forked from OSchip/llvm-project
[ELF] Don't special case symbolic relocations with 0 addend to ifunc in writable locations
Currently the following 3 relocation types do not trigger the creation of a canonical PLT (which changes STT_GNU_IFUNC to STT_FUNC and redirects all references): 1) GOT-generating (`needsGot`) 2) PLT-generating (`needsPlt`) 3) R_ABS with 0 addend in a writable location. This is used for for ifunc function pointers in writable sections such as .data and .toc. This patch deletes case 3) to simplify the R_*_IRELATIVE generating logic added in D57371. Other advantages: * It is guaranteed no more than 1 R_*_IRELATIVE is created for an ifunc. * PPC64: no need to special case ifunc in toc-indirect to toc-relative relaxation. See D65755 The deleted elf::addIRelativeRelocs demonstrates that one-pass scan through relocations makes several optimizations difficult. This is something we can think about in the future. Reviewed By: peter.smith Differential Revision: https://reviews.llvm.org/D65995 llvm-svn: 368661
This commit is contained in:
Fangrui Song
parent
5390d25f2b
commit
dc06b0bc9a
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@ -172,13 +172,13 @@ bool elf::tryRelaxPPC64TocIndirection(RelType type, const Relocation &rel,
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: getRelaTocSymAndAddend<ELF64BE>(tocISB, rel.addend);
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// Only non-preemptable defined symbols can be relaxed.
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//
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// The toc entry of a non-preemptable ifunc is relocated by R_PPC64_IRELATIVE,
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// which will run at load time to determine the relocated value. It is not
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// known until load time, so the access cannot be relaxed.
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if (!d || d->isPreemptible || d->isGnuIFunc())
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if (!d || d->isPreemptible)
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return false;
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// R_PPC64_ADDR64 should have created a canonical PLT for the non-preemptable
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// ifunc and changed its type to STT_FUNC.
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assert(!d->isGnuIFunc());
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// Two instructions can materialize a 32-bit signed offset from the toc base.
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uint64_t tocRelative = d->getVA(addend) - getPPC64TocBase();
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if (!isInt<32>(tocRelative))
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@ -1089,15 +1089,6 @@ static void processRelocAux(InputSectionBase &sec, RelExpr expr, RelType type,
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getLocation(sec, sym, offset));
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}
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struct IRelativeReloc {
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RelType type;
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InputSectionBase *sec;
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uint64_t offset;
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Symbol *sym;
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};
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static std::vector<IRelativeReloc> iRelativeRelocs;
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template <class ELFT, class RelTy>
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static void scanReloc(InputSectionBase &sec, OffsetGetter &getOffset, RelTy *&i,
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RelTy *end) {
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@ -1265,12 +1256,6 @@ static void scanReloc(InputSectionBase &sec, OffsetGetter &getOffset, RelTy *&i,
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// correctly, the IRELATIVE relocations are stored in an array which a
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// statically linked executable's startup code must enumerate using the
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// linker-defined symbols __rela?_iplt_{start,end}.
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//
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// - An absolute relocation to a non-preemptible ifunc (such as a global
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// variable containing a pointer to the ifunc) needs to be relocated in
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// the exact same way as a GOT entry, so we can avoid needing to make the
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// PLT entry canonical by translating such relocations into IRELATIVE
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// relocations in the relaIplt.
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if (!sym.isInPlt()) {
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// Create PLT and GOTPLT slots for the symbol.
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sym.isInIplt = true;
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@ -1287,17 +1272,6 @@ static void scanReloc(InputSectionBase &sec, OffsetGetter &getOffset, RelTy *&i,
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*directSym);
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sym.pltIndex = directSym->pltIndex;
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}
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if (expr == R_ABS && addend == 0 && (sec.flags & SHF_WRITE)) {
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// We might be able to represent this as an IRELATIVE. But we don't know
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// yet whether some later relocation will make the symbol point to a
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// canonical PLT, which would make this either a dynamic RELATIVE (PIC) or
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// static (non-PIC) relocation. So we keep a record of the information
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// required to process the relocation, and after scanRelocs() has been
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// called on all relocations, the relocation is resolved by
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// addIRelativeRelocs().
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iRelativeRelocs.push_back({type, &sec, offset, &sym});
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return;
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}
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if (needsGot(expr)) {
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// Redirect GOT accesses to point to the Igot.
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//
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@ -1365,21 +1339,6 @@ template <class ELFT> void elf::scanRelocations(InputSectionBase &s) {
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scanRelocs<ELFT>(s, s.rels<ELFT>());
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}
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// Figure out which representation to use for any absolute relocs to
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// non-preemptible ifuncs that we visited during scanRelocs().
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void elf::addIRelativeRelocs() {
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for (IRelativeReloc &r : iRelativeRelocs) {
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if (r.sym->type == STT_GNU_IFUNC)
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in.relaIplt->addReloc(
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{target->iRelativeRel, r.sec, r.offset, true, r.sym, 0});
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else if (config->isPic)
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addRelativeReloc(r.sec, r.offset, r.sym, 0, R_ABS, r.type);
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else
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r.sec->relocations.push_back({R_ABS, r.type, r.offset, 0, r.sym});
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}
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iRelativeRelocs.clear();
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}
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static bool mergeCmp(const InputSection *a, const InputSection *b) {
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// std::merge requires a strict weak ordering.
