[libc-commits] [libc] [libc] Enable ifunc support in static startup (PR #182841)
via libc-commits
libc-commits at lists.llvm.org
Mon Feb 23 04:42:49 PST 2026
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
@llvm/pr-subscribers-backend-risc-v
Author: Jeff Bailey (kaladron)
<details>
<summary>Changes</summary>
Resolves ifunc targets before `main()` runs in static libc
This enables static binaries to use ifunc-based dispatch during early process startup, so optimized implementations can be selected based on CPU features. Without this relocation step in startup, those targets are not ready when program code begins executing.
This change:
- adds IRELATIVE relocation handling for x86_64, AArch64, and RISC-V,
- reads `AT_HWCAP` / `AT_HWCAP2` from auxv and passes them to resolvers where required (notably AArch64),
- runs IRELATIVE processing after base-address discovery and before TLS setup,
- adds integration tests for both the ifunc path and the no-ifunc path.
- Changed the load bias type for ptrdiff_t to intptr_t to align with IRELATIVE handling, which uses intptr_t for load bias calculations.
---
Full diff: https://github.com/llvm/llvm-project/pull/182841.diff
14 Files Affected:
- (modified) libc/hdr/CMakeLists.txt (+9)
- (modified) libc/include/llvm-libc-macros/elf-macros.h (+7-1)
- (modified) libc/startup/linux/CMakeLists.txt (+4)
- (modified) libc/startup/linux/aarch64/CMakeLists.txt (+16)
- (added) libc/startup/linux/aarch64/irelative.cpp (+39)
- (modified) libc/startup/linux/do_start.cpp (+18-3)
- (added) libc/startup/linux/irelative.h (+34)
- (modified) libc/startup/linux/riscv/CMakeLists.txt (+16)
- (added) libc/startup/linux/riscv/irelative.cpp (+46)
- (modified) libc/startup/linux/x86_64/CMakeLists.txt (+16)
- (added) libc/startup/linux/x86_64/irelative.cpp (+36)
- (modified) libc/test/integration/startup/linux/CMakeLists.txt (+14)
- (added) libc/test/integration/startup/linux/irelative_no_ifunc_test.cpp (+16)
- (added) libc/test/integration/startup/linux/irelative_test.cpp (+48)
``````````diff
diff --git a/libc/hdr/CMakeLists.txt b/libc/hdr/CMakeLists.txt
index 6362c10788b2b..cb8ade0604ba7 100644
--- a/libc/hdr/CMakeLists.txt
+++ b/libc/hdr/CMakeLists.txt
@@ -266,6 +266,15 @@ add_proxy_header_library(
libc.include.stdint
)
+add_proxy_header_library(
+ elf_macros
+ HDRS
+ elf_macros.h
+ FULL_BUILD_DEPENDS
+ libc.include.llvm-libc-macros.elf_macros
+ libc.include.elf
+)
+
add_gen_header(
elf_proxy
YAML_FILE
diff --git a/libc/include/llvm-libc-macros/elf-macros.h b/libc/include/llvm-libc-macros/elf-macros.h
index 6857b6681bfa2..625f64cc6335a 100644
--- a/libc/include/llvm-libc-macros/elf-macros.h
+++ b/libc/include/llvm-libc-macros/elf-macros.h
@@ -1,4 +1,4 @@
-//===-- Definition of macros from elf.h -----------------------------------===//
+//===-- Definition of macros from elf.h ------------------------ *- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
@@ -32,4 +32,10 @@
#define ELF64_R_TYPE(i) ((i) & 0xffffffffL)
#define ELF64_R_INFO(s, t) (((s) << 32) + ((t) & 0xffffffffL))
+// Architecture-specific IRELATIVE relocation types
+
+#define R_X86_64_IRELATIVE 37
+#define R_AARCH64_IRELATIVE 1032
+#define R_RISCV_IRELATIVE 58
+
#endif // LLVM_LIBC_MACROS_ELF_MACROS_H
diff --git a/libc/startup/linux/CMakeLists.txt b/libc/startup/linux/CMakeLists.txt
