[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
Fri Mar 21 08:40:26 PDT 2025
https://github.com/Il-Capitano created https://github.com/llvm/llvm-project/pull/132412
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.
>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] [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
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