[lld] [LLD][ELF] Allow merging XO and RX sections, and add `--[no-]xosegment` flag (PR #132412)
Csanád Hajdú via llvm-commits
llvm-commits at lists.llvm.org
Sun Mar 23 01:30:37 PDT 2025
https://github.com/Il-Capitano updated https://github.com/llvm/llvm-project/pull/132412
>From 074e15784950548531e184ac5d1e4005b54afff0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Csan=C3=A1d=20Hajd=C3=BA?= <csanad.hajdu at arm.com>
Date: Fri, 21 Mar 2025 16:26:28 +0100
Subject: [PATCH 1/5] [LLD][ELF] Allow merging XO and RX sections, and add
`--[no-]xosegment` flag
Following from the discussion in #132224, this seems like the best
approach to deal with a mix of XO and RX output sections in the same
binary. This change will also simplify the implementation of the
PURECODE section flag for AArch64.
To control this behaviour, the `--[no-]xosegment` flag is added to LLD
(similarly to `--[no-]rosegment`), which determines whether to allow
merging XO and RX sections in the same segment. The default value is
`--no-xosegment`, which is a breaking change compared to the previous
behaviour.
Release notes are also added, since this will be a breaking change.
---
lld/ELF/Config.h | 1 +
lld/ELF/Driver.cpp | 1 +
lld/ELF/Options.td | 4 ++++
lld/ELF/Writer.cpp | 14 ++++++++++----
lld/docs/ReleaseNotes.rst | 4 ++++
lld/test/ELF/aarch64-execute-only.s | 4 ++--
lld/test/ELF/arm-execute-only.s | 4 ++--
7 files changed, 24 insertions(+), 8 deletions(-)
diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index e07c7dd4ca1b6..c1925cec1429c 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -340,6 +340,7 @@ struct Config {
llvm::DenseSet<llvm::StringRef> saveTempsArgs;
llvm::SmallVector<std::pair<llvm::GlobPattern, uint32_t>, 0> shuffleSections;
bool singleRoRx;
+ bool singleXoRx;
bool shared;
bool symbolic;
bool isStatic = false;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 3e7e05746483a..9c80f8793aae1 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -1486,6 +1486,7 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
ctx.arg.randomizeSectionPadding =
args::getInteger(args, OPT_randomize_section_padding, 0);
ctx.arg.singleRoRx = !args.hasFlag(OPT_rosegment, OPT_no_rosegment, true);
+ ctx.arg.singleXoRx = !args.hasFlag(OPT_xosegment, OPT_no_xosegment, false);
ctx.arg.soName = args.getLastArgValue(OPT_soname);
ctx.arg.sortSection = getSortSection(ctx, args);
ctx.arg.splitStackAdjustSize =
diff --git a/lld/ELF/Options.td b/lld/ELF/Options.td
index b3b12a0646875..35879bcc52bf6 100644
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -432,6 +432,10 @@ defm rosegment: BB<"rosegment",
"Put read-only non-executable sections in their own segment (default)",
"Do not put read-only non-executable sections in their own segment">;
+defm xosegment: BB<"xosegment",
+ "Put execute-only sections in their own segment",
+ "Do not put execute-only sections in their own segment (default)">;
+
defm rpath: Eq<"rpath", "Add a DT_RUNPATH to the output">;
def relocatable: F<"relocatable">, HelpText<"Create relocatable object file">;
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 2cea6a44b391a..28b24f90716b8 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -2379,10 +2379,16 @@ Writer<ELFT>::createPhdrs(Partition &part) {
// so when hasSectionsCommand, since we cannot introduce the extra alignment
// needed to create a new LOAD)
uint64_t newFlags = computeFlags(ctx, sec->getPhdrFlags());
- // When --no-rosegment is specified, RO and RX sections are compatible.
- uint32_t incompatible = flags ^ newFlags;
- if (ctx.arg.singleRoRx && !(newFlags & PF_W))
- incompatible &= ~PF_X;
+ uint64_t incompatible = flags ^ newFlags;
+ if (!(newFlags & PF_W)) {
+ // When --no-rosegment is specified, RO and RX sections are compatible.
+ if (ctx.arg.singleRoRx)
+ incompatible &= ~PF_X;
+ // When --no-xosegment is specified (the default), XO and RX sections are
+ // compatible.