index 1caabcd9179d0..015b08780da23 100644
--- a/libc/startup/linux/CMakeLists.txt
+++ b/libc/startup/linux/CMakeLists.txt
@@ -107,6 +107,7 @@ add_object_library(
do_start.cpp
HDRS
do_start.h
+ irelative.h
DEPENDS
libc.config.app_h
libc.hdr.elf_proxy
@@ -117,6 +118,8 @@ add_object_library(
libc.src.__support.OSUtil.linux.auxv
libc.src.__support.OSUtil.osutil
libc.src.__support.threads.thread
+ libc.src.__support.macros.config
+ libc.startup.linux.${LIBC_TARGET_ARCHITECTURE}.irelative
libc.src.stdlib.exit
libc.src.stdlib.atexit
libc.src.unistd.environ
@@ -131,6 +134,7 @@ merge_relocatable_object(
crt1
.${LIBC_TARGET_ARCHITECTURE}.start
.${LIBC_TARGET_ARCHITECTURE}.tls
+ .${LIBC_TARGET_ARCHITECTURE}.irelative
.do_start
.gnu_property_section
)
diff --git a/libc/startup/linux/aarch64/CMakeLists.txt b/libc/startup/linux/aarch64/CMakeLists.txt
index 5564f0a8f687e..d6e7662d158e2 100644
--- a/libc/startup/linux/aarch64/CMakeLists.txt
+++ b/libc/startup/linux/aarch64/CMakeLists.txt
@@ -23,3 +23,19 @@ add_startup_object(
-fno-omit-frame-pointer
-ffreestanding # To avoid compiler warnings about calling the main function.
)
+
+add_startup_object(
+ irelative
+ SRC
+ irelative.cpp
+ DEPENDS
+ libc.hdr.link_macros
+ libc.hdr.elf_macros
+ libc.hdr.stdint_proxy
+ libc.src.__support.macros.config
+ COMPILE_OPTIONS
+ -fno-stack-protector
+ -fno-omit-frame-pointer
+ -ffreestanding
+ -fno-builtin
+)
diff --git a/libc/startup/linux/aarch64/irelative.cpp b/libc/startup/linux/aarch64/irelative.cpp
new file mode 100644
index 0000000000000..22ba78e636418
--- /dev/null
+++ b/libc/startup/linux/aarch64/irelative.cpp
@@ -0,0 +1,39 @@
+//===-- Implementation of apply_irelative_relocs (AArch64) ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "startup/linux/irelative.h"
+#include "hdr/elf_macros.h"
+#include "hdr/link_macros.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+void apply_irelative_relocs(intptr_t base, unsigned long hwcap,
+ unsigned long hwcap2) {
+ for (const ElfW(Rela) *rela = __rela_iplt_start; rela != __rela_iplt_end;
+ ++rela) {
+ if (ELF64_R_TYPE(rela->r_info) != R_AARCH64_IRELATIVE)
+ continue;
+
+ // AArch64 resolvers receive hwcap and hwcap2.
+ // Use unsigned arithmetic to avoid undefined behavior on signed overflow,
+ // which can occur with very large binaries or high load addresses.
+ uintptr_t resolver_addr =
+ static_cast<uintptr_t>(base) + static_cast<uintptr_t>(rela->r_addend);
+ auto resolver =
+ reinterpret_cast<uintptr_t (*)(unsigned long, unsigned long)>(
+ resolver_addr);
+ uintptr_t result = resolver(hwcap, hwcap2);
+
+ // Write the resolved function pointer to the target location.
+ uintptr_t target_addr = static_cast<uintptr_t>(base) + rela->r_offset;
+ *reinterpret_cast<uintptr_t *>(target_addr) = result;
+ }
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/startup/linux/do_start.cpp b/libc/startup/linux/do_start.cpp
index 65adfb50bccba..b1faaf7d90dd3 100644
--- a/libc/startup/linux/do_start.cpp
+++ b/libc/startup/linux/do_start.cpp
@@ -18,6 +18,7 @@
#include "src/stdlib/exit.h"
#include "src/unistd/environ.h"
#include "startup/linux/gnu_property_section.h"
+#include "startup/linux/irelative.h"
#include <sys/mman.h>
#include <sys/syscall.h>
@@ -86,6 +87,8 @@ static TLSDescriptor tls;
// denoted by an AT_NULL entry.
