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[ELF] Support placing .lbss/.lrodata/.ldata after .bss
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https://reviews.llvm.org/D150510 places .lrodata before .rodata to
minimize the number of permission transitions in the memory image.
However, this layout is less ideal for -fno-pic code (which is still
important).

Small code model -fno-pic code has R_X86_64_32S relocations with a range
of `[0,2**31)` (if we ignore the negative area). Placing `.lrodata`
earlier exerts relocation pressure on such code. Non-x86 64-bit
architectures generally have a similar `[0,2**31)` limitation if they
don't use PC-relative relocations.

If we place .lrodata later, we will need one extra PT_LOAD. Two layouts
are appealing:

* .bss/.lbss/.lrodata/.ldata (GNU ld)
* .bss/.ldata/.lbss/.lrodata

The GNU ld layout has the nice property that there is only one BSS
(except .tbss/.relro_padding). Add -z lrodata-after-bss to support
this layout.

Since a read-only PT_LOAD segment (for large data sections) may appear
after RW PT_LOAD segments. The placement of `_etext` has to be adjusted.

Pull Request: llvm/llvm-project#81224
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@MaskRay
MaskRay committed Feb 20, 2024
1 parent 96b1704 commit 7876235
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Showing 6 changed files with 77 additions and 22 deletions.
1 change: 1 addition & 0 deletions lld/ELF/Config.h
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Expand Up @@ -310,6 +310,7 @@ struct Config {
bool zInitfirst;
bool zInterpose;
bool zKeepTextSectionPrefix;
bool zLrodataAfterBss;
bool zNodefaultlib;
bool zNodelete;
bool zNodlopen;
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2 changes: 2 additions & 0 deletions lld/ELF/Driver.cpp
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Expand Up @@ -1436,6 +1436,8 @@ static void readConfigs(opt::InputArgList &args) {
config->zInterpose = hasZOption(args, "interpose");
config->zKeepTextSectionPrefix = getZFlag(
args, "keep-text-section-prefix", "nokeep-text-section-prefix", false);
config->zLrodataAfterBss =
getZFlag(args, "lrodata-after-bss", "nolrodata-after-bss", false);
config->zNodefaultlib = hasZOption(args, "nodefaultlib");
config->zNodelete = hasZOption(args, "nodelete");
config->zNodlopen = hasZOption(args, "nodlopen");
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47 changes: 30 additions & 17 deletions lld/ELF/Writer.cpp
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Expand Up @@ -911,11 +911,12 @@ enum RankFlags {
RF_NOT_ALLOC = 1 << 26,
RF_PARTITION = 1 << 18, // Partition number (8 bits)
RF_NOT_SPECIAL = 1 << 17,
RF_WRITE = 1 << 16,
RF_EXEC_WRITE = 1 << 15,
RF_EXEC = 1 << 14,
RF_RODATA = 1 << 13,
RF_LARGE = 1 << 12,
RF_LARGE_ALT = 1 << 15,
RF_WRITE = 1 << 14,
RF_EXEC_WRITE = 1 << 13,
RF_EXEC = 1 << 12,
RF_RODATA = 1 << 11,
RF_LARGE = 1 << 10,
RF_NOT_RELRO = 1 << 9,
RF_NOT_TLS = 1 << 8,
RF_BSS = 1 << 7,
Expand Down Expand Up @@ -974,8 +975,14 @@ static unsigned getSectionRank(OutputSection &osec) {
if (osec.type == SHT_PROGBITS)
rank |= RF_RODATA;
// Among PROGBITS sections, place .lrodata further from .text.
if (!(osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64))
rank |= RF_LARGE;
// For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This
// layout has one extra PT_LOAD, but alleviates relocation overflow
// pressure for absolute relocations referencing small data from -fno-pic
// relocatable files.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64)
rank |= config->zLrodataAfterBss ? RF_LARGE_ALT : 0;
else
rank |= config->zLrodataAfterBss ? 0 : RF_LARGE;
} else if (isExec) {
rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
} else {
Expand All @@ -988,10 +995,15 @@ static unsigned getSectionRank(OutputSection &osec) {
osec.relro = true;
else
rank |= RF_NOT_RELRO;
// Place .ldata and .lbss after .bss. Making .bss closer to .text alleviates
// relocation overflow pressure.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64)
rank |= RF_LARGE;
// Place .ldata and .lbss after .bss. Making .bss closer to .text
// alleviates relocation overflow pressure.
// For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss.
// .bss/.lbss being adjacent reuses the NOBITS size optimization.
if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64) {
rank |= config->zLrodataAfterBss
? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT)
: RF_LARGE;
}
}