+ if (ctx.arg.singleXoRx)
+ incompatible &= ~PF_R;
+ }
if (incompatible)
load = nullptr;
diff --git a/lld/docs/ReleaseNotes.rst b/lld/docs/ReleaseNotes.rst
index 133a6fe87402d..4ec5c42b0f31f 100644
--- a/lld/docs/ReleaseNotes.rst
+++ b/lld/docs/ReleaseNotes.rst
@@ -29,9 +29,13 @@ ELF Improvements
GNU GCS Attribute Flags in Dynamic Objects when GCS is enabled. Inherits value
from ``-zgcs-report`` (capped at ``warning`` level) unless user-defined,
ensuring compatibility with GNU ld linker.
+* Added ``--xosegment`` and ``--no-xosegment`` flags to control whether to place
+ XO and RX sections in the same segment. The default value is ``--no-xosegment``.
Breaking changes
----------------
+* XO and RX sections are now allowed to be placed in the same segment by default.
+ Pass ``--xosegment`` to lld in order to get the old behavior back.
COFF Improvements
-----------------
diff --git a/lld/test/ELF/aarch64-execute-only.s b/lld/test/ELF/aarch64-execute-only.s
index 20908ba9f754f..d4ee783e2c578 100644
--- a/lld/test/ELF/aarch64-execute-only.s
+++ b/lld/test/ELF/aarch64-execute-only.s
@@ -1,12 +1,12 @@
// REQUIRES: aarch64
// RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t.o
-// RUN: ld.lld %t.o -o %t.so -shared
+// RUN: ld.lld --xosegment %t.o -o %t.so -shared
// RUN: llvm-readelf -l %t.so | FileCheck --implicit-check-not=LOAD %s
// RUN: echo ".section .foo,\"ax\"; ret" > %t.s
// RUN: llvm-mc -filetype=obj -triple=aarch64 %t.s -o %t2.o
-// RUN: ld.lld %t.o %t2.o -o %t.so -shared
+// RUN: ld.lld --xosegment %t.o %t2.o -o %t.so -shared
// RUN: llvm-readelf -l %t.so | FileCheck --check-prefix=DIFF --implicit-check-not=LOAD %s
// CHECK: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000245 0x000245 R 0x10000
diff --git a/lld/test/ELF/arm-execute-only.s b/lld/test/ELF/arm-execute-only.s
index e938be5e64a4b..3bb89ad0620f8 100644
--- a/lld/test/ELF/arm-execute-only.s
+++ b/lld/test/ELF/arm-execute-only.s
@@ -1,13 +1,13 @@
// REQUIRES: arm
// RUN: llvm-mc -filetype=obj -triple=armv7-pc-linux %s -o %t.o
-// RUN: ld.lld %t.o -o %t.so -shared
+// RUN: ld.lld --xosegment %t.o -o %t.so -shared
// RUN: llvm-readelf -l %t.so | FileCheck --implicit-check-not=LOAD %s
// RUN: echo ".section .foo,\"ax\"; \
// RUN: bx lr" > %t.s
// RUN: llvm-mc -filetype=obj -triple=armv7-pc-linux %t.s -o %t2.o
-// RUN: ld.lld %t.o %t2.o -o %t.so -shared
+// RUN: ld.lld --xosegment %t.o %t2.o -o %t.so -shared
// RUN: llvm-readelf -l %t.so | FileCheck --check-prefix=DIFF --implicit-check-not=LOAD %s
// CHECK: LOAD 0x000000 0x00000000 0x00000000 0x0016d 0x0016d R 0x10000
>From 9613bb86c6fbedcb099ee6ebf5ca97a7ef0bf8df Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Csan=C3=A1d=20Hajd=C3=BA?= <csanad.hajdu at arm.com>
Date: Fri, 21 Mar 2025 17:23:49 +0100
Subject: [PATCH 2/5] Add missing test cases
---
lld/test/ELF/aarch64-execute-only-mixed.s | 55 +++++++++++++++++++++++
lld/test/ELF/arm-execute-only-mixed.s | 55 +++++++++++++++++++++++
2 files changed, 110 insertions(+)
create mode 100644 lld/test/ELF/aarch64-execute-only-mixed.s
create mode 100644 lld/test/ELF/arm-execute-only-mixed.s
diff --git a/lld/test/ELF/aarch64-execute-only-mixed.s b/lld/test/ELF/aarch64-execute-only-mixed.s
new file mode 100644
index 0000000000000..d12a8828dc29f
--- /dev/null
+++ b/lld/test/ELF/aarch64-execute-only-mixed.s
@@ -0,0 +1,55 @@
+// 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 xo.s -o xo.o
+// RUN: llvm-mc -filetype=obj -triple=aarch64 rx.s -o rx.o
+// RUN: ld.lld start.o xo.o -o xo
+// RUN: ld.lld start.o rx.o -o rx-default
+// RUN: ld.lld --xosegment start.o rx.o -o rx-xosegment
+// RUN: ld.lld --no-xosegment start.o rx.o -o rx-no-xosegment
+// RUN: llvm-readelf -l xo | FileCheck --check-prefix=CHECK-XO %s
+// RUN: llvm-readelf -l rx-default | FileCheck --check-prefix=CHECK-MERGED %s
+// RUN: llvm-readelf -l rx-xosegment | FileCheck --check-prefix=CHECK-SEPARATE %s
+// RUN: llvm-readelf -l rx-no-xosegment | FileCheck --check-prefix=CHECK-MERGED %s
+
+// CHECK-XO: PHDR
+// CHECK-XO-NEXT: LOAD
+// CHECK-XO-NEXT: LOAD 0x000120 0x0000000000210120 0x0000000000210120 0x00000c 0x00000c E 0x10000
+/// Index should match the index of the LOAD section above.
