[lld] [ELF] Implement -z dynamic-undefined-weak (PR #143831)
via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 11 21:55:35 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-lld-elf
Author: Fangrui Song (MaskRay)
<details>
<summary>Changes</summary>
The behavior of an undefined weak reference is implementation defined.
For static -no-pie linking, dynamic relocations are generally avoided (except
IRELATIVE). -shared linking generally emits dynamic relocations.
Dynamic -no-pie linking and -pie allow flexibility. Changes adjust the
behavior for better consistency and simpler internal representation,
e.g. https://reviews.llvm.org/D63003 https://reviews.llvm.org/D105164
(generalized to undefined non-weak in
2fcaa00d1e2317a90c9071b735eb0e758b5dd58b).
This patch implements -z [no]dynamic-undefined-weak option to fine-tune
this behavior. The effects are summarized as follows:
* Static -no-pie: no-op
* Dynamic -no-pie: nodynamic-undefined-weak suppresses GLOB_DAT/JUMP_SLOT
* Static -pie: dynamic-undefined-weak generates ABS/GLOB_DAT/JUMP_SLOT.
https://discourse.llvm.org/t/lld-weak-undefined-symbols-in-vdso-only/86749
* Dynamic -pie: nodynamic-undefined-weak suppresses ABS/GLOB_DAT/JUMP_SLOT
The -pie behavior likely stays stable while -no-pie (`!ctx.arg.isPic` in
`isStaticLinkTimeConstant`) behavior will likely change in the future.
The current default value of ctx.arg.zDynamicUndefined is selected to
prevent behavior changes.
Documentation in ld.lld.1 has not been updated yet, as the behavior is
complex and challenging to describe concisely.
---
Full diff: https://github.com/llvm/llvm-project/pull/143831.diff
7 Files Affected:
- (modified) lld/ELF/Config.h (+1)
- (modified) lld/ELF/Driver.cpp (+8)
- (modified) lld/ELF/Symbols.cpp (+8-6)
- (modified) lld/ELF/Writer.cpp (+2-4)
- (modified) lld/test/ELF/driver.test (+2-1)
- (modified) lld/test/ELF/weak-undef-got-plt.s (+6)
- (modified) lld/test/ELF/weak-undef-rw.s (+16-3)
``````````diff
diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 0a52dfe6901bd..3a9001d2cc8b8 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -368,6 +368,7 @@ struct Config {
bool writeAddends;
bool zCombreloc;
bool zCopyreloc;
+ bool zDynamicUndefined;
bool zForceBti;
bool zForceIbt;
bool zGlobal;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 12dac82c614a7..87b19cf543d9f 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -591,6 +591,7 @@ static void checkZOptions(Ctx &ctx, opt::InputArgList &args) {
args::getZOptionValue(args, OPT_z, "max-page-size", 0);
args::getZOptionValue(args, OPT_z, "common-page-size", 0);
getZFlag(args, "rel", "rela", false);
+ getZFlag(args, "dynamic-undefined-weak", "nodynamic-undefined-weak", false);
for (auto *arg : args.filtered(OPT_z))
if (!arg->isClaimed())
Warn(ctx) << "unknown -z value: " << StringRef(arg->getValue());
@@ -3058,6 +3059,13 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
ctx.hasDynsym = !ctx.sharedFiles.empty() || ctx.arg.isPic;
ctx.arg.exportDynamic &= ctx.hasDynsym;
+ // Preemptibility of undefined symbols when ctx.hasDynsym is true. Default is
+ // true for dynamic linking.
+ ctx.arg.zDynamicUndefined =
+ getZFlag(args, "dynamic-undefined-weak", "nodynamic-undefined-weak",
+ ctx.sharedFiles.size() || ctx.arg.shared) &&
+ ctx.hasDynsym;
+
// If an entry symbol is in a static archive, pull out that file now.
if (Symbol *sym = ctx.symtab->find(ctx.arg.entry))
handleUndefined(ctx, sym, "--entry");
diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index c461dfed0d741..de839795c50d7 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -333,10 +333,13 @@ bool elf::computeIsPreemptible(Ctx &ctx, const Symbol &sym) {
if (sym.visibility() != STV_DEFAULT)
return false;
- // At this point copy relocations have not been created yet, so any
- // symbol that is not defined locally is preemptible.
+ // At this point copy relocations have not been created yet.
+ // Shared symbols are preemptible. Undefined symbols are preemptible
+ // when zDynamicUndefined (default in dynamic linking). Weakness is not
+ // checked, though undefined non-weak would typically trigger relocation
+ // errors unless options like -z undefs are used.
if (!sym.isDefined())
- return true;
+ return !sym.isUndefined() || ctx.arg.zDynamicUndefined;
if (!ctx.arg.shared)
return false;
@@ -360,7 +363,6 @@ void elf::parseVersionAndComputeIsPreemptible(Ctx &ctx) {
// can contain versions in the form of <name>@<version>.
// Let them parse and update their names to exclude version suffix.
// In addition, compute isExported and isPreemptible.
