riscv: support the elf-fdpic binfmt loader

Add support for enabling and using the binfmt_elf_fdpic program loader
on RISC-V platforms. The most important change is to setup registers
during program load to pass the mapping addresses to the new process.

One of the interesting features of the elf-fdpic loader is that it
also allows appropriately compiled ELF format binaries to be loaded on
nommu systems. Appropriate being those compiled with -pie.

Signed-off-by: Greg Ungerer <gerg@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230711130754.481209-3-gerg@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/elf.h

@@ -41,6 +41,7 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
 #define compat_elf_check_arch	compat_elf_check_arch
 
 #define CORE_DUMP_USE_REGSET
+#define ELF_FDPIC_CORE_EFLAGS	0
 #define ELF_EXEC_PAGESIZE	(PAGE_SIZE)
 
 /*
@@ -69,6 +70,13 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
 #define ELF_HWCAP	riscv_get_elf_hwcap()
 extern unsigned long elf_hwcap;
 
+#define ELF_FDPIC_PLAT_INIT(_r, _exec_map_addr, _interp_map_addr, dynamic_addr) \
+	do { \
+		(_r)->a1 = _exec_map_addr; \
+		(_r)->a2 = _interp_map_addr; \
+		(_r)->a3 = dynamic_addr; \
+	} while (0)
+
 /*
  * This yields a string that ld.so will use to load implementation
  * specific libraries for optimization.  This is more specific in
@@ -78,7 +86,6 @@ extern unsigned long elf_hwcap;
 
 #define COMPAT_ELF_PLATFORM	(NULL)
 
-#ifdef CONFIG_MMU
 #define ARCH_DLINFO						\
 do {								\
 	/*							\
@@ -115,6 +122,8 @@ do {								\
 	else							 \
 		NEW_AUX_ENT(AT_IGNORE, 0);			 \
 } while (0)
+
+#ifdef CONFIG_MMU
 #define ARCH_HAS_SETUP_ADDITIONAL_PAGES
 struct linux_binprm;
 extern int arch_setup_additional_pages(struct linux_binprm *bprm,

arch/riscv/include/asm/mmu.h

@@ -20,6 +20,10 @@ typedef struct {
 	/* A local icache flush is needed before user execution can resume. */
 	cpumask_t icache_stale_mask;
 #endif
+#ifdef CONFIG_BINFMT_ELF_FDPIC
+	unsigned long exec_fdpic_loadmap;
+	unsigned long interp_fdpic_loadmap;
+#endif
 } mm_context_t;
 
 void __init create_pgd_mapping(pgd_t *pgdp, uintptr_t va, phys_addr_t pa,

arch/riscv/include/uapi/asm/ptrace.h

@@ -10,6 +10,11 @@
 
 #include <linux/types.h>
 
+#define PTRACE_GETFDPIC		33
+
+#define PTRACE_GETFDPIC_EXEC	0
+#define PTRACE_GETFDPIC_INTERP	1
+
 /*
  * User-mode register state for core dumps, ptrace, sigcontext
  *

fs/Kconfig.binfmt

@@ -58,7 +58,7 @@ config ARCH_USE_GNU_PROPERTY
 config BINFMT_ELF_FDPIC
 	bool "Kernel support for FDPIC ELF binaries"
 	default y if !BINFMT_ELF
-	depends on ARM || ((M68K || SUPERH || XTENSA) && !MMU)
+	depends on ARM || ((M68K || RISCV || SUPERH || XTENSA) && !MMU)
 	select ELFCORE
 	help
 	  ELF FDPIC binaries are based on ELF, but allow the individual load