[lld] [LLD][AArch64] Mark .plt with PURECODE flag if all input sections also have it (PR #132224)
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Thu Mar 20 07:46:13 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-lld
Author: Csanád Hajdú (Il-Capitano)
<details>
<summary>Changes</summary>
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.
---
Full diff: https://github.com/llvm/llvm-project/pull/132224.diff
2 Files Affected:
- (modified) lld/ELF/SyntheticSections.cpp (+12)
- (added) lld/test/ELF/aarch64-execute-only-plt.s (+115)
``````````diff
diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index b03c4282ab1aa..a7ff8ed9b16d1 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -2610,6 +2610,18 @@ PltSection::PltSection(Ctx &ctx)
// modify the instructions in the PLT entries.
if (ctx.arg.emachine == EM_SPARCV9)
this->flags |= SHF_WRITE;
+
+ // On AArch64, PLT entries only do loads from the .got.plt section, so the
+ // .plt section can be marked with the SHF_AARCH64_PURECODE section flag. We
+ // only do this if all other executable sections also have the same section
+ // flag set, because otherwise .plt can't be allocated in the same segment as
+ // the other executable sections.
+ if (ctx.arg.emachine == EM_AARCH64 &&
+ all_of(ctx.inputSections, [](InputSectionBase *sec) {
+ return !(sec->flags & SHF_EXECINSTR) ||
+ (sec->flags & SHF_AARCH64_PURECODE);
+ }))
+ this->flags |= SHF_AARCH64_PURECODE;
}
void PltSection::writeTo(uint8_t *buf) {
diff --git a/lld/test/ELF/aarch64-execute-only-plt.s b/lld/test/ELF/aarch64-execute-only-plt.s
new file mode 100644
index 0000000000000..08e69fba8fb0c
--- /dev/null
+++ b/lld/test/ELF/aarch64-execute-only-plt.s
@@ -0,0 +1,115 @@
+// REQUIRES: aarch64
+// RUN: rm -rf %t && split-file %s %t && cd %t
+
+// RUN: llvm-mc -filetype=obj -triple=aarch64 start.s -o start.o
+// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-xo-same-section.s -o foo-xo-same-section.o
+// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-rx-same-section.s -o foo-rx-same-section.o
+// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-xo-different-section.s -o foo-xo-different-section.o
+// RUN: llvm-mc -filetype=obj -triple=aarch64 foo-rx-different-section.s -o foo-rx-different-section.o
+// RUN: llvm-mc -filetype=obj -triple=aarch64 %p/Inputs/plt-aarch64.s -o plt.o
+// RUN: ld.lld -shared plt.o -soname=t2.so -o plt.so
+// RUN: ld.lld start.o foo-xo-same-section.o plt.so -o xo-same-section
+// RUN: ld.lld start.o foo-rx-same-section.o plt.so -o rx-same-section
+// RUN: ld.lld start.o foo-xo-different-section.o plt.so -o xo-different-section
+// RUN: ld.lld start.o foo-rx-different-section.o plt.so -o rx-different-section
+// RUN: llvm-readobj -S -l xo-same-section | FileCheck --check-prefix=CHECK-XO %s
+// RUN: llvm-readobj -S -l rx-same-section | FileCheck --check-prefix=CHECK-RX %s
+// RUN: llvm-readobj -S -l xo-different-section | FileCheck --check-prefix=CHECK-XO %s
+// RUN: llvm-readobj -S -l rx-different-section | FileCheck --check-prefix=CHECK-RX %s
+// RUN: llvm-objdump -d --no-show-raw-insn xo-same-section | FileCheck --check-prefix=DISASM %s
+// RUN: llvm-objdump -d --no-show-raw-insn rx-same-section | FileCheck --check-prefix=DISASM %s
+// RUN: llvm-objdump -d --no-show-raw-insn xo-different-section | FileCheck --check-prefix=DISASM %s
+// RUN: llvm-objdump -d --no-show-raw-insn rx-different-section | FileCheck --check-prefix=DISASM %s
+
+// CHECK-XO: Name: .plt
+// CHECK-XO-NEXT: Type: SHT_PROGBITS
+// CHECK-XO-NEXT: Flags [
+// CHECK-XO-NEXT: SHF_AARCH64_PURECODE
+// CHECK-XO-NEXT: SHF_ALLOC
+// CHECK-XO-NEXT: SHF_EXECINSTR
+// CHECK-XO-NEXT: ]
+// CHECK-XO-NEXT: Address: 0x2102E0
+
+/// The address of .plt above should be within this program header.