// Within TLS sections, or within other RelRo sections, or within non-RelRo
Expand Down Expand Up @@ -1103,7 +1115,7 @@ template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
}

PhdrEntry *last = nullptr;
PhdrEntry *lastRO = nullptr;
OutputSection *lastRO = nullptr;
auto isLarge = [](OutputSection *osec) {
return config->emachine == EM_X86_64 && osec->flags & SHF_X86_64_LARGE;
};
Expand All @@ -1112,17 +1124,18 @@ template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
if (p->p_type != PT_LOAD)
continue;
last = p;
if (!(p->p_flags & PF_W))
lastRO = p;
if (!(p->p_flags & PF_W) && p->lastSec && !isLarge(p->lastSec))
lastRO = p->lastSec;
}
}

if (lastRO) {
// _etext is the first location after the last read-only loadable segment.
// _etext is the first location after the last read-only loadable segment
// that does not contain large sections.
if (ElfSym::etext1)
ElfSym::etext1->section = lastRO->lastSec;
ElfSym::etext1->section = lastRO;
if (ElfSym::etext2)
ElfSym::etext2->section = lastRO->lastSec;
ElfSym::etext2->section = lastRO;
}

if (last) {
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3 changes: 3 additions & 0 deletions lld/docs/ld.lld.1
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Expand Up @@ -791,6 +791,9 @@ flag to indicate to the runtime linker that the object is an interposer.
During symbol resolution interposers are searched after the application
but before other dependencies.
.Pp
.It Cm lrodata-after-bss
Place .lrodata after .bss.
.Pp
.It Cm muldefs
Do not error if a symbol is defined multiple times.
The first definition will be used.
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8 changes: 4 additions & 4 deletions lld/test/ELF/lto/codemodel.ll
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Expand Up @@ -2,8 +2,8 @@
; RUN: llvm-as %s -o %t.o
; RUN: ld.lld %t.o -o %ts -mllvm -code-model=small
; RUN: ld.lld %t.o -o %tl -mllvm -code-model=large
; RUN: llvm-objdump --no-print-imm-hex -d %ts | FileCheck %s --check-prefix=CHECK-SMALL
; RUN: llvm-objdump --no-print-imm-hex -d %tl | FileCheck %s --check-prefix=CHECK-LARGE
; RUN: llvm-objdump -d %ts | FileCheck %s --check-prefix=CHECK-SMALL
; RUN: llvm-objdump -d %tl | FileCheck %s --check-prefix=CHECK-LARGE

target triple = "x86_64-unknown-linux-gnu"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
Expand All @@ -13,8 +13,8 @@ target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16
define ptr @_start() nounwind readonly {
entry:
; CHECK-SMALL-LABEL: <_start>:
; CHECK-SMALL: movl $2097440, %eax
; CHECK-SMALL: movl ${{.*}}, %eax
; CHECK-LARGE-LABEL: <_start>:
; CHECK-LARGE: movabsq $2097440, %rax
; CHECK-LARGE: movabsq ${{.*}}, %rax
ret ptr @data
}
38 changes: 37 additions & 1 deletion lld/test/ELF/x86-64-section-layout.s
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Expand Up @@ -12,9 +12,12 @@
# RUN: ld.lld --section-start=.note=0x200300 a1.o -o a1
# RUN: llvm-readelf -S -sX a1 | FileCheck %s --check-prefix=CHECK1

# RUN: ld.lld -T b.lds -z norelro a.o -o b
# RUN: ld.lld -T b.lds -z norelro a.o -z lrodata-after-bss -z nolrodata-after-bss -o b --fatal-warnings
# RUN: llvm-readelf -S -l b | FileCheck %s --check-prefix=CHECK2

# RUN: ld.lld --section-start=.note=0x200300 a.o -z lrodata-after-bss -o a3
# RUN: llvm-readelf -S -l -sX a3 | FileCheck %s --check-prefix=CHECK3

