Message-ID: <20260204124542.523567-4-daniel@thingy.jp>
Date: Wed,  4 Feb 2026 21:45:35 +0900
From: Daniel Palmer <daniel@...ngy.jp>
To: w@....eu,
	linux@...ssschuh.net
Cc: linux-kernel@...r.kernel.org,
	Daniel Palmer <daniel@...ngy.jp>
Subject: [RFC PATCH v2 03/10] tools/nolibc: Add basic ELF self-relocation support for static PIE

Currently nolibc programs cannot be compiled with -static-pie.
Which is basically no shared libraries, no interpreter, but contain
relocation information in the ELF to allow the program to be fixed
up to run at the address that the kernel loaded it to.

There might be other use cases for static PIE but mine is for nommu.
The ELF FDPIC loader can actually load normal ELFs is long as they
can be relocated.

This very basic implementation does the following:

- Works out if we are PIE and need to be relocated. ELF type == ET_DYN
- Works out if we are static PIE, have no interpreter, and need to
  relocate ourselves.
- Calculates the base address using the ELF program header address
  given to us by the kernel and the size of the ELF header that
  should be directly above.
- Finds the ELF relocation data.
- Calls an arch specific function to handle each of the relocations.

Note that from testing a lot of archs don't produce static PIE
binaries with the -static-pie option and you need to compile with
-pie -Wl,--no-dynamic-linker to get a static PIE binary.

Currently REL and RELA formats are supported.

Signed-off-by: Daniel Palmer <daniel@...ngy.jp>
---
 tools/include/nolibc/Makefile |   1 +
 tools/include/nolibc/crt.h    |   7 ++
 tools/include/nolibc/reloc.h  | 224 ++++++++++++++++++++++++++++++++++
 3 files changed, 232 insertions(+)
 create mode 100644 tools/include/nolibc/reloc.h

diff --git a/tools/include/nolibc/Makefile b/tools/include/nolibc/Makefile
index 8118e22844f1..2b968a097854 100644
--- a/tools/include/nolibc/Makefile
+++ b/tools/include/nolibc/Makefile
@@ -38,6 +38,7 @@ all_files := \
 		math.h \
 		nolibc.h \
 		poll.h \
+		reloc.h \
 		sched.h \
 		signal.h \
 		stackprotector.h \
diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
index fab042f1ff62..4b5be276b83b 100644
--- a/tools/include/nolibc/crt.h
+++ b/tools/include/nolibc/crt.h
@@ -10,6 +10,7 @@
 #ifndef NOLIBC_NO_RUNTIME
 
 #include "compiler.h"
+#include "reloc.h"
 
 char **environ __attribute__((weak));
 const unsigned long *_auxv __attribute__((weak));
@@ -94,6 +95,12 @@ void __no_sanitize __no_stack_protector _start_c(long *sp)
 	for (auxv = (void *)envp; *auxv++;)
 		;
 
+	/*
+	 * Do relocation if required and supported, this must happen before any
+	 * global variables are updated or used.
+	 */
+	_relocate(auxv);
+
 	_start_c_global_data(argc, argv, envp, auxv);
 }
 
