[lld] r294816 - Create only one section symbol per section.
Peter Collingbourne via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 14 11:34:45 PST 2017
This change causes check-llvm to fail for me when testing (ironically
enough) LLVM's Go bindings, as a result of a segfault in lld. I've attached
a repro file.
It looks like the problem is that we segfault when looking up the output
section if the .eh_frame section contains a relocation to a discarded
section symbol in an FDE. I haven't really thought about how we need to
handle FDEs referring to discarded sections in -r mode.
Peter
On Fri, Feb 10, 2017 at 5:40 PM, Rafael Espindola via llvm-commits <
llvm-commits at lists.llvm.org> wrote:
> Author: rafael
> Date: Fri Feb 10 19:40:49 2017
> New Revision: 294816
>
> URL: http://llvm.org/viewvc/llvm-project?rev=294816&view=rev
> Log:
> Create only one section symbol per section.
>
> Unfortunately some consumers of our .o files produced with -r expect
> only one section symbol per section. That is true of at least of go's
> own linker.
>
> Combining them is a somewhat convoluted process. We have to create a
> symbol for every section since we don't know which ones will be
> needed. The relocation sections also have to be written first to
> handle the Elf_Rel addend.
>
> I did consider a completely different approach:
>
> We could remove the -r special case of relocation sections when
> reading. We would instead have a copyRelocs function that is used
> instead of scanRelocs. It would create a DynamicReloc for each
> relocation and a RelocationSection for each input relocation section.
>
> A complication of such change is that DynamicReloc would have to take
> a section index and a input section instead of a symbol since with
> -emit-relocs some DynamicReloc would hold relocations referring to the
> dynamic symbol table and other to the static symbol table.
>
> That would be a pretty big change, and if we do it it is probably
> better to do it as a refactoring.
>
> Added:
> lld/trunk/test/ELF/relocatable-section-symbol.s
> Modified:
> lld/trunk/ELF/InputSection.cpp
> lld/trunk/ELF/SyntheticSections.cpp
> lld/trunk/ELF/Writer.cpp
> lld/trunk/test/ELF/emit-relocs.s
> lld/trunk/test/ELF/mips-sto-pic-flag.s
> lld/trunk/test/ELF/relocatable-bss.s
>
> Modified: lld/trunk/ELF/InputSection.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
> InputSection.cpp?rev=294816&r1=294815&r2=294816&view=diff
> ============================================================
> ==================
> --- lld/trunk/ELF/InputSection.cpp (original)
> +++ lld/trunk/ELF/InputSection.cpp Fri Feb 10 19:40:49 2017
> @@ -236,6 +236,22 @@ void InputSection<ELFT>::copyRelocations
> if (Config->Rela)
> P->r_addend = getAddend<ELFT>(Rel);
>
> + if (Body.Type == STT_SECTION) {
> + // We combine multiple section symbols into only one per
> + // section. This means we have to update the addend. That is
> + // trivial for Elf_Rela, but for Elf_Rel we have to write to the
> + // section data. We do that by adding to the Relocation vector.
> + if (Config->Rela) {
> + P->r_addend += Body.getVA<ELFT>() -
> + cast<DefinedRegular<ELFT>>(
> Body).Section->OutSec->Addr;
> + } else if (Config->Relocatable) {
> + const uint8_t *BufLoc = RelocatedSection->Data.begin() +
> Rel.r_offset;
> + uint64_t Implicit = Target->getImplicitAddend(BufLoc, Type);
> + RelocatedSection->Relocations.push_back(
> + {R_ABS, Type, Rel.r_offset, Implicit, &Body});
> + }
> + }
> +
> // Output section VA is zero for -r, so r_offset is an offset within
> the
> // section, but for --emit-relocs it is an virtual address.
> P->r_offset = RelocatedSection->OutSec->Addr +
>
> Modified: lld/trunk/ELF/SyntheticSections.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
> SyntheticSections.cpp?rev=294816&r1=294815&r2=294816&view=diff
> ============================================================
> ==================
> --- lld/trunk/ELF/SyntheticSections.cpp (original)
> +++ lld/trunk/ELF/SyntheticSections.cpp Fri Feb 10 19:40:49 2017
> @@ -1127,8 +1127,16 @@ template <class ELFT> void SymbolTableSe
>
> template <class ELFT>
> size_t SymbolTableSection<ELFT>::getSymbolIndex(SymbolBody *Body) {
> - auto I = llvm::find_if(
> - Symbols, [&](const SymbolTableEntry &E) { return E.Symbol == Body;
> });
> + auto I = llvm::find_if(Symbols, [&](const SymbolTableEntry &E) {
> + if (E.Symbol == Body)
> + return true;
> + // This is used for -r, so we have to handle multiple section
> + // symbols being combined.
> + if (Body->Type == STT_SECTION && E.Symbol->Type == STT_SECTION)
> + return cast<DefinedRegular<ELFT>>(Body)->Section->OutSec ==
> + cast<DefinedRegular<ELFT>>(E.Symbol)->Section->OutSec;
> + return false;
> + });
> if (I == Symbols.end())
> return 0;
> return I - Symbols.begin() + 1;
>
> Modified: lld/trunk/ELF/Writer.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Writer.
