1 //===----------------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #ifndef LLD_ELF_ARCH_TARGETIMPL_H
10 #define LLD_ELF_ARCH_TARGETIMPL_H
11
12 #include "InputFiles.h"
13 #include "InputSection.h"
14 #include "Relocations.h"
15 #include "Symbols.h"
16 #include "llvm/BinaryFormat/ELF.h"
17
18 namespace lld::elf {
19
20 // getControlTransferAddend: If this relocation is used for control transfer
21 // instructions (e.g. branch, branch-link or call) or code references (e.g.
22 // virtual function pointers) and indicates an address-insignificant reference,
23 // return the effective addend for the relocation, otherwise return
24 // std::nullopt. The effective addend for a relocation is the addend that is
25 // used to determine its branch destination.
26 //
27 // getBranchInfoAtTarget: If a control transfer relocation referring to
28 // is+offset directly transfers control to a relocated branch instruction in the
29 // specified section, return the relocation for the branch target as well as its
30 // effective addend (see above). Otherwise return {nullptr, 0}.
31 //
32 // redirectControlTransferRelocations: Given r1, a relocation for which
33 // getControlTransferAddend() returned a value, and r2, a relocation returned by
34 // getBranchInfo(), modify r1 so that it branches directly to the target of r2.
35 template <typename GetControlTransferAddend, typename GetBranchInfoAtTarget,
36 typename RedirectControlTransferRelocations>
applyBranchToBranchOptImpl(Ctx & ctx,GetControlTransferAddend getControlTransferAddend,GetBranchInfoAtTarget getBranchInfoAtTarget,RedirectControlTransferRelocations redirectControlTransferRelocations)37 inline void applyBranchToBranchOptImpl(
38 Ctx &ctx, GetControlTransferAddend getControlTransferAddend,
39 GetBranchInfoAtTarget getBranchInfoAtTarget,
40 RedirectControlTransferRelocations redirectControlTransferRelocations) {
41 // Needs to run serially because it writes to the relocations array as well as
42 // reading relocations of other sections.
43 for (ELFFileBase *f : ctx.objectFiles) {
44 auto getRelocBranchInfo =
45 [&getBranchInfoAtTarget](
46 Relocation &r,
47 uint64_t addend) -> std::pair<Relocation *, uint64_t> {
48 auto *target = dyn_cast_or_null<Defined>(r.sym);
49 // We don't allow preemptible symbols or ifuncs (may go somewhere else),
50 // absolute symbols (runtime behavior unknown), non-executable or writable
51 // memory (ditto) or non-regular sections (no section data).
52 if (!target || target->isPreemptible || target->isGnuIFunc() ||
53 !target->section ||
54 !(target->section->flags & llvm::ELF::SHF_EXECINSTR) ||
55 (target->section->flags & llvm::ELF::SHF_WRITE) ||
56 target->section->kind() != SectionBase::Regular)
57 return {nullptr, 0};
58 return getBranchInfoAtTarget(*cast<InputSection>(target->section),
59 target->value + addend);
60 };
61 for (InputSectionBase *sb : f->getSections()) {
62 auto *s = dyn_cast_or_null<InputSection>(sb);
63 if (!s)
64 continue;
65 for (Relocation &r : s->relocations) {
66 std::optional<uint64_t> addend = getControlTransferAddend(*s, r);
67 if (!addend)
68 continue;
69 std::pair<Relocation *, uint64_t> targetAndAddend =
70 getRelocBranchInfo(r, *addend);
71 if (!targetAndAddend.first)
72 continue;
73 // Avoid getting stuck in an infinite loop if we encounter a branch
74 // that (possibly indirectly) branches to itself. It is unlikely
75 // that more than 5 iterations will ever be needed in practice.
76 size_t iterations = 5;
77 while (iterations--) {
78 std::pair<Relocation *, uint64_t> nextTargetAndAddend =
79 getRelocBranchInfo(*targetAndAddend.first,
80 targetAndAddend.second);
81 if (!nextTargetAndAddend.first)
82 break;
83 targetAndAddend = nextTargetAndAddend;
84 }
85 redirectControlTransferRelocations(r, *targetAndAddend.first);
86 }
87 }
88 }
89 }
90
91 } // namespace lld::elf
92
93 #endif
94