ElfW(Phdr) *program_hdr_table = nullptr;
uintptr_t program_hdr_count = 0;
+ unsigned long hwcap = 0;
+ unsigned long hwcap2 = 0;
auxv::Vector::initialize_unsafe(
reinterpret_cast<const auxv::Entry *>(env_end_marker + 1));
auxv::Vector auxvec;
@@ -100,12 +103,18 @@ static TLSDescriptor tls;
case AT_PAGESZ:
app.page_size = aux_entry.val;
break;
+ case AT_HWCAP:
+ hwcap = aux_entry.val;
+ break;
+ case AT_HWCAP2:
+ hwcap2 = aux_entry.val;
+ break;
default:
break; // TODO: Read other useful entries from the aux vector.
}
}
- ptrdiff_t base = 0;
+ intptr_t base = 0;
app.tls.size = 0;
ElfW(Phdr) *tls_phdr = nullptr;
[[maybe_unused]] ElfW(Phdr) *gnu_property_phdr = nullptr;
@@ -113,9 +122,9 @@ static TLSDescriptor tls;
for (uintptr_t i = 0; i < program_hdr_count; ++i) {
ElfW(Phdr) &phdr = program_hdr_table[i];
if (phdr.p_type == PT_PHDR)
- base = reinterpret_cast<ptrdiff_t>(program_hdr_table) - phdr.p_vaddr;
+ base = reinterpret_cast<intptr_t>(program_hdr_table) - phdr.p_vaddr;
if (phdr.p_type == PT_DYNAMIC && _DYNAMIC)
- base = reinterpret_cast<ptrdiff_t>(_DYNAMIC) - phdr.p_vaddr;
+ base = reinterpret_cast<intptr_t>(_DYNAMIC) - phdr.p_vaddr;
if (phdr.p_type == PT_TLS)
tls_phdr = &phdr;
if (phdr.p_type == PT_GNU_PROPERTY)
@@ -123,6 +132,12 @@ static TLSDescriptor tls;
// TODO: adjust PT_GNU_STACK
}
+ // Process IRELATIVE relocations (ifunc resolvers).
+ // Skips when no ifuncs are present in the binary.
+ if (reinterpret_cast<uintptr_t>(__rela_iplt_start) !=
+ reinterpret_cast<uintptr_t>(__rela_iplt_end))
+ apply_irelative_relocs(base, hwcap, hwcap2);
+
app.tls.address = tls_phdr->p_vaddr + base;
app.tls.size = tls_phdr->p_memsz;
app.tls.init_size = tls_phdr->p_filesz;
diff --git a/libc/startup/linux/irelative.h b/libc/startup/linux/irelative.h
new file mode 100644
index 0000000000000..0845ce22a7e68
--- /dev/null
+++ b/libc/startup/linux/irelative.h
@@ -0,0 +1,34 @@
+//===-- Implementation header for IRELATIVE relocations -------- *- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_STARTUP_LINUX_IRELATIVE_H
+#define LLVM_LIBC_STARTUP_LINUX_IRELATIVE_H
+
+#include "hdr/link_macros.h"
+#include "hdr/stdint_proxy.h"
+#include "src/__support/macros/config.h"
+
+extern "C" {
+[[gnu::weak, gnu::visibility("hidden")]] extern const ElfW(Rela)
+ __rela_iplt_start[]; // NOLINT
+[[gnu::weak, gnu::visibility("hidden")]] extern const ElfW(Rela)
+ __rela_iplt_end[]; // NOLINT
+}
+
+namespace LIBC_NAMESPACE_DECL {
+
+// Process IRELATIVE relocations (ifunc resolvers).
+// base is the load bias (actual load address − link-time address). It is
+// intptr_t (signed) because it is a difference; it is negative if the binary
+// loaded below its link address. (unlikely but possible in principle)
+void apply_irelative_relocs(intptr_t base, unsigned long hwcap,
+ unsigned long hwcap2);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_STARTUP_LINUX_IRELATIVE_H
diff --git a/libc/startup/linux/riscv/CMakeLists.txt b/libc/startup/linux/riscv/CMakeLists.txt
index 2a61f8289067d..61c912909dbb4 100644
--- a/libc/startup/linux/riscv/CMakeLists.txt
+++ b/libc/startup/linux/riscv/CMakeLists.txt
@@ -24,3 +24,19 @@ add_startup_object(
-fno-omit-frame-pointer
-ffreestanding # To avoid compiler warnings about calling the main function.