+// CHECK-XO: 02 .text .foo
+
+// CHECK-MERGED: PHDR
+// CHECK-MERGED-NEXT: LOAD
+// CHECK-MERGED-NEXT: LOAD 0x000120 0x0000000000210120 0x0000000000210120 0x00000c 0x00000c R E 0x10000
+/// Index should match the index of the LOAD section above.
+// CHECK-MERGED: 02 .text .foo
+
+// CHECK-SEPARATE: PHDR
+// CHECK-SEPARATE-NEXT: LOAD
+// CHECK-SEPARATE-NEXT: LOAD 0x000158 0x0000000000210158 0x0000000000210158 0x000008 0x000008 E 0x10000
+// CHECK-SEPARATE-NEXT: LOAD 0x000160 0x0000000000220160 0x0000000000220160 0x000004 0x000004 R E 0x10000
+/// Index should match the index of the LOAD section above.
+// CHECK-SEPARATE: 02 .text
+// CHECK-SEPARATE: 03 .foo
+
+//--- start.s
+.section .text,"axy", at progbits,unique,0
+.global _start
+_start:
+ bl foo
+ ret
+
+//--- xo.s
+.section .foo,"axy", at progbits,unique,0
+.global foo
+foo:
+ ret
+
+//--- rx.s
+/// An empty .text section is implicitly created without the SHF_AARCH64_PURECODE flag without this.
+.section .text,"axy", at progbits,unique,0
+.section .foo,"ax", at progbits,unique,0
+.global foo
+foo:
+ ret
diff --git a/lld/test/ELF/arm-execute-only-mixed.s b/lld/test/ELF/arm-execute-only-mixed.s
new file mode 100644
index 0000000000000..3df898e7c2bae
--- /dev/null
+++ b/lld/test/ELF/arm-execute-only-mixed.s
@@ -0,0 +1,55 @@
+// REQUIRES: arm
+// RUN: rm -rf %t && split-file %s %t && cd %t
+
+// RUN: llvm-mc -filetype=obj -triple=armv7 start.s -o start.o
+// RUN: llvm-mc -filetype=obj -triple=armv7 xo.s -o xo.o
+// RUN: llvm-mc -filetype=obj -triple=armv7 rx.s -o rx.o
+// RUN: ld.lld start.o xo.o -o xo
+// RUN: ld.lld start.o rx.o -o rx-default
+// RUN: ld.lld --xosegment start.o rx.o -o rx-xosegment
+// RUN: ld.lld --no-xosegment start.o rx.o -o rx-no-xosegment
+// RUN: llvm-readelf -l xo | FileCheck --check-prefix=CHECK-XO %s
+// RUN: llvm-readelf -l rx-default | FileCheck --check-prefix=CHECK-MERGED %s
+// RUN: llvm-readelf -l rx-xosegment | FileCheck --check-prefix=CHECK-SEPARATE %s
+// RUN: llvm-readelf -l rx-no-xosegment | FileCheck --check-prefix=CHECK-MERGED %s
+
+// CHECK-XO: PHDR
+// CHECK-XO-NEXT: LOAD
+// CHECK-XO-NEXT: LOAD 0x0000b4 0x000200b4 0x000200b4 0x0000c 0x0000c E 0x10000
+/// Index should match the index of the LOAD section above.
