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[LLD][AArch64] Mark .plt with PURECODE flag if all input sections also have it
Mark the synthetic `.plt` section with the `SHF_AARCH64_PURECODE` section flag if all executable input sections also have that flag. Without this change, if we were to compile a binary with `-mexecute-only`, the final executable will only have `.plt` not marked with the section flag, causing it to be placed in a different load segment. This leads to an extra page's worth of memory usage unnecessarily when running the executable. A similar issue happens if we always set the section flag on `.plt` and compile a binary without `-mexecute-only`, so the solution should match the `SHF_AARCH64_PURECODE` section flags between `.plt` and all other executable sections.
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‎lld/ELF/SyntheticSections.cpp‎

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@@ -2610,6 +2610,18 @@ PltSection::PltSection(Ctx &ctx)
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// modify the instructions in the PLT entries.
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if (ctx.arg.emachine == EM_SPARCV9)
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this->flags |= SHF_WRITE;
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// On AArch64, PLT entries only do loads from the .got.plt section, so the
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// .plt section can be marked with the SHF_AARCH64_PURECODE section flag. We
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// only do this if all other executable sections also have the same section
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// flag set, because otherwise .plt can't be allocated in the same segment as
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// the other executable sections.
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if (ctx.arg.emachine == EM_AARCH64 &&
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all_of(ctx.inputSections, [](InputSectionBase *sec) {
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return !(sec->flags & SHF_EXECINSTR) ||
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(sec->flags & SHF_AARCH64_PURECODE);
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}))
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this->flags |= SHF_AARCH64_PURECODE;
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}
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void PltSection::writeTo(uint8_t *buf) {

‎lld/test/ELF/aarch64-execute-only-plt.s‎

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// REQUIRES: aarch64
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// RUN: rm -rf %t && split-file %s %t && cd %t
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// RUN: llvm-mc -filetype=obj -triple=aarch64 start.s -o start.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-xo-same-section.s -o foo-xo-same-section.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-rx-same-section.s -o foo-rx-same-section.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-xo-different-section.s -o foo-xo-different-section.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-rx-different-section.s -o foo-rx-different-section.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 %p/Inputs/plt-aarch64.s -o plt.o
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// RUN: ld.lld -shared plt.o -soname=t2.so -o plt.so
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// RUN: ld.lld start.o foo-xo-same-section.o plt.so -o xo-same-section
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// RUN: ld.lld start.o foo-rx-same-section.o plt.so -o rx-same-section
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// RUN: ld.lld start.o foo-xo-different-section.o plt.so -o xo-different-section
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// RUN: ld.lld start.o foo-rx-different-section.o plt.so -o rx-different-section
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// RUN: llvm-readobj -S -l xo-same-section | FileCheck --check-prefix=CHECK-XO %s
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// RUN: llvm-readobj -S -l rx-same-section | FileCheck --check-prefix=CHECK-RX %s
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// RUN: llvm-readobj -S -l xo-different-section | FileCheck --check-prefix=CHECK-XO %s
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// RUN: llvm-readobj -S -l rx-different-section | FileCheck --check-prefix=CHECK-RX %s
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// RUN: llvm-objdump -d --no-show-raw-insn xo-same-section | FileCheck --check-prefix=DISASM %s
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// RUN: llvm-objdump -d --no-show-raw-insn rx-same-section | FileCheck --check-prefix=DISASM %s
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// RUN: llvm-objdump -d --no-show-raw-insn xo-different-section | FileCheck --check-prefix=DISASM %s
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// RUN: llvm-objdump -d --no-show-raw-insn rx-different-section | FileCheck --check-prefix=DISASM %s
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// CHECK-XO: Name: .plt
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// CHECK-XO-NEXT: Type: SHT_PROGBITS
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// CHECK-XO-NEXT: Flags [
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// CHECK-XO-NEXT: SHF_AARCH64_PURECODE
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// CHECK-XO-NEXT: SHF_ALLOC
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// CHECK-XO-NEXT: SHF_EXECINSTR
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// CHECK-XO-NEXT: ]
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// CHECK-XO-NEXT: Address: 0x2102E0
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/// The address of .plt above should be within this program header.
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// CHECK-XO: VirtualAddress: 0x2102C8
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// CHECK-XO-NEXT: PhysicalAddress: 0x2102C8
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// CHECK-XO-NEXT: FileSize: 88
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// CHECK-XO-NEXT: MemSize: 88
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// CHECK-XO-NEXT: Flags [
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// CHECK-XO-NEXT: PF_X
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// CHECK-XO-NEXT: ]
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// CHECK-RX: Name: .plt
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// CHECK-RX-NEXT: Type: SHT_PROGBITS
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// CHECK-RX-NEXT: Flags [
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// CHECK-RX-NEXT: SHF_ALLOC
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// CHECK-RX-NEXT: SHF_EXECINSTR
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// CHECK-RX-NEXT: ]
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// CHECK-RX-NEXT: Address: 0x2102E0
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/// The address of .plt above should be within this program header.
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// CHECK-RX: VirtualAddress: 0x2102C8
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// CHECK-RX-NEXT: PhysicalAddress: 0x2102C8
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// CHECK-RX-NEXT: FileSize: 88
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// CHECK-RX-NEXT: MemSize: 88
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// CHECK-RX-NEXT: Flags [
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// CHECK-RX-NEXT: PF_R
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// CHECK-RX-NEXT: PF_X
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// CHECK-RX-NEXT: ]
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// DISASM-LABEL: Disassembly of section .plt:
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// DISASM-LABEL: <.plt>:
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// DISASM-NEXT: 2102e0: stp x16, x30, [sp, #-0x10]!
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// DISASM-NEXT: adrp x16, 0x230000 <weak+0x230000>
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// DISASM-NEXT: ldr x17, [x16, #0x400]
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// DISASM-NEXT: add x16, x16, #0x400
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// DISASM-NEXT: br x17
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// DISASM-NEXT: nop
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// DISASM-NEXT: nop
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// DISASM-NEXT: nop
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// DISASM-LABEL: <bar@plt>:
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// DISASM-NEXT: 210300: adrp x16, 0x230000 <weak+0x230000>
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// DISASM-NEXT: ldr x17, [x16, #0x408]
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// DISASM-NEXT: add x16, x16, #0x408
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// DISASM-NEXT: br x17
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// DISASM-LABEL: <weak@plt>:
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// DISASM-NEXT: 210310: adrp x16, 0x230000 <weak+0x230000>
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// DISASM-NEXT: ldr x17, [x16, #0x410]
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// DISASM-NEXT: add x16, x16, #0x410
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// DISASM-NEXT: br x17
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//--- start.s
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.section .text,"axy",@progbits,unique,0
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.global _start, foo, bar
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.weak weak
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_start:
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bl foo
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bl bar
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bl weak
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ret
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//--- foo-xo-same-section.s
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.section .text,"axy",@progbits,unique,0
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.global foo
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foo:
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ret
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//--- foo-rx-same-section.s
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.section .text,"ax",@progbits,unique,0
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.global foo
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foo:
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ret
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//--- foo-xo-different-section.s
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.section .foo,"axy",@progbits,unique,0
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.global foo
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foo:
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ret
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//--- foo-rx-different-section.s
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.section .foo,"ax",@progbits,unique,0
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.global foo
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foo:
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ret

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