> cpp?rev=294816&r1=294815&r2=294816&view=diff
> ============================================================
> ==================
> --- lld/trunk/ELF/Writer.cpp (original)
> +++ lld/trunk/ELF/Writer.cpp Fri Feb 10 19:40:49 2017
> @@ -51,6 +51,7 @@ public:
> private:
> void createSyntheticSections();
> void copyLocalSymbols();
> + void addSectionSymbols();
> void addReservedSymbols();
> void addInputSec(InputSectionBase<ELFT> *S);
> void createSections();
> @@ -228,6 +229,9 @@ template <class ELFT> void Writer<ELFT>:
> if (Config->Discard != DiscardPolicy::All)
> copyLocalSymbols();
>
> + if (Config->copyRelocs())
> + addSectionSymbols();
> +
> // Now that we have a complete set of output sections. This function
> // completes section contents. For example, we need to add strings
> // to the string table, and add entries to .got and .plt.
> @@ -446,13 +450,9 @@ template <class ELFT> void Writer<ELFT>:
> template <class ELFT>
> static bool shouldKeepInSymtab(InputSectionBase<ELFT> *Sec, StringRef
> SymName,
> const SymbolBody &B) {
> - if (B.isFile())
> + if (B.isFile() || B.isSection())
> return false;
>
> - // We keep sections in symtab for relocatable output and --emit-reloc.
> - if (B.isSection())
> - return Config->copyRelocs();
> -
> // If sym references a section in a discarded group, don't keep it.
> if (Sec == &InputSection<ELFT>::Discarded)
> return false;
> @@ -518,6 +518,27 @@ template <class ELFT> void Writer<ELFT>:
> }
> }
>
> +template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
> + // Create one STT_SECTION symbol for each output section we might
> + // have a relocation with.
> + for (OutputSectionBase *Sec : OutputSections) {
> + InputSectionData *First = nullptr;
> + Sec->forEachInputSection([&](InputSectionData *D) {
> + if (!First)
> + First = D;
> + });
> + auto *IS = dyn_cast_or_null<InputSection<ELFT>>(First);
> + if (!IS || isa<SyntheticSection<ELFT>>(IS) || IS->Type == SHT_REL ||
> + IS->Type == SHT_RELA)
> + continue;
> + auto *B = new (BAlloc)
> + DefinedRegular<ELFT>("", /*IsLocal=*/true, /*StOther*/ 0,
> STT_SECTION,
> + /*Value*/ 0, /*Size*/ 0, IS, nullptr);
> +
> + In<ELFT>::SymTab->addLocal(B);
> + }
> +}
> +
> // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE
> sections that
> // we would like to make sure appear is a specific order to maximize their
> // coverage by a single signed 16-bit offset from the TOC base pointer.
> @@ -1782,8 +1803,17 @@ template <class ELFT> void Writer<ELFT>:
>
> OutputSectionBase *EhFrameHdr =
> In<ELFT>::EhFrameHdr ? In<ELFT>::EhFrameHdr->OutSec : nullptr;
> +
> + // In -r or -emit-relocs mode, write the relocation sections first as in
> + // ELf_Rel targets we might find out that we need to modify the
> relocated
> + // section while doing it.
> + for (OutputSectionBase *Sec : OutputSections)
> + if (Sec->Type == SHT_REL || Sec->Type == SHT_RELA)
> + Sec->writeTo(Buf + Sec->Offset);
> +
> for (OutputSectionBase *Sec : OutputSections)
> - if (Sec != Out<ELFT>::Opd && Sec != EhFrameHdr)
> + if (Sec != Out<ELFT>::Opd && Sec != EhFrameHdr && Sec->Type !=
> SHT_REL &&
> + Sec->Type != SHT_RELA)
> Sec->writeTo(Buf + Sec->Offset);
>
> // The .eh_frame_hdr depends on .eh_frame section contents, therefore
>
> Modified: lld/trunk/test/ELF/emit-relocs.s
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/
> emit-relocs.s?rev=294816&r1=294815&r2=294816&view=diff
> ============================================================
> ==================
> --- lld/trunk/test/ELF/emit-relocs.s (original)
> +++ lld/trunk/test/ELF/emit-relocs.s Fri Feb 10 19:40:49 2017
> @@ -15,7 +15,7 @@
> # CHECK-NEXT: Section ({{.*}}) .rela.text {
> # CHECK-NEXT: 0x201002 R_X86_64_32 .text 0x1
