[lld] [LLD][ELF][AArch64] Add BTI Aware long branch thunks (PR #108989)

[Fangrui Song via llvm-commits]

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@@ -2326,6 +2334,21 @@ bool ThunkCreator::createThunks(uint32_t pass,
                 ts = getISDThunkSec(os, isec, isd, rel, src);
               ts->addThunk(t);
               thunks[t->getThunkTargetSym()] = t;
+
+              // When indirect branches are restricted, such as AArch64 BTI
+              // Thunks may need to target a linker generated landing pad
+              // instead of the target.
+              if (t->needsSyntheticLandingPad()) {
+                Thunk *lpt;
+                auto &dr = cast<Defined>(t->destination);
+                std::tie(lpt, isNew) = getSyntheticLandingPad(dr, t->addend);
+                if (isNew) {
+                  InputSection *targetsec = dyn_cast<InputSection>(dr.section);
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MaskRay wrote:

It seems that `targetSec` could be nullptr in erroneous non-InputSection cases (MergeInputSection) and would cause a null pointer dereference in `getISThunkSec`.

If we don't write code guarding against such cases, perhaps just use `cast<>`.

https://github.com/llvm/llvm-project/pull/108989