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[lld][ELF] Allow data.rel.ro.hot and .data.rel.ro.unlikely to be RELRO #148920

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Merged
merged 6 commits into from
Jul 23, 2025
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[lld][ELF] Allow data.rel.ro.hot and .data.rel.ro.unlikely to be RELRO #148920

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00b0ddb
allow data.rel.ro.unlikely as an relro section
mingmingl-llvm Jul 15, 2025
9cdf86d
regard both .hot and .unlikely as section suffix if lld option keep-d…
mingmingl-llvm Jul 15, 2025
27778e5
resolve comments
mingmingl-llvm Jul 16, 2025
eb439b1
omit ByName as suggested
mingmingl-llvm Jul 17, 2025
3b23fde
apply suggestions
mingmingl-llvm Jul 18, 2025
0f42025
resolve comments
mingmingl-llvm Jul 21, 2025
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1 change: 1 addition & 0 deletions lld/ELF/Config.h
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Original file line number Diff line number Diff line change
Expand Up @@ -404,6 +404,7 @@ struct Config {
bool zIfuncNoplt;
bool zInitfirst;
bool zInterpose;
bool zKeepDataSectionPrefix;
bool zKeepTextSectionPrefix;
bool zLrodataAfterBss;
bool zNoBtCfi;
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2 changes: 2 additions & 0 deletions lld/ELF/Driver.cpp
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Expand Up @@ -1625,6 +1625,8 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
ctx.arg.zIfuncNoplt = hasZOption(args, "ifunc-noplt");
ctx.arg.zInitfirst = hasZOption(args, "initfirst");
ctx.arg.zInterpose = hasZOption(args, "interpose");
ctx.arg.zKeepDataSectionPrefix = getZFlag(
args, "keep-data-section-prefix", "nokeep-data-section-prefix", false);
ctx.arg.zKeepTextSectionPrefix = getZFlag(
args, "keep-text-section-prefix", "nokeep-text-section-prefix", false);
ctx.arg.zLrodataAfterBss =
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23 changes: 22 additions & 1 deletion lld/ELF/Writer.cpp
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Expand Up @@ -553,6 +553,27 @@ template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
}
}

// Returns true if the section is a data section that's read only and
// relocatable per its section name.
static bool isRelRoDataSection(Ctx &ctx, StringRef secName) {
// The section name should start with ".data.rel.ro".
if (!secName.consume_front(".data.rel.ro"))
return false;

// If the section name is .data.rel.ro, it is a relocatable read-only data
// section.
if (secName.empty())
return true;
// If -z keep-data-section-prefix is given, '.data.rel.ro.hot' and
// '.data.rel.ro.unlikely' are considered a split of '.data.rel.ro' based on
// hotness.
if (ctx.arg.zKeepDataSectionPrefix) {
return secName == ".hot" || secName == ".unlikely";
}

return false;
}

// Today's loaders have a feature to make segments read-only after
// processing dynamic relocations to enhance security. PT_GNU_RELRO
// is defined for that.
Expand Down Expand Up @@ -629,7 +650,7 @@ static bool isRelroSection(Ctx &ctx, const OutputSection *sec) {
// magic section names.
StringRef s = sec->name;

bool abiAgnostic = s == ".data.rel.ro" || s == ".bss.rel.ro" ||
bool abiAgnostic = isRelRoDataSection(ctx, s) || s == ".bss.rel.ro" ||
s == ".ctors" || s == ".dtors" || s == ".jcr" ||
s == ".eh_frame" || s == ".fini_array" ||
s == ".init_array" || s == ".preinit_array";
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89 changes: 89 additions & 0 deletions lld/test/ELF/keep-text-section-prefix.s
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@@ -0,0 +1,89 @@
# REQUIRES: x86
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Why is this x86 only? The assembly below is consumed with the x86 linux triple so it shouldn't matter?

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We use x86 as a placeholder for target-agnostic behavior. llvm-mc depends on llvm/lib/Target/X86, and we use REQUIRES: x86 to prevent test execution when LLVM_TARGETS_TO_BUILD skips x86.

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I didn't know the reason to use 'x86' before. Thanks for the info!


