elf: support ELF binaries in noMMU

The Linux kernels ELF-FDPIC binfmt program loader can support loading and
running conventional ELF format binaries on noMMU kernels when compiled
appropriately. That is when they are constant displacement binaries such
as generated using the -pie compile option.

Add a configure option to allow selecting ELF binary support in noMMU
mode configurations on architectures that support this. The main
requirement is to generate the ldso run-time loader to perform relocation
at load time. These configurations do not support shared libraries, so
there is no need to generate a full shared library, only the static
version is required.

The use of ELF format binaries does mean a slightly simpler toolchain
generation (does not require a -uclinux- for some architectures) and does
not require an extra tool like elf2flt.

This initial support targets M68K, ARM and RISC-V architectures. No kernel
changes are required, the required support for this is already in mainline
kernels (certainly as of linux-6.6).

Note that for the M68K and ARM architectures that the initialized
registers and stack layout at process startup is slightly different for
the flat format loader and the ELF/ELF-FDPIC loaders. So we need some
changes to the startup code (crt1.S) for them.

I have not done extensive testing outside of M68K, ARM and RISC-V.
I had to make changes to a couple of the dl-startup.h architecture files
to get them to build for this noMMU case. I did not dig down too deep on
the reasons, but they still seem ok for the MMU case as well.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

Makerules

@@ -22,6 +22,9 @@ ifeq ($(UCLIBC_FORMAT_SHARED_FLAT),y)
 libs: $(lib-gdb-y)
 endif
 libs: $(lib-a-y)
+ifeq ($(HAVE_LDSO),y)
+$(lib-a-y): | $(ldso)
+endif
 endif
 objs: all_objs
 $(lib-so-y) $(lib-a-y): | $(top_builddir)lib

Rules.mak

@@ -167,6 +167,8 @@ endif
 
 ifneq ($(HAVE_SHARED),y)
 libc :=
+endif
+ifneq ($(HAVE_LDSO),y)
 interp :=
 ldso :=
 endif

extra/Configs/Config.in

@@ -333,8 +333,9 @@ config STATIC_PIE
 	bool "Add support for Static Position Independent Executables (PIE)"
 	default n
 	depends on DOPIC && !UCLIBC_FORMAT_FDPIC_ELF && \
-		(TARGET_arm || TARGET_i386 || TARGET_x86_64 || TARGET_aarch64 || \
-		 TARGET_mips || TARGET_xtensa || TARGET_powerpc)
+		(TARGET_aarch64 || TARGET_arm || TARGET_i386 || \
+		 TARGET_m68k || TARGET_mips || TARGET_powerpc || \
+		 TARGET_riscv64 || TARGET_x86_64 || TARGET_xtensa)
 
 config ARCH_HAS_NO_SHARED
 	bool
@@ -346,9 +347,13 @@ config ARCH_HAS_NO_LDSO
 config ARCH_HAS_UCONTEXT
 	bool
 
+config HAVE_LDSO
+	bool
+
 config HAVE_SHARED
 	bool "Enable shared libraries"
 	depends on !ARCH_HAS_NO_SHARED
+	select HAVE_LDSO
 	default y
 	help
 	  If you wish to build uClibc with support for shared libraries then

extra/Configs/Config.in.arch

@@ -10,6 +10,14 @@
 if !ARCH_USE_MMU
 choice
 	prompt "Target File Format"
+config UCLIBC_FORMAT_ELF
+	bool "ELF (using ELF_FDPIC loader)"
+	depends on !ARCH_USE_MMU && (TARGET_arm || TARGET_m68k || \
+				     TARGET_riscv64)
+	select DOPIC
+	select STATIC_PIE
+	select ARCH_HAS_NO_SHARED
+	select HAVE_LDSO
 config UCLIBC_FORMAT_FDPIC_ELF
 	bool "FDPIC ELF"
 	depends on !ARCH_USE_MMU && (TARGET_bfin || TARGET_frv || TARGET_arm)

