);
   syms.push_back(d);
   return d;
 }

lld/test/ELF/aarch64-thunk-bti-multipass.s

@@ -29,6 +29,7 @@
 _start:
 /// Call that requires a thunk.
  bl fn1
+ bl fn2
 /// padding so that the thunk for fn1 is placed after this section is
 /// sufficiently close to the target to be within short range, but only
 /// just so that a small displacement will mean a long thunk is needed.
@@ -39,13 +40,20 @@ _start:
 
 // CHECK-LABEL: <_start>:
 // CHECK-NEXT: 10001000: bl  0x10002008 <__AArch64AbsLongThunk_fn1>
+// CHECK-NEXT:           bl  0x10002018 <__AArch64AbsLongThunk_fn2>
 
 // CHECK-LABEL: <__AArch64AbsLongThunk_fn1>:
 // CHECK-NEXT: 10002008: ldr     x16, 0x10002010 <__AArch64AbsLongThunk_fn1+0x8>
 // CHECK-NEXT:           br      x16
 // CHECK-NEXT:           00 30 00 18    .word   0x18003000
 // CHECK-NEXT:           00 00 00 00    .word   0x00000000
 
+// CHECK-LABEL: <__AArch64AbsLongThunk_fn2>:
+// CHECK-NEXT: 10002018: ldr     x16, 0x10002020 <__AArch64AbsLongThunk_fn2+0x8>
+// CHECK-NEXT:           br      x16
+// CHECK-NEXT:           04 40 00 18    .word   0x18004004
+// CHECK-NEXT:           00 00 00 00    .word   0x00000000
+
 .section .text.1, "ax", %progbits
 .balign 0x1000
 .global farcall
@@ -75,6 +83,12 @@ farcall:
 fn1:
  ret
 
+.section .text.3, "ax", %progbits
+.global fn2
+.type fn2, %function
+fn2:
+ ret
+
 .section .text.far, "ax", %progbits
 .type far, %function
 .global far
@@ -88,6 +102,12 @@ far:
 // CHECK-LABEL: <fn1>:
 // CHECK-NEXT: 18004000: ret
 
+// CHECK-LABEL: <__AArch64BTIThunk_fn2>:
+// CHECK-NEXT: 18004004:       bti     c
+
+// CHECK-LABEL: <fn2>:
+// CHECK-NEXT: 18004008:       ret
+
 // CHECK-LABEL: <__AArch64BTIThunk_far>:
 // CHECK-NEXT: 30000000: bti     c
 
@@ -104,6 +124,6 @@ SECTIONS {
   .rodata 0x10000000 : { *(.note.gnu.property) } :low
   .text_low : { *(.text.0) } :low
   .text 0x18001000 : { *(.text.1) } :mid
-  .text_aligned : { *(.text.2) } :mid
+  .text_aligned : { *(.text.2) *(.text.3) } :mid
   .text_high 0x30000000 : { *(.text.far) } :high
 }