+// CHECK-XO: 02 .text .foo
+
+// CHECK-MERGED: PHDR
+// CHECK-MERGED-NEXT: LOAD
+// CHECK-MERGED-NEXT: LOAD 0x0000b4 0x000200b4 0x000200b4 0x0000c 0x0000c R E 0x10000
+/// Index should match the index of the LOAD section above.
+// CHECK-MERGED: 02 .text .foo
+
+// CHECK-SEPARATE: PHDR
+// CHECK-SEPARATE-NEXT: LOAD
+// CHECK-SEPARATE-NEXT: LOAD 0x0000d4 0x000200d4 0x000200d4 0x00008 0x00008 E 0x10000
+// CHECK-SEPARATE-NEXT: LOAD 0x0000dc 0x000300dc 0x000300dc 0x00004 0x00004 R E 0x10000
+/// Index should match the index of the LOAD section above.
+// CHECK-SEPARATE: 02 .text
+// CHECK-SEPARATE: 03 .foo
+
+//--- start.s
+.section .text,"axy",%progbits,unique,0
+.global _start
+_start:
+ bl foo
+ bx lr
+
+//--- xo.s
+.section .foo,"axy",%progbits,unique,0
+.global foo
+foo:
+ bx lr
+
+//--- rx.s
+/// An empty .text section is implicitly created without the SHF_ARM_PURECODE flag without this.
+.section .text,"axy",%progbits,unique,0
+.section .foo,"ax",%progbits,unique,0
+.global foo
+foo:
+ bx lr
>From 3c4287e1bb75d7a778f3bb97baec2aa92e4bb6fd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Csan=C3=A1d=20Hajd=C3=BA?= <csanad.hajdu at arm.com>
Date: Fri, 21 Mar 2025 17:48:44 +0100
Subject: [PATCH 3/5] Add PR number to release notes
---
lld/docs/ReleaseNotes.rst | 1 +
1 file changed, 1 insertion(+)
diff --git a/lld/docs/ReleaseNotes.rst b/lld/docs/ReleaseNotes.rst
index 4ec5c42b0f31f..8986ff83b9621 100644
--- a/lld/docs/ReleaseNotes.rst
+++ b/lld/docs/ReleaseNotes.rst
@@ -31,6 +31,7 @@ ELF Improvements
ensuring compatibility with GNU ld linker.
* Added ``--xosegment`` and ``--no-xosegment`` flags to control whether to place
XO and RX sections in the same segment. The default value is ``--no-xosegment``.
+ (`#132412 <https://github.com/llvm/llvm-project/pull/132412>`_)
Breaking changes
----------------
>From ffb7b094f132674ab3eadd1f4bbe9736051378a8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Csan=C3=A1d=20Hajd=C3=BA?= <csanad.hajdu at arm.com>
Date: Sun, 23 Mar 2025 09:00:23 +0100
Subject: [PATCH 4/5] Update wording in release notes
---
lld/docs/ReleaseNotes.rst | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/lld/docs/ReleaseNotes.rst b/lld/docs/ReleaseNotes.rst
index 8986ff83b9621..0febb5d0f07f6 100644
--- a/lld/docs/ReleaseNotes.rst
+++ b/lld/docs/ReleaseNotes.rst
@@ -30,13 +30,15 @@ ELF Improvements
from ``-zgcs-report`` (capped at ``warning`` level) unless user-defined,
ensuring compatibility with GNU ld linker.
* Added ``--xosegment`` and ``--no-xosegment`` flags to control whether to place
- XO and RX sections in the same segment. The default value is ``--no-xosegment``.
+ executable-only and readable-executable sections in the same segment. The
+ default value is ``--no-xosegment``.
(`#132412 <https://github.com/llvm/llvm-project/pull/132412>`_)
Breaking changes
----------------
-* XO and RX sections are now allowed to be placed in the same segment by default.
- Pass ``--xosegment`` to lld in order to get the old behavior back.
+* Executable-only and readable-executable sections are now allowed to be placed
+ in the same segment by default. Pass ``--xosegment`` to lld in order to get
+ the old behavior back.
