xtensa: add FDPIC and static PIE support for noMMU

Define ELFOSABI_XTENSA_FDPIC and use it as an OSABI tag in the ELF
header to distinguish FDPIC ELF files from regular ELF files.
Define ELF_FDPIC_PLAT_INIT and put executable map, interpreter map and
executable dynamic section addresses into registers a4..a6.
Update start_thread macro to preserve register values in the current
register window.
Add definitions for PTRACE_GETFDPIC, PTRACE_GETFDPIC_EXEC and
PTRACE_GETFDPIC_INTERP.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/elf.h

@@ -93,6 +93,10 @@ typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];
 #define elf_check_arch(x) ( ( (x)->e_machine == EM_XTENSA )  || \
 			    ( (x)->e_machine == EM_XTENSA_OLD ) )
 
+#define ELFOSABI_XTENSA_FDPIC 65
+#define elf_check_fdpic(x) ((x)->e_ident[EI_OSABI] == ELFOSABI_XTENSA_FDPIC)
+#define ELF_FDPIC_CORE_EFLAGS 0
+
 /*
  * These are used to set parameters in the core dumps.
  */
@@ -164,6 +168,13 @@ typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];
 		(_r)->areg[14] = 0; (_r)->areg[15] = 0; \
 	} while (0)
 
+#define ELF_FDPIC_PLAT_INIT(_r, _exec_map_addr, _interp_map_addr, dynamic_addr) \
+	do { \
+		(_r)->areg[4] = _exec_map_addr; \
+		(_r)->areg[5] = _interp_map_addr; \
+		(_r)->areg[6] = dynamic_addr; \
+	} while (0)
+
 typedef struct {
 	xtregs_opt_t	opt;
 	xtregs_user_t	user;

arch/xtensa/include/asm/processor.h

@@ -205,9 +205,12 @@ struct thread_struct {
 #define start_thread(regs, new_pc, new_sp) \
 	do { \
 		unsigned long syscall = (regs)->syscall; \
+		unsigned long current_aregs[16]; \
+		memcpy(current_aregs, (regs)->areg, sizeof(current_aregs)); \
 		memset((regs), 0, sizeof(*(regs))); \
 		(regs)->pc = (new_pc); \
 		(regs)->ps = USER_PS_VALUE; \
+		memcpy((regs)->areg, current_aregs, sizeof(current_aregs)); \
 		(regs)->areg[1] = (new_sp); \
 		(regs)->areg[0] = 0; \
 		(regs)->wmask = 1; \

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

@@ -37,6 +37,10 @@
 #define PTRACE_SETXTREGS	19
 #define PTRACE_GETHBPREGS	20
 #define PTRACE_SETHBPREGS	21
+#define PTRACE_GETFDPIC		22
+
+#define PTRACE_GETFDPIC_EXEC	0
+#define PTRACE_GETFDPIC_INTERP	1
 
 #ifndef __ASSEMBLY__
 

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) && !MMU)
+	depends on ARM || ((M68K || SUPERH || XTENSA) && !MMU)
 	select ELFCORE
 	help
 	  ELF FDPIC binaries are based on ELF, but allow the individual load