+// CHECK-XO: VirtualAddress: 0x2102C8
+// CHECK-XO-NEXT: PhysicalAddress: 0x2102C8
+// CHECK-XO-NEXT: FileSize: 88
+// CHECK-XO-NEXT: MemSize: 88
+// CHECK-XO-NEXT: Flags [
+// CHECK-XO-NEXT: PF_X
+// CHECK-XO-NEXT: ]
+
+// CHECK-RX: Name: .plt
+// CHECK-RX-NEXT: Type: SHT_PROGBITS
+// CHECK-RX-NEXT: Flags [
+// CHECK-RX-NEXT: SHF_ALLOC
+// CHECK-RX-NEXT: SHF_EXECINSTR
+// CHECK-RX-NEXT: ]
+// CHECK-RX-NEXT: Address: 0x2102E0
+
+/// The address of .plt above should be within this program header.
+// CHECK-RX: VirtualAddress: 0x2102C8
+// CHECK-RX-NEXT: PhysicalAddress: 0x2102C8
+// CHECK-RX-NEXT: FileSize: 88
+// CHECK-RX-NEXT: MemSize: 88
+// CHECK-RX-NEXT: Flags [
+// CHECK-RX-NEXT: PF_R
+// CHECK-RX-NEXT: PF_X
+// CHECK-RX-NEXT: ]
+
+// DISASM-LABEL: Disassembly of section .plt:
+// DISASM-LABEL: <.plt>:
+// DISASM-NEXT: 2102e0: stp x16, x30, [sp, #-0x10]!
+// DISASM-NEXT: adrp x16, 0x230000 <weak+0x230000>
+// DISASM-NEXT: ldr x17, [x16, #0x400]
+// DISASM-NEXT: add x16, x16, #0x400
+// DISASM-NEXT: br x17
+// DISASM-NEXT: nop
+// DISASM-NEXT: nop
+// DISASM-NEXT: nop
+
+// DISASM-LABEL: <bar at plt>:
+// DISASM-NEXT: 210300: adrp x16, 0x230000 <weak+0x230000>
+// DISASM-NEXT: ldr x17, [x16, #0x408]
+// DISASM-NEXT: add x16, x16, #0x408
+// DISASM-NEXT: br x17
+
+// DISASM-LABEL: <weak at plt>:
+// DISASM-NEXT: 210310: adrp x16, 0x230000 <weak+0x230000>
+// DISASM-NEXT: ldr x17, [x16, #0x410]
+// DISASM-NEXT: add x16, x16, #0x410
+// DISASM-NEXT: br x17
+
+//--- start.s
+.section .text,"axy", at progbits,unique,0
+.global _start, foo, bar
+.weak weak
+_start:
+ bl foo
+ bl bar
+ bl weak
+ ret
+
+//--- foo-xo-same-section.s
+.section .text,"axy", at progbits,unique,0
+.global foo
+foo:
+ ret
+
+//--- foo-rx-same-section.s
+.section .text,"ax", at progbits,unique,0
+.global foo
+foo:
+ ret
+
+//--- foo-xo-different-section.s
+.section .foo,"axy", at progbits,unique,0
+.global foo
+foo:
+ ret
+
+//--- foo-rx-different-section.s
+.section .foo,"ax", at progbits,unique,0
+.global foo
+foo:
+ ret
``````````
</details>
https://github.com/llvm/llvm-project/pull/132224
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