COFF Improvements
-----------------
>From 58968bbbed8a5e45810ca4b05f4d5fd71a060231 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Csan=C3=A1d=20Hajd=C3=BA?= <csanad.hajdu at arm.com>
Date: Sun, 23 Mar 2025 09:01:14 +0100
Subject: [PATCH 5/5] Update comments in tests
---
lld/test/ELF/aarch64-execute-only-mixed.s | 8 ++++----
lld/test/ELF/arm-execute-only-mixed.s | 8 ++++----
2 files changed, 8 insertions(+), 8 deletions(-)
diff --git a/lld/test/ELF/aarch64-execute-only-mixed.s b/lld/test/ELF/aarch64-execute-only-mixed.s
index d12a8828dc29f..f95a1547bfba2 100644
--- a/lld/test/ELF/aarch64-execute-only-mixed.s
+++ b/lld/test/ELF/aarch64-execute-only-mixed.s
@@ -16,20 +16,20 @@
// CHECK-XO: PHDR
// CHECK-XO-NEXT: LOAD
// CHECK-XO-NEXT: LOAD 0x000120 0x0000000000210120 0x0000000000210120 0x00000c 0x00000c E 0x10000
-/// Index should match the index of the LOAD section above.
+/// Index should match the index of the LOAD segment above.
// CHECK-XO: 02 .text .foo
// CHECK-MERGED: PHDR
// CHECK-MERGED-NEXT: LOAD
// CHECK-MERGED-NEXT: LOAD 0x000120 0x0000000000210120 0x0000000000210120 0x00000c 0x00000c R E 0x10000
-/// Index should match the index of the LOAD section above.
+/// Index should match the index of the LOAD segment above.
// CHECK-MERGED: 02 .text .foo
// CHECK-SEPARATE: PHDR
// CHECK-SEPARATE-NEXT: LOAD
// CHECK-SEPARATE-NEXT: LOAD 0x000158 0x0000000000210158 0x0000000000210158 0x000008 0x000008 E 0x10000
// CHECK-SEPARATE-NEXT: LOAD 0x000160 0x0000000000220160 0x0000000000220160 0x000004 0x000004 R E 0x10000
-/// Index should match the index of the LOAD section above.
+/// Index should match the index of the LOAD segment above.
// CHECK-SEPARATE: 02 .text
// CHECK-SEPARATE: 03 .foo
@@ -47,7 +47,7 @@ foo:
ret
//--- rx.s
-/// An empty .text section is implicitly created without the SHF_AARCH64_PURECODE flag without this.
+/// Ensure that the implicitly-created .text section has the SHF_AARCH64_PURECODE flag.
.section .text,"axy", at progbits,unique,0
.section .foo,"ax", at progbits,unique,0
.global foo
diff --git a/lld/test/ELF/arm-execute-only-mixed.s b/lld/test/ELF/arm-execute-only-mixed.s
index 3df898e7c2bae..17c227cf69983 100644
--- a/lld/test/ELF/arm-execute-only-mixed.s
+++ b/lld/test/ELF/arm-execute-only-mixed.s
@@ -16,20 +16,20 @@
// CHECK-XO: PHDR
// CHECK-XO-NEXT: LOAD
// CHECK-XO-NEXT: LOAD 0x0000b4 0x000200b4 0x000200b4 0x0000c 0x0000c E 0x10000
-/// Index should match the index of the LOAD section above.
+/// Index should match the index of the LOAD segment above.
// CHECK-XO: 02 .text .foo
// CHECK-MERGED: PHDR
// CHECK-MERGED-NEXT: LOAD
// CHECK-MERGED-NEXT: LOAD 0x0000b4 0x000200b4 0x000200b4 0x0000c 0x0000c R E 0x10000
-/// Index should match the index of the LOAD section above.
+/// Index should match the index of the LOAD segment above.
// CHECK-MERGED: 02 .text .foo
// CHECK-SEPARATE: PHDR
// CHECK-SEPARATE-NEXT: LOAD
// CHECK-SEPARATE-NEXT: LOAD 0x0000d4 0x000200d4 0x000200d4 0x00008 0x00008 E 0x10000
// CHECK-SEPARATE-NEXT: LOAD 0x0000dc 0x000300dc 0x000300dc 0x00004 0x00004 R E 0x10000
-/// Index should match the index of the LOAD section above.
+/// Index should match the index of the LOAD segment above.
// CHECK-SEPARATE: 02 .text
// CHECK-SEPARATE: 03 .foo
@@ -47,7 +47,7 @@ foo:
bx lr
//--- rx.s
-/// An empty .text section is implicitly created without the SHF_ARM_PURECODE flag without this.
+/// Ensure that the implicitly-created .text section has the SHF_ARM_PURECODE flag.
.section .text,"axy",%progbits,unique,0
.section .foo,"ax",%progbits,unique,0
.global foo
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