diff --git a/tools/include/nolibc/reloc.h b/tools/include/nolibc/reloc.h
new file mode 100644
index 000000000000..aead8cc680f5
--- /dev/null
+++ b/tools/include/nolibc/reloc.h
@@ -0,0 +1,224 @@
+/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
+/*
+ * Self relocation support for NOLIBC
+ * Copyright (C) 2026 Daniel Palmer<daniel@...ngy.jp>
+ *
+ * This allows a PIE compiled nolibc binary relocate itself
+ * instead of relying on a dynamic linker. So called "static PIE".
+ *
+ * With some care binaries produced with this relocation code
+ * can run via the FDPIC ELF loader on nommu systems.
+ */
+
+#ifndef _NOLIBC_RELOC_H
+#define _NOLIBC_RELOC_H
+
+/*
+ * If NOLIBC_WANT_RELOC is not defined decide based on the
+ * compiler defining __pie__ or not.
+ */
+#ifndef NOLIBC_WANT_RELOC
+#ifdef __pie__
+#define NOLIBC_WANT_RELOC
+#endif
+#endif
+
+#ifdef NOLIBC_WANT_RELOC
+#ifndef _NOLIBC_ARCH_HAS_RELOC
+#error "Relocation requested but arch does not support it"
+#endif
+
+#include <linux/auxvec.h>
+#include "elf.h"
+
+#ifdef _NOLIBC_ARCH_ELF32
+#define _nolibc_elf_ehdr		Elf32_Ehdr
+#define _nolibc_elf_phdr		Elf32_Phdr
+/* 32bit ARM, x86 uses REL instead of RELA */
+#ifdef _NOLIBC_ARCH_ELF_REL
+#define _nolibc_elf_rel			Elf32_Rel
+#else
+#define _nolibc_elf_rela		Elf32_Rela
+#endif
+#define _nolibc_elf_dyn			Elf32_Dyn
+#define _nolibc_elf_addr		Elf32_Addr
+#define _nolibc_elf_r_type(_x)		ELF32_R_TYPE(_x)
+#else
+#define _nolibc_elf_ehdr		Elf64_Ehdr
+#define _nolibc_elf_phdr		Elf64_Phdr
+#define _nolibc_elf_dyn			Elf64_Dyn
+#define _nolibc_elf_rela		Elf64_Rela
+#define _nolibc_elf_addr		Elf64_Addr
+#define _nolibc_elf_r_type(_x)		ELF64_R_TYPE(_x)
+#endif
+
+#ifdef _NOLIBC_ARCH_ELF_REL
+#define _nolibc_elf_dt_rel_rela		DT_REL
+#define _nolibc_elf_dt_rel_rela_count	DT_RELCOUNT
+#else
+#define _nolibc_elf_dt_rel_rela		DT_RELA
+#define _nolibc_elf_dt_rel_rela_count	DT_RELACOUNT
+#endif
+
+#ifdef _NOLIBC_ARCH_ELF_REL
+/*
+ * Your arch needs to provide this to actually handle doing each of the
+ * relocations if it uses the REL format.
+ */
+static int __relocate_rel(unsigned long base, _nolibc_elf_rel *entry);
+
+/* Generic implementation of R_x_RELATIVE for REL */
+static __inline__ void __relocate_rel_relative(unsigned long base, _nolibc_elf_rel *entry)
+{
+	_nolibc_elf_addr *addr;
+	int addend;
+
+	addr = (_nolibc_elf_addr *)(base + entry->r_offset);
+	addend = *addr;
+	*addr = base + addend;
+}
+
+static __inline__ int __relocate(unsigned long base,
+				 unsigned long rel_off,
+				 unsigned long rel_count)
+{
+	_nolibc_elf_rel *rel = (_nolibc_elf_rel *)(base + rel_off);
+	unsigned long i;
+
+	for (i = 0; i < rel_count; i++) {
+		if (__relocate_rel(base, &rel[i]))
+			return -1;
+	}
+
+	return 0;
+}
+#else
+/*
+ * Your arch needs to provide this to actually handle doing each of the
+ * relocations if it uses the RELA format.
+ */
+static int __relocate_rela(unsigned long base, _nolibc_elf_rela *entry);
+
+/* Generic implementation of R_x_RELATIVE for RELA */
+static __inline__ void __relocate_rela_relative(unsigned long base, _nolibc_elf_rela *entry)
+{
+	_nolibc_elf_addr *addr;
+
+	addr = (_nolibc_elf_addr *) (base + entry->r_offset);
+	*addr = (_nolibc_elf_addr) (base + entry->r_addend);
+}
+
+static __inline__ int __relocate(unsigned long base,
+				 unsigned long rela_off,
+				 unsigned long rela_count)
+{
+	_nolibc_elf_rela *rela = (_nolibc_elf_rela *)(base + rela_off);
+	unsigned long i;
+
+	for (i = 0; i < rela_count; i++) {
+		if (__relocate_rela(base, &rela[i]))
+			return -1;
+	}
+
+	return 0;
+}
+#endif
+
+static __inline__ void _relocate(const unsigned long *auxv)
+{
+	_nolibc_elf_phdr *phdr_dyn = NULL;
+	unsigned long rel_rela_count = 0;
+	unsigned long rel_rela_off = 0;
+	unsigned long phdr_addr = 0;
+	unsigned long phdr_num = 0;
+	_nolibc_elf_ehdr *ehdr;
+	_nolibc_elf_phdr *phdr;
+	_nolibc_elf_dyn *dyn;
+	unsigned long base;
+	unsigned long i;
+	int remaining;
+
+	for (remaining = 2; remaining; ) {
+		if (!auxv[0] && !auxv[1])
+			break;
+
+		switch (auxv[0]) {
+		case AT_NOTELF:
+			return;
+
+		case AT_PHDR:
+			phdr_addr = auxv[1];
+			remaining--;
+			break;
+
+		case AT_PHNUM:
+			phdr_num = auxv[1];
+			remaining--;
+			break;
+		}
+
+		auxv += 2;
+	}
+
+	if (remaining)
+		goto failed;
+
+	/*
+	 * The ELF header and the start of our image in memory
+	 * should be the size of the ELF header above the program
+	 * headers.
+	 */
+	base = phdr_addr - sizeof(_nolibc_elf_ehdr);
+
+	/* Check that we are PIE */
+	ehdr = (_nolibc_elf_ehdr *) base;
+	if (ehdr->e_type != ET_DYN)
+		return;
+
+	phdr = (_nolibc_elf_phdr *) phdr_addr;
+	for (i = 0; i < phdr_num; i++, phdr++) {
+		switch (phdr->p_type) {
+		case PT_INTERP:
+			/* Interp was set, we were relocated already?, return */
+			return;
+		case PT_DYNAMIC:
+			phdr_dyn = phdr;
+			break;
+		}
+	}
+
+	if (!phdr_dyn)
+		goto failed;
+
+	dyn = (_nolibc_elf_dyn *)(base + phdr_dyn->p_vaddr);
+	for (remaining = 2; dyn->d_tag != DT_NULL; dyn++) {
+		switch (dyn->d_tag) {
+		case _nolibc_elf_dt_rel_rela:
+			rel_rela_off = dyn->d_un.d_ptr;
+			remaining--;
+			break;
+		case _nolibc_elf_dt_rel_rela_count:
+			rel_rela_count = dyn->d_un.d_val;
+			remaining--;
+			break;
+		}
+	}
+
+	if (remaining)
+		goto failed;
+
+	if (__relocate(base, rel_rela_off, rel_rela_count))
+		goto failed;
+
+	return;
+
+failed:
+	__builtin_trap();
+}
+#else
+static void _relocate(const unsigned long *auxv __attribute__((unused)))
+{
+}
+#endif /* NOLIBC_WANT_RELOC */
+
+#endif /* _NOLIBC_RELOC_H */
-- 
2.51.0

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