# RUN: rm -rf %t && split-file %s %t && cd %t

# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o

# RUN: ld.lld -z keep-data-section-prefix -T x.lds a.o -o out1
# RUN: llvm-readelf -l out1 | FileCheck --check-prefixes=SEG,LS %s
# RUN: llvm-readelf -S out1 | FileCheck %s --check-prefix=CHECK-LS

# RUN: ld.lld -z keep-data-section-prefix a.o -o out2
# RUN: llvm-readelf -l out2 | FileCheck --check-prefixes=SEG,PRE %s
# RUN: llvm-readelf -S out2 | FileCheck %s --check-prefix=CHECK-PRE

# RUN: ld.lld a.o -o out3
# RUN: llvm-readelf -l out3 | FileCheck --check-prefixes=SEG,PRE %s
# RUN: llvm-readelf -S out3 | FileCheck %s --check-prefix=CHECK-PRE

# RUN: not ld.lld -T x.lds a.o 2>&1 | FileCheck %s
# CHECK: error: section: .relro_padding is not contiguous with other relro sections

## The first RW PT_LOAD segment has FileSiz 0x126f (0x1000 + 0x200 + 0x60 + 0xf),
## and its p_offset p_vaddr p_paddr p_filesz should match PT_GNU_RELRO.
# Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
# SEG: LOAD 0x0001c8 0x00000000002011c8 0x00000000002011c8 0x000001 0x000001 R E 0x1000
# SEG-NEXT: LOAD 0x0001c9 0x00000000002021c9 0x00000000002021c9 0x00126f 0x001e37 RW 0x1000
# SEG-NEXT: LOAD 0x001438 0x0000000000204438 0x0000000000204438 0x000001 0x000002 RW 0x1000
# SEG-NEXT: GNU_RELRO 0x0001c9 0x00000000002021c9 0x00000000002021c9 0x00126f 0x001e37 R 0x1
# SEG-NEXT: GNU_STACK 0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW 0x0

## Input to output mapping per linker script
## .data.rel.ro.split -> .data.rel.ro
## .data.rel.ro -> .data.rel.ro
## .data.rel.ro.hot -> .data.rel.ro.hot
## .data.rel.ro.unlikely -> .data.rel.ro.unlikely
# LS: .text
# LS-NEXT: .data.rel.ro.hot .data.rel.ro .data.rel.ro.unlikely .relro_padding
# LS-NEXT: .data .bss

# [Nr] Name Type Address Off Size
# CHECK-LS: .data.rel.ro.hot PROGBITS 00000000002021c9 0001c9 00000f
# CHECK-LS-NEXT: .data.rel.ro PROGBITS 00000000002021d8 0001d8 000260
# CHECK-LS-NEXT: .data.rel.ro.unlikely PROGBITS 0000000000202438 000438 001000
# CHECK-LS-NEXT: .relro_padding NOBITS 0000000000203438 001438 000bc8
# CHECK-LS-NEXT: .data PROGBITS 0000000000204438 001438 000001
# CHECK-LS-NEXT: .bss NOBITS 0000000000204439 001439 000001

## Linker script is not provided to map data sections.
## So all input sections with prefix .data.rel.ro will map to .data.rel.ro in the output.
# PRE: .text
# PRE-NEXT: .data.rel.ro .relro_padding
# PRE-NEXT: .data .bss

# [Nr] Name Type Address Off Size
# CHECK-PRE: .data.rel.ro PROGBITS 00000000002021c9 0001c9 00126f
# CHECK-PRE-NEXT: .relro_padding NOBITS 0000000000203438 001438 000bc8
# CHECK-PRE-NEXT: .data PROGBITS 0000000000204438 001438 000001
# CHECK-PRE-NEXT: .bss NOBITS 0000000000204439 001439 000001

#--- x.lds
SECTIONS {
.data.rel.ro.hot : { *(.data.rel.ro.hot) }
.data.rel.ro : { .data.rel.ro }
.data.rel.ro.unlikely : { *(.data.rel.ro.unlikely) }
} INSERT AFTER .text


#--- a.s
.globl _start
_start:
ret

.section .data.rel.ro.hot, "aw"
.space 15

.section .data.rel.ro, "aw"
.space 96

.section .data.rel.ro.split,"aw"
.space 512

.section .data.rel.ro.unlikely, "aw"
.space 4096

.section .data, "aw"
.space 1

.section .bss, "aw"
.space 1
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