)
+
+add_startup_object(
+ irelative
+ SRC
+ irelative.cpp
+ DEPENDS
+ libc.hdr.link_macros
+ libc.hdr.elf_macros
+ libc.hdr.stdint_proxy
+ libc.src.__support.macros.config
+ COMPILE_OPTIONS
+ -fno-stack-protector
+ -fno-omit-frame-pointer
+ -ffreestanding
+ -fno-builtin
+)
diff --git a/libc/startup/linux/riscv/irelative.cpp b/libc/startup/linux/riscv/irelative.cpp
new file mode 100644
index 0000000000000..b8c36bbc7d353
--- /dev/null
+++ b/libc/startup/linux/riscv/irelative.cpp
@@ -0,0 +1,46 @@
+//===-- Implementation of apply_irelative_relocs (RISC-V) -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "startup/linux/irelative.h"
+#include "hdr/elf_macros.h"
+#include "hdr/link_macros.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+// RISC-V may be 32-bit or 64-bit. ElfW(Rela) handles the struct type,
+// but we need the correct R_TYPE extraction macro.
+static LIBC_INLINE constexpr unsigned get_r_type(uintptr_t info) {
+#ifdef __LP64__
+ return ELF64_R_TYPE(info);
+#else
+ return ELF32_R_TYPE(info);
+#endif
+}
+
+void apply_irelative_relocs(intptr_t base, unsigned long /*hwcap*/,
+ unsigned long /*hwcap2*/) {
+ for (const ElfW(Rela) *rela = __rela_iplt_start; rela != __rela_iplt_end;
+ ++rela) {
+ if (get_r_type(rela->r_info) != R_RISCV_IRELATIVE)
+ continue;
+
+ // RISC-V resolvers take no arguments (same as x86_64).
+ // Use unsigned arithmetic to avoid undefined behavior on signed overflow,
+ // which can occur with very large binaries or high load addresses.
+ uintptr_t resolver_addr =
+ static_cast<uintptr_t>(base) + static_cast<uintptr_t>(rela->r_addend);
+ auto resolver = reinterpret_cast<uintptr_t (*)(void)>(resolver_addr);
+ uintptr_t result = resolver();
+
+ uintptr_t target_addr = static_cast<uintptr_t>(base) + rela->r_offset;
+ *reinterpret_cast<uintptr_t *>(target_addr) = result;
+ }
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/startup/linux/x86_64/CMakeLists.txt b/libc/startup/linux/x86_64/CMakeLists.txt
index 4f482eaf5d18e..6eebc734a06a7 100644
--- a/libc/startup/linux/x86_64/CMakeLists.txt
+++ b/libc/startup/linux/x86_64/CMakeLists.txt
@@ -28,3 +28,19 @@ add_startup_object(
-ffreestanding
-fno-builtin
)
+
+add_startup_object(
+ irelative
+ SRC
+ irelative.cpp
+ DEPENDS
+ libc.hdr.link_macros
+ libc.hdr.elf_macros
+ libc.hdr.stdint_proxy
+ libc.src.__support.macros.config
+ COMPILE_OPTIONS
+ -fno-stack-protector
+ -fno-omit-frame-pointer
+ -ffreestanding
+ -fno-builtin
+)
diff --git a/libc/startup/linux/x86_64/irelative.cpp b/libc/startup/linux/x86_64/irelative.cpp
new file mode 100644
index 0000000000000..60d5c81ac36a7
--- /dev/null
+++ b/libc/startup/linux/x86_64/irelative.cpp
@@ -0,0 +1,36 @@
+//===-- Implementation of apply_irelative_relocs (x86_64) -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "startup/linux/irelative.h"
+#include "hdr/elf_macros.h"
+#include "hdr/link_macros.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+void apply_irelative_relocs(intptr_t base, unsigned long /*hwcap*/,
+ unsigned long /*hwcap2*/) {
+ for (const ElfW(Rela) *rela = __rela_iplt_start; rela != __rela_iplt_end;
+ ++rela) {
+ if (ELF64_R_TYPE(rela->r_info) != R_X86_64_IRELATIVE)
+ continue;
+
+ // x86_64 resolvers take no arguments.
