[llvm] [MC][BPF] Avoid generating .note.GNU-stack section (PR #159960)
via llvm-commits
llvm-commits at lists.llvm.org
Sat Sep 20 17:44:34 PDT 2025
https://github.com/yonghong-song created https://github.com/llvm/llvm-project/pull/159960
The kernel libbpf does not need .note.GNU-stack section. If not filtering out in llvm, the section will be filtered out in libbpf. So let us filter it out as early as possible which is in llvm.
Change function getNonexecutableStackSection() in MCAsmInfoELF.h from 'final' to 'override' so target (e.g. BPF) can decide whether '.note.GNU-stack' section should be emitted or not.
The following is an example.
```
$ cat t.c
int test() { return 5; }
```
Without this change:
```
$ llvm-readelf -S t.o
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000110 000047 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000010 00 AX 0 0 8
[ 3] .comment PROGBITS 0000000000000000 000050 000072 01 MS 0 0 1
[ 4] .note.GNU-stack PROGBITS 0000000000000000 0000c2 000000 00 0 0 1
[ 5] .llvm_addrsig LLVM_ADDRSIG 0000000000000000 000110 000000 00 E 6 0 1
[ 6] .symtab SYMTAB 0000000000000000 0000c8 000048 18 1 2 8
```
With this change:
```
$ llvm-readelf -S t.o
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000110 000037 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000010 00 AX 0 0 8
[ 3] .comment PROGBITS 0000000000000000 000050 000072 01 MS 0 0 1
[ 4] .llvm_addrsig LLVM_ADDRSIG 0000000000000000 000110 000000 00 E 5 0 1
[ 5] .symtab SYMTAB 0000000000000000 0000c8 000048 18 1 2 8
```
>From 84ab322b685955222893492ad406244cda333cb5 Mon Sep 17 00:00:00 2001
From: Yonghong Song <yonghong.song at linux.dev>
Date: Sat, 20 Sep 2025 17:08:49 -0700
Subject: [PATCH] [MC][BPF] Avoid generating .note.GNU-stack section
The kernel libbpf does not need .note.GNU-stack section. If not
filtering out in llvm, the section will be filtered out in libbpf.
So let us filter it out as early as possible which is in llvm.
Change function getNonexecutableStackSection() in MCAsmInfoELF.h
from 'final' to 'override' so target (e.g. BPF) can decide
whether '.note.GNU-stack' section should be emitted or not.
The following is an example.
$ cat t.c
int test() { return 5; }
Without this change:
$ llvm-readelf -S t.o
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000110 000047 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000010 00 AX 0 0 8
[ 3] .comment PROGBITS 0000000000000000 000050 000072 01 MS 0 0 1
[ 4] .note.GNU-stack PROGBITS 0000000000000000 0000c2 000000 00 0 0 1
[ 5] .llvm_addrsig LLVM_ADDRSIG 0000000000000000 000110 000000 00 E 6 0 1
[ 6] .symtab SYMTAB 0000000000000000 0000c8 000048 18 1 2 8
With this change:
$ llvm-readelf -S t.o
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000110 000037 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000010 00 AX 0 0 8
[ 3] .comment PROGBITS 0000000000000000 000050 000072 01 MS 0 0 1
[ 4] .llvm_addrsig LLVM_ADDRSIG 0000000000000000 000110 000000 00 E 5 0 1
[ 5] .symtab SYMTAB 0000000000000000 0000c8 000048 18 1 2 8
---
llvm/include/llvm/MC/MCAsmInfoELF.h | 2 +-
llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h | 4 ++++
2 files changed, 5 insertions(+), 1 deletion(-)
diff --git a/llvm/include/llvm/MC/MCAsmInfoELF.h b/llvm/include/llvm/MC/MCAsmInfoELF.h
index c05e4ad78ecd1..e0678888d1003 100644
--- a/llvm/include/llvm/MC/MCAsmInfoELF.h
+++ b/llvm/include/llvm/MC/MCAsmInfoELF.h
@@ -15,7 +15,7 @@ namespace llvm {
class MCAsmInfoELF : public MCAsmInfo {
virtual void anchor();
- MCSection *getNonexecutableStackSection(MCContext &Ctx) const final;
+ MCSection *getNonexecutableStackSection(MCContext &Ctx) const override;
void printSwitchToSection(const MCSection &, uint32_t, const Triple &,
raw_ostream &) const final;
bool useCodeAlign(const MCSection &Sec) const final;
diff --git a/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h b/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
index 63d6e6fb630a6..2198ed8c9034b 100644
--- a/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
+++ b/llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
@@ -47,6 +47,10 @@ class BPFMCAsmInfo : public MCAsmInfoELF {
void setDwarfUsesRelocationsAcrossSections(bool enable) {
DwarfUsesRelocationsAcrossSections = enable;
}
+
+ MCSection *getNonexecutableStackSection(MCContext &Ctx) const override {
+ return nullptr;
+ }
};
}
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