# CHECK: Name Type Address Off Size ES Flg Lk Inf Al
# CHECK-NEXT: NULL 0000000000000000 000000 000000 00 0 0 0
# CHECK-NEXT: .note NOTE 0000000000200300 000300 000001 00 A 0 0 1
Expand Down Expand Up @@ -80,6 +83,39 @@
# CHECK2-NEXT: LOAD 0x000305 0x0000000000200305 0x0000000000200305 0x001805 0x002a06 RW 0x1000
# CHECK2-NEXT: TLS 0x000305 0x0000000000200305 0x0000000000200305 0x000001 0x000003 R 0x1

# CHECK3: Name Type Address Off Size ES Flg Lk Inf Al
# CHECK3-NEXT: NULL 0000000000000000 000000 000000 00 0 0 0
# CHECK3-NEXT: .note NOTE 0000000000200300 000300 000001 00 A 0 0 1
# CHECK3-NEXT: .rodata PROGBITS 0000000000200301 000301 000001 00 A 0 0 1
# CHECK3-NEXT: .text PROGBITS 0000000000201304 000304 000001 00 AX 0 0 4
# CHECK3-NEXT: .tdata PROGBITS 0000000000202305 000305 000001 00 WAT 0 0 1
# CHECK3-NEXT: .tbss NOBITS 0000000000202306 000306 000002 00 WAT 0 0 1
# CHECK3-NEXT: .relro_padding NOBITS 0000000000202306 000306 000cfa 00 WA 0 0 1
# CHECK3-NEXT: .data PROGBITS 0000000000203306 000306 000001 00 WA 0 0 1
# CHECK3-NEXT: .bss NOBITS 0000000000203307 000307 001800 00 WA 0 0 1
## We spend (size(.bss) + size(.lbss)) % MAXPAGESIZE bytes.
# CHECK3-NEXT: .lbss NOBITS 0000000000204b07 000307 001201 00 WAl 0 0 1
# CHECK3-NEXT: .lrodata PROGBITS 0000000000206d08 000d08 000002 00 Al 0 0 1
# CHECK3-NEXT: .ldata PROGBITS 0000000000207d0a 000d0a 000002 00 WAl 0 0 1
# CHECK3-NEXT: .ldata2 PROGBITS 0000000000207d0c 000d0c 000001 00 WAl 0 0 1
# CHECK3-NEXT: .comment PROGBITS 0000000000000000 000d0d {{.*}} 01 MS 0 0 1

# CHECK3: Program Headers:
# CHECK3-NEXT: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
# CHECK3-NEXT: PHDR 0x000040 0x0000000000200040 0x0000000000200040 {{.*}} {{.*}} R 0x8
# CHECK3-NEXT: LOAD 0x000000 0x0000000000200000 0x0000000000200000 0x000302 0x000302 R 0x1000
# CHECK3-NEXT: LOAD 0x000304 0x0000000000201304 0x0000000000201304 0x000001 0x000001 R E 0x1000
# CHECK3-NEXT: LOAD 0x000305 0x0000000000202305 0x0000000000202305 0x000001 0x000cfb RW 0x1000
# CHECK3-NEXT: LOAD 0x000306 0x0000000000203306 0x0000000000203306 0x000001 0x002a02 RW 0x1000
# CHECK3-NEXT: LOAD 0x000d08 0x0000000000206d08 0x0000000000206d08 0x000002 0x000002 R 0x1000
# CHECK3-NEXT: LOAD 0x000d0a 0x0000000000207d0a 0x0000000000207d0a 0x000003 0x000003 RW 0x1000
# CHECK3-NEXT: TLS 0x000305 0x0000000000202305 0x0000000000202305 0x000001 0x000003 R 0x1

# CHECK3: 0000000000201304 0 NOTYPE GLOBAL DEFAULT [[#]] (.text) _start
# CHECK3-NEXT: 0000000000201305 0 NOTYPE GLOBAL DEFAULT [[#]] (.text) _etext
# CHECK3-NEXT: 0000000000203307 0 NOTYPE GLOBAL DEFAULT [[#]] (.data) _edata
# CHECK3-NEXT: 0000000000207d0d 0 NOTYPE GLOBAL DEFAULT [[#]] (.ldata2) _end

#--- a.s
.globl _start, _etext, _edata, _end
_start:
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