+ // Use unsigned arithmetic to avoid undefined behavior on signed overflow,
+ // which can occur with very large binaries or high load addresses.
+ uintptr_t resolver_addr =
+ static_cast<uintptr_t>(base) + static_cast<uintptr_t>(rela->r_addend);
+ auto resolver = reinterpret_cast<uintptr_t (*)(void)>(resolver_addr);
+ uintptr_t result = resolver();
+
+ uintptr_t target_addr = static_cast<uintptr_t>(base) + rela->r_offset;
+ *reinterpret_cast<uintptr_t *>(target_addr) = result;
+ }
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/integration/startup/linux/CMakeLists.txt b/libc/test/integration/startup/linux/CMakeLists.txt
index 511b16d613026..83baa8e1b2844 100644
--- a/libc/test/integration/startup/linux/CMakeLists.txt
+++ b/libc/test/integration/startup/linux/CMakeLists.txt
@@ -49,3 +49,17 @@ add_integration_test(
SRCS
init_fini_array_test.cpp
)
+
+add_integration_test(
+ startup_irelative_test
+ SUITE libc-startup-tests
+ SRCS
+ irelative_test.cpp
+)
+
+add_integration_test(
+ startup_irelative_no_ifunc_test
+ SUITE libc-startup-tests
+ SRCS
+ irelative_no_ifunc_test.cpp
+)
diff --git a/libc/test/integration/startup/linux/irelative_no_ifunc_test.cpp b/libc/test/integration/startup/linux/irelative_no_ifunc_test.cpp
new file mode 100644
index 0000000000000..631c5bb92c77e
--- /dev/null
+++ b/libc/test/integration/startup/linux/irelative_no_ifunc_test.cpp
@@ -0,0 +1,16 @@
+//===-- Implementation of apply_irelative_relocs (no ifunc) test ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "startup/linux/irelative.h"
+#include "test/IntegrationTest/test.h"
+
+TEST_MAIN() {
+ ASSERT_EQ(reinterpret_cast<uintptr_t>(__rela_iplt_start),
+ reinterpret_cast<uintptr_t>(__rela_iplt_end));
+ return 0;
+}
diff --git a/libc/test/integration/startup/linux/irelative_test.cpp b/libc/test/integration/startup/linux/irelative_test.cpp
new file mode 100644
index 0000000000000..0fda65c95660b
--- /dev/null
+++ b/libc/test/integration/startup/linux/irelative_test.cpp
@@ -0,0 +1,48 @@
+//===-- Implementation of apply_irelative_relocs test ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "test/IntegrationTest/test.h"
+
+#ifndef __has_attribute
+#define __has_attribute(x) 0
+#endif
+
+#if __has_attribute(ifunc)
+
+// Two trivial implementations that return different constants.
+static int impl_a() { return 42; }
+static int impl_b() { return 84; }
+
+// The resolver function. On x86_64 this takes no arguments; on AArch64 it
+// receives (hwcap, hwcap2). We use extern "C" to ensure C linkage for the
+// ifunc attribute.
+extern "C" {
+// Declare the resolver to return a generic function pointer.
+// For testing, unconditionally select impl_a.
+void *my_ifunc_resolver() {
+ (void)impl_b; // Suppress unused warning.
+ return reinterpret_cast<void *>(impl_a);
+}
+}
+
+// Declare the ifunc. The compiler and linker will generate an IRELATIVE
+// relocation that calls my_ifunc_resolver at startup.
+extern "C" int my_ifunc() __attribute__((ifunc("my_ifunc_resolver")));
+
+TEST_MAIN() {
+ // If IRELATIVE processing works correctly, my_ifunc() should call impl_a
+ // and return 42.
+ ASSERT_EQ(my_ifunc(), 42);
+ return 0;
+}
+
+#else
+
+TEST_MAIN() { return 0; }
+
+#endif
``````````
</details>
https://github.com/llvm/llvm-project/pull/182841
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