- bool maybePreemptible = ctx.sharedFiles.size() || ctx.arg.shared;
for (Symbol *sym : ctx.symtab->getSymbols()) {
if (sym->hasVersionSuffix)
sym->parseSymbolVersion(ctx);
@@ -369,11 +371,11 @@ void elf::parseVersionAndComputeIsPreemptible(Ctx &ctx) {
continue;
}
if (!sym->isDefined() && !sym->isCommon()) {
- sym->isPreemptible = maybePreemptible && computeIsPreemptible(ctx, *sym);
+ sym->isPreemptible = computeIsPreemptible(ctx, *sym);
} else if (ctx.arg.exportDynamic &&
(sym->isUsedInRegularObj || !sym->ltoCanOmit)) {
sym->isExported = true;
- sym->isPreemptible = maybePreemptible && computeIsPreemptible(ctx, *sym);
+ sym->isPreemptible = computeIsPreemptible(ctx, *sym);
}
}
}
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 10dc688160d1e..f430f4e59a3fc 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -285,7 +285,6 @@ static void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
static void demoteSymbolsAndComputeIsPreemptible(Ctx &ctx) {
llvm::TimeTraceScope timeScope("Demote symbols");
DenseMap<InputFile *, DenseMap<SectionBase *, size_t>> sectionIndexMap;
- bool maybePreemptible = ctx.sharedFiles.size() || ctx.arg.shared;
for (Symbol *sym : ctx.symtab->getSymbols()) {
if (auto *d = dyn_cast<Defined>(sym)) {
if (d->section && !d->section->isLive())
@@ -301,9 +300,8 @@ static void demoteSymbolsAndComputeIsPreemptible(Ctx &ctx) {
}
}
- if (maybePreemptible)
- sym->isPreemptible = (sym->isUndefined() || sym->isExported) &&
- computeIsPreemptible(ctx, *sym);
+ sym->isPreemptible = (sym->isUndefined() || sym->isExported) &&
+ computeIsPreemptible(ctx, *sym);
}
}
diff --git a/lld/test/ELF/driver.test b/lld/test/ELF/driver.test
index 45d73607c8ac6..6d5761212cc38 100644
--- a/lld/test/ELF/driver.test
+++ b/lld/test/ELF/driver.test
@@ -47,7 +47,8 @@
# ERR9: error: cannot open output file utput=/no/such/file
# RUN: ld.lld %t -z foo -o /dev/null 2>&1 | FileCheck -check-prefix=ERR10 %s --implicit-check-not=warning:
-# RUN: ld.lld %t -z foo -z rel -z rela -z max-page-size=1 -z common-page-size=1 -o /dev/null --version 2>&1 | \
+# RUN: ld.lld %t -z foo -z rel -z rela -z max-page-size=1 -z common-page-size=1 -z dynamic-undefined-weak \
+# RUN: -z nodynamic-undefined-weak -o /dev/null --version 2>&1 | \
# RUN: FileCheck -check-prefix=ERR10 %s --implicit-check-not=warning:
# ERR10: warning: unknown -z value: foo
diff --git a/lld/test/ELF/weak-undef-got-plt.s b/lld/test/ELF/weak-undef-got-plt.s
index 0ee3da2cd3b40..48a7914e5b987 100644
--- a/lld/test/ELF/weak-undef-got-plt.s
+++ b/lld/test/ELF/weak-undef-got-plt.s
@@ -6,11 +6,17 @@
# RUN: ld.lld a.o -o a
# RUN: llvm-readelf -r a | FileCheck %s --check-prefix=NORELOC
+# RUN: ld.lld a.o -o a -z dynamic-undefined-weak
+# RUN: llvm-readelf -r a | FileCheck %s --check-prefix=NORELOC
# RUN: ld.lld a.o s.so -o as
# RUN: llvm-objdump -dR as | FileCheck %s
+# RUN: ld.lld a.o s.so -o as -z nodynamic-undefined-weak
+# RUN: llvm-readelf -r a | FileCheck %s --check-prefix=NORELOC
# RUN: ld.lld -pie a.o s.so -o as.pie
# RUN: llvm-objdump -dR as.pie | FileCheck %s
+# RUN: ld.lld -pie a.o s.so -o as.pie -z nodynamic-undefined-weak
+# RUN: llvm-readelf -r as.pie | FileCheck --check-prefix=NORELOC %s
# RUN: ld.lld -shared a.o -o a.so
# RUN: llvm-objdump -dR a.so | FileCheck %s
diff --git a/lld/test/ELF/weak-undef-rw.s b/lld/test/ELF/weak-undef-rw.s
index 497228a3cf905..8d777669b7e16 100644
--- a/lld/test/ELF/weak-undef-rw.s
+++ b/lld/test/ELF/weak-undef-rw.s
@@ -18,9 +18,22 @@
## gABI leaves the behavior of weak undefined references implementation defined.
## We choose to resolve them statically for static linking and produce dynamic relocations
## for dynamic linking (-shared or at least one input DSO).
-##
-## Note: Some ports of GNU ld support -z nodynamic-undefined-weak that we don't
-## implement.
+
+## -z dynamic-undefined-weak is ignored if .dynsym is absent (-no-pie without DSO)
+# RUN: ld.lld a.o -o a.d -z dynamic-undefined-weak 2>&1 | count 0
+# RUN: llvm-readelf -r --hex-dump=.data a.d | FileCheck %s --check-prefix=STATIC
+
+## Currently no effect for S+A relocations.
+# RUN: ld.lld a.o s.so -o as.d -z dynamic-undefined-weak
+# RUN: llvm-readelf -r --hex-dump=.data as.d | FileCheck %s --check-prefix=STATIC
+
+## -z dynamic-undefined-weak forces dynamic relocations if .dynsym is present.
+# RUN: ld.lld a.o -o a.pie.d -pie -z dynamic-undefined-weak
+# RUN: llvm-readelf -r a.pie.d | FileCheck %s --check-prefix=DYN
+
+## -z nodynamic-undefined-weak suppresses dynamic relocations.
+# RUN: ld.lld a.o -o a.so.n -shared -z dynamic-undefined-weak -z nodynamic-undefined-weak
+# RUN: llvm-readelf -r --hex-dump=.data a.so.n | FileCheck %s --check-prefix=STATIC
# STATIC: no relocations
# STATIC: Hex dump of section '.data':
``````````
</details>
https://github.com/llvm/llvm-project/pull/143831
More information about the llvm-commits
mailing list