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if (a->outSecOff < b->outSecOff)
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@ -1738,8 +1738,6 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
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reportUndefinedSymbols<ELFT>();
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}
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addIRelativeRelocs();
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if (in.plt && in.plt->isNeeded())
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in.plt->addSymbols();
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if (in.iplt && in.iplt->isNeeded())
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@ -0,0 +1,36 @@
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# REQUIRES: aarch64
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# RUN: llvm-mc -filetype=obj -triple=aarch64-none-linux-gnu %s -o %t.o
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# RUN: ld.lld %t.o -o %t
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# RUN: llvm-readelf -S -s %t | FileCheck %s --check-prefix=SEC
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# RUN: llvm-readelf -x .rodata -x .data %t | FileCheck --check-prefix=HEX %s
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# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=RELOC
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## ifunc is a non-preemptable STT_GNU_IFUNC. Check we create a canonical PLT
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## and redirect .rodata and .data references to it.
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# SEC: .text PROGBITS 0000000000210000
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# SEC: .got.plt PROGBITS 0000000000220008
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# SEC: 0000000000210010 0 FUNC GLOBAL DEFAULT 4 ifunc
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## .rodata[0] and .data[0] store the address of the canonical PLT.
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# HEX: section '.rodata':
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# HEX-NEXT: 0x00200170 10002100 00000000
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# HEX: section '.data':
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# HEX-NEXT: 0x00220000 10002100 00000000
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# RELOC: .rela.dyn {
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# RELOC-NEXT: 0x220008 R_AARCH64_IRELATIVE - 0x210000
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# RELOC-NEXT: }
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.globl ifunc
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.type ifunc,@gnu_indirect_function
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ifunc:
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ret
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.rodata
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.p2align 3
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.xword ifunc
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.data
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.p2align 3
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.xword ifunc
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@ -4,7 +4,7 @@
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// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/gnu-ifunc-canon-ro-abs.s -o %t-ro-abs.o
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// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/gnu-ifunc-canon-rw-addend.s -o %t-rw-addend.o
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// RUN: ld.lld %t.o -o %t1
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// RUN: llvm-readobj -r %t1 | FileCheck --check-prefix=IREL2 %s
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// RUN: llvm-readobj -r %t1 | FileCheck --check-prefix=IREL1 %s
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// RUN: ld.lld %t.o %t-ro-pcrel.o -o %t2
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// RUN: llvm-readobj -r %t2 | FileCheck --check-prefix=IREL1 %s
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// RUN: ld.lld %t.o %t-ro-abs.o -o %t3
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@ -22,7 +22,7 @@
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// RUN: ld.lld %t-rw-addend.o %t.o -o %t7
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// RUN: llvm-readobj -r %t7 | FileCheck --check-prefix=IREL1 %s
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// RUN: ld.lld %t.o -o %t8 -pie
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// RUN: llvm-readobj -r %t8 | FileCheck --check-prefix=IREL2 %s
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// RUN: llvm-readobj -r %t8 | FileCheck --check-prefix=IREL1-REL2 %s
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// RUN: ld.lld %t.o %t-ro-pcrel.o -o %t9 -pie
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// RUN: llvm-readobj -r %t9 | FileCheck --check-prefix=IREL1-REL2 %s
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// RUN: ld.lld %t.o %t-rw-addend.o -o %t10 -pie
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@ -32,13 +32,6 @@
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// RUN: ld.lld %t-rw-addend.o %t.o -o %t12 -pie
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// RUN: llvm-readobj -r %t12 | FileCheck --check-prefix=IREL1-REL3 %s
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// Two relocs, one for the GOT and the other for .data.
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// IREL2-NOT: R_X86_64_
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// IREL2: .rela.dyn
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// IREL2-NEXT: R_X86_64_IRELATIVE
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// IREL2-NEXT: R_X86_64_IRELATIVE
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// IREL2-NOT: R_X86_64_
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// One reloc for the canonical PLT.
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// IREL1-NOT: R_X86_64_
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// IREL1: .rela.dyn
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@ -4,14 +4,28 @@
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# RUN: echo '.globl ifunc; .type ifunc, %gnu_indirect_function; ifunc:' | \
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# RUN: llvm-mc -filetype=obj -triple=powerpc64le - -o %t1.o
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# RUN: ld.lld %t.o %t1.o -o %t
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# RUN: llvm-objdump -d %t | FileCheck %s
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# RUN: llvm-readelf -S -s %t | FileCheck --check-prefix=SEC %s
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# RUN: llvm-readelf -x .toc %t | FileCheck --check-prefix=HEX %s
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# RUN: llvm-objdump -d %t | FileCheck --check-prefix=DIS %s
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## ifunc is a non-preemptable STT_GNU_IFUNC. Its toc entry will be
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## relocated by R_PPC64_IRELATIVE, not representable by a toc-relative value.
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## Check the toc-indirect access is not relaxed.
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## ifunc is a non-preemptable STT_GNU_IFUNC. The R_PPC64_ADDR64 in .toc
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## creates a canonical PLT for it and changes its type to STT_FUNC. We can thus
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## still perform toc-indirect to toc-relative relaxation because the distance
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## to the address of the canonical PLT is fixed.
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# CHECK: nop
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# CHECK-NEXT: ld 3, -32768(2)
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# SEC: .text PROGBITS 0000000010010000
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# SEC: .plt NOBITS 0000000010030000
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# SEC: 0000000010010010 0 FUNC GLOBAL DEFAULT 3 ifunc
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## .toc[0] stores the address of the canonical PLT.
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# HEX: section '.toc':
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# HEX-NEXT: 0x10020000 10000110 00000000
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# REL: .rela.dyn {
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# REL-NEXT: 0x10030000 R_PPC64_IRELATIVE - 0x10010008
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# REL-NEXT: }
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# DIS: addi 3, 3,
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addis 3, 2, .toc@toc@ha
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ld 3, .toc@toc@l(3)
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