ldso/include/dl-defs.h

@@ -72,7 +72,7 @@ typedef struct {
 #endif
 
 #ifdef _LIBC
-#ifndef __ARCH_HAS_NO_SHARED__
+#ifndef __ARCH_HAS_NO_LDSO__
 /* arch specific defines */
 #include <dl-sysdep.h>
 #endif

ldso/include/ldso.h

@@ -48,7 +48,7 @@
 /* Pull in the MIN macro */
 #include <sys/param.h>
 /* Pull in the ldso syscalls and string functions */
-#ifndef __ARCH_HAS_NO_SHARED__
+#if !defined(__ARCH_HAS_NO_SHARED__) || !defined(__ARCH_HAS_NO_LDSO__)
 #include <dl-syscall.h>
 #include <dl-string.h>
 /* Now the ldso specific headers */

ldso/ldso/arm/dl-startup.h

@@ -301,3 +301,5 @@ int raise(int sig)
   _dl_exit(1);
 }
 #endif /* __FDPIC__ */
+
+#define DL_UPDATE_LOADADDR_HDR(LOADADDR, ADDR, PHDR)

ldso/ldso/m68k/dl-startup.h

@@ -55,6 +55,9 @@ _dl_start_user:\n\
  * do something a little more subtle here.  */
 #define GET_ARGV(ARGVP, ARGS) ARGVP = (((unsigned long *) ARGS) + 1)
 
+/* We can't call functions earlier in the dl startup process */
+#define NO_FUNCS_BEFORE_BOOTSTRAP
+
 /* Handle relocation of the symbols in the dynamic loader. */
 static __always_inline
 void PERFORM_BOOTSTRAP_RELOC(ELF_RELOC *rpnt, unsigned long *reloc_addr,

ldso/ldso/m68k/dl-sysdep.h

@@ -83,3 +83,5 @@ elf_machine_relative (Elf32_Addr load_off, const Elf32_Addr rel_addr,
 		*reloc_addr = load_off + rpnt->r_addend;
 	} while (--relative_count);
 }
+
+#define DL_UPDATE_LOADADDR_HDR(LOADADDR, ADDR, PHDR)

ldso/ldso/riscv64/dl-startup.h

@@ -88,3 +88,5 @@ void PERFORM_BOOTSTRAP_RELOC(ELF_RELOC *rpnt, ElfW(Addr) *reloc_addr,
 			_dl_exit(1);
 	}
 }
+
+#define DL_UPDATE_LOADADDR_HDR(LOADADDR, ADDR, PHDR)

libc/sysdeps/linux/arm/crt1.S

@@ -245,7 +245,7 @@ _start:
 	mov fp, #0
 	mov lr, #0
 
-#ifdef __ARCH_USE_MMU__
+#if defined(__ARCH_USE_MMU__) || defined(__UCLIBC_FORMAT_ELF__)
 #ifdef L_rcrt1
 	/* We don't need to save a1 since no dynamic linker should have run */
 	ldr a1, .L_GOT          /* Get value at .L_GOT + 0  (offset to GOT)*/

libc/sysdeps/linux/m68k/crt1.S

@@ -78,9 +78,13 @@ _start:
 	sub.l %fp, %fp
 
 #if !defined __ARCH_USE_MMU__ && defined __PIC__
+#ifdef UCLIBC_FORMAT_ELF
+	move.l #_GLOBAL_OFFSET_TABLE_, %a5
+#else
 	/* Set up the global pointer.  The GOT is at the beginning of the
 	   data segment, whose address is in %d5.  */
 	move.l %d5,%a5
+#endif
 	.equ have_current_got, 1
 #endif
 	
@@ -92,11 +96,11 @@ _start:
 	   arguments for `main': argc, argv.  envp will be determined
 	   later in __libc_start_main.  */
 	move.l (%sp)+, %d0	/* Pop the argument count.  */
-#ifndef __ARCH_USE_MMU__
-	move.l (%sp)+, %a0
-#else
+#if defined(__ARCH_USE_MMU__) || defined(__UCLIBC_FORMAT_ELF__)
 	move.l %sp, %a0		/* The argument vector starts just at the
 				   current stack top.  */
+#else
+	move.l (%sp)+, %a0
 #endif
 
 	/* Provide the highest stack address to the user code (for stacks