[ELF] Don't resolve relocations referencing SHN_ABS to tombstone in n…

…on-SHF_ALLOC sections (llvm#79238)

A SHN_ABS symbol has never been considered for
InputSection::relocateNonAlloc.
Before llvm#74686, the code did made it work in the absence of `-z
dead-reloc-in-nonalloc=`.
There is now a report about such SHN_ABS uses

(llvm#74686 (comment))
and I think it makes sense for non-SHF_ALLOC to support SHN_ABS, like
SHF_ALLOC sections do.

```
// clang -g
__attribute__((weak)) int symbol;
int *foo() { return &symbol; }

0x00000023:   DW_TAG_variable [2]   (0x0000000c)
                ...
                DW_AT_location [DW_FORM_exprloc]        (DW_OP_addrx 0x0)

```

.debug_addr references `symbol`, which can be redefined by a symbol
assignment or --defsym to become a SHN_ABS symbol.

The problem is that `!sym.getOutputSection()` cannot discern SHN_ABS
from a symbol whose section has been discarded. Since commit
1981b1b, a symbol relative to a
discarded section is changed to `Undefined`, so the `SHN_ABS` check
become trivial.

We currently apply tombstone for a relocation referencing
`SharedSymbol`. This patch does not change the behavior.

(cherry picked from commit 8abf8d1)

lld/ELF/InputSection.cpp

@@ -961,20 +961,19 @@ void InputSection::relocateNonAlloc(uint8_t *buf, ArrayRef<RelTy> rels) {
       // vector. The computed value is st_value plus a non-negative offset.
       // Negative values are invalid, so -1 can be used as the tombstone value.
       //
-      // If the referenced symbol is discarded (made Undefined), or the
-      // section defining the referenced symbol is garbage collected,
-      // sym.getOutputSection() is nullptr. `ds->folded` catches the ICF folded
-      // case. However, resolving a relocation in .debug_line to -1 would stop
-      // debugger users from setting breakpoints on the folded-in function, so
-      // exclude .debug_line.
+      // If the referenced symbol is relative to a discarded section (due to
+      // --gc-sections, COMDAT, etc), it has been converted to a Undefined.
+      // `ds->folded` catches the ICF folded case. However, resolving a
+      // relocation in .debug_line to -1 would stop debugger users from setting
+      // breakpoints on the folded-in function, so exclude .debug_line.
       //
       // For pre-DWARF-v5 .debug_loc and .debug_ranges, -1 is a reserved value
       // (base address selection entry), use 1 (which is used by GNU ld for
       // .debug_ranges).
       //
       // TODO To reduce disruption, we use 0 instead of -1 as the tombstone
       // value. Enable -1 in a future release.
-      if (!sym.getOutputSection() || (ds && ds->folded && !isDebugLine)) {
+      if (!ds || (ds->folded && !isDebugLine)) {
         // If -z dead-reloc-in-nonalloc= is specified, respect it.
         uint64_t value = SignExtend64<bits>(*tombstone);
         // For a 32-bit local TU reference in .debug_names, X86_64::relocate

lld/test/ELF/dead-reloc-in-nonalloc.s

@@ -17,7 +17,7 @@
 # AA:          Contents of section .debug_info:
 # AA-NEXT:      0000 [[ADDR]] 00000000 aaaaaaaa 00000000
 # AA:          Contents of section .not_debug:
-# AA-NEXT:      0000 bbbbbbbb bbbbbbbb 00000000          .
+# AA-NEXT:      0000 bbbbbbbb 2a000000 00000000          .
 
 ## Specifying zero can get a behavior similar to GNU ld.
 # RUN: ld.lld --icf=all -z dead-reloc-in-nonalloc=.debug_info=0 %t.o %tabs.o -o %tzero