> # CHECK-NEXT: 0x201007 R_X86_64_PLT32 fn 0xFFFFFFFFFFFFFFFC
> -# CHECK-NEXT: 0x20100E R_X86_64_32 .text 0x1
> +# CHECK-NEXT: 0x20100E R_X86_64_32 .text 0xD
> # CHECK-NEXT: 0x201013 R_X86_64_PLT32 fn2 0xFFFFFFFFFFFFFFFC
> # CHECK-NEXT: }
> # CHECK-NEXT: ]
> @@ -53,15 +53,6 @@
> # CHECK-NEXT: Size: 0
> # CHECK-NEXT: Binding: Local
> # CHECK-NEXT: Type: Section
> -# CHECK-NEXT: Other: 0
> -# CHECK-NEXT: Section: .text
> -# CHECK-NEXT: }
> -# CHECK-NEXT: Symbol {
> -# CHECK-NEXT: Name:
> -# CHECK-NEXT: Value: 0x20100C
> -# CHECK-NEXT: Size: 0
> -# CHECK-NEXT: Binding: Local
> -# CHECK-NEXT: Type: Section
> # CHECK-NEXT: Other: 0
> # CHECK-NEXT: Section: .text
> # CHECK-NEXT: }
>
> Modified: lld/trunk/test/ELF/mips-sto-pic-flag.s
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/
> mips-sto-pic-flag.s?rev=294816&r1=294815&r2=294816&view=diff
> ============================================================
> ==================
> --- lld/trunk/test/ELF/mips-sto-pic-flag.s (original)
> +++ lld/trunk/test/ELF/mips-sto-pic-flag.s Fri Feb 10 19:40:49 2017
> @@ -19,8 +19,8 @@
> # CHECK-NEXT: Other: 0
> # CHECK-NEXT: Section: .text
> # CHECK-NEXT: }
> -# CHECK-NEXT: Symbol {
> -# CHECK-NEXT: Name: foo1a
> +# CHECK: Symbol {
> +# CHECK: Name: foo1a
> # CHECK-NEXT: Value:
> # CHECK-NEXT: Size:
> # CHECK-NEXT: Binding: Global
>
> Modified: lld/trunk/test/ELF/relocatable-bss.s
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/
> relocatable-bss.s?rev=294816&r1=294815&r2=294816&view=diff
> ============================================================
> ==================
> --- lld/trunk/test/ELF/relocatable-bss.s (original)
> +++ lld/trunk/test/ELF/relocatable-bss.s Fri Feb 10 19:40:49 2017
> @@ -20,7 +20,7 @@
> # CHECK-NEXT: Version:
> # CHECK-NEXT: Entry:
> # CHECK-NEXT: ProgramHeaderOffset:
> -# CHECK-NEXT: SectionHeaderOffset: 0xA8
> +# CHECK-NEXT: SectionHeaderOffset: 0xD8
> # CHECK-NEXT: Flags [
> # CHECK-NEXT: ]
> # CHECK-NEXT: HeaderSize:
>
> Added: lld/trunk/test/ELF/relocatable-section-symbol.s
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/
> relocatable-section-symbol.s?rev=294816&view=auto
> ============================================================
> ==================
> --- lld/trunk/test/ELF/relocatable-section-symbol.s (added)
> +++ lld/trunk/test/ELF/relocatable-section-symbol.s Fri Feb 10 19:40:49
> 2017
> @@ -0,0 +1,49 @@
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
> +# RUN: ld.lld -r -o %t %t.o %t.o
> +# RUN: llvm-readobj -r %t | FileCheck --check-prefix=RELA %s
> +
> +# RELA: Relocations [
> +# RELA-NEXT: Section ({{.*}}) .rela.data {
> +# RELA-NEXT: 0x0 R_X86_64_32 .text 0x1
> +# RELA-NEXT: 0x4 R_X86_64_32 .text 0x5
> +# RELA-NEXT: }
> +# RELA-NEXT: ]
> +
> +
> +# RUN: llvm-mc -filetype=obj -triple=i686-pc-linux %s -o %t.o
> +# RUN: ld.lld -r -o %t %t.o %t.o
> +# RUN: llvm-readobj -r -s -section-data %t | FileCheck --check-prefix=REL
> %s
> +
> +
> +# REL: Section {
> +# REL: Index:
> +# REL: Name: .data
> +# REL-NEXT: Type: SHT_PROGBITS
> +# REL-NEXT: Flags [
> +# REL-NEXT: SHF_ALLOC
> +# REL-NEXT: SHF_WRITE
> +# REL-NEXT: ]
> +# REL-NEXT: Address:
> +# REL-NEXT: Offset:
> +# REL-NEXT: Size:
> +# REL-NEXT: Link:
> +# REL-NEXT: Info:
> +# REL-NEXT: AddressAlignment:
> +# REL-NEXT: EntrySize:
> +# REL-NEXT: SectionData (
> +# REL-NEXT: 0000: 01000000 05000000 |
> +# REL-NEXT: )
> +# REL-NEXT: }
> +
> +
> +# REL: Relocations [
> +# REL-NEXT: Section ({{.*}}) .rel.data {
> +# REL-NEXT: 0x0 R_386_32 .text 0x0
> +# REL-NEXT: 0x4 R_386_32 .text 0x0
> +# REL-NEXT: }
> +# REL-NEXT: ]
> +
> +
> +.long 42
> +.data
> +.long .text + 1
>
>
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>
--
--
Peter
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