powerpc: Add a proper syscall for switching endianness

We currently have a "special" syscall for switching endianness. This is
syscall number 0x1ebe, which is handled explicitly in the 64-bit syscall
exception entry.

That has a few problems, firstly the syscall number is outside of the
usual range, which confuses various tools. For example strace doesn't
recognise the syscall at all.

Secondly it's handled explicitly as a special case in the syscall
exception entry, which is complicated enough without it.

As a first step toward removing the special syscall, we need to add a
regular syscall that implements the same functionality.

The logic is simple, it simply toggles the MSR_LE bit in the userspace
MSR. This is the same as the special syscall, with the caveat that the
special syscall clobbers fewer registers.

This version clobbers r9-r12, XER, CTR, and CR0-1,5-7.

Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/include/asm/systbl.h

···
 SYSCALL_SPU(memfd_create)
 SYSCALL_SPU(bpf)
 COMPAT_SYS(execveat)
+PPC64ONLY(switch_endian)

arch/powerpc/include/asm/unistd.h

···
 #include <uapi/asm/unistd.h>
 
 
-#define __NR_syscalls		363
+#define __NR_syscalls		364
 
 #define __NR__exit __NR_exit
 #define NR_syscalls	__NR_syscalls

arch/powerpc/include/uapi/asm/unistd.h

···
 #define __NR_memfd_create	360
 #define __NR_bpf		361
 #define __NR_execveat		362
+#define __NR_switch_endian	363
 
 #endif /* _UAPI_ASM_POWERPC_UNISTD_H_ */

arch/powerpc/kernel/entry_64.S

···
 	bl	sys_swapcontext
 	b	.Lsyscall_exit
 
+_GLOBAL(ppc_switch_endian)
+	bl	save_nvgprs
+	bl	sys_switch_endian
+	b	.Lsyscall_exit
+
 _GLOBAL(ret_from_fork)
 	bl	schedule_tail
 	REST_NVGPRS(r1)

arch/powerpc/kernel/syscalls.c

···
 	return sys_fadvise64(fd, (u64)offset_high << 32 | offset_low,
 			     (u64)len_high << 32 | len_low, advice);
 }
+
+long sys_switch_endian(void)
+{
+	struct thread_info *ti;
+
+	current->thread.regs->msr ^= MSR_LE;
+
+	/*
+	 * Set TIF_RESTOREALL so that r3 isn't clobbered on return to
+	 * userspace. That also has the effect of restoring the non-volatile
+	 * GPRs, so we saved them on the way in here.
+	 */
+	ti = current_thread_info();
+	ti->flags |= _TIF_RESTOREALL;
+
+	return 0;
+}

arch/powerpc/kernel/systbl.S

···
 #define PPC_SYS(func)		.llong	DOTSYM(ppc_##func),DOTSYM(ppc_##func)
 #define OLDSYS(func)		.llong	DOTSYM(sys_ni_syscall),DOTSYM(sys_ni_syscall)
 #define SYS32ONLY(func)		.llong	DOTSYM(sys_ni_syscall),DOTSYM(compat_sys_##func)
+#define PPC64ONLY(func)		.llong	DOTSYM(ppc_##func),DOTSYM(sys_ni_syscall)
 #define SYSX(f, f3264, f32)	.llong	DOTSYM(f),DOTSYM(f3264)
 #else
 #define SYSCALL(func)		.long	sys_##func
···
 #define PPC_SYS(func)		.long	ppc_##func
 #define OLDSYS(func)		.long	sys_##func
 #define SYS32ONLY(func)		.long	sys_##func
+#define PPC64ONLY(func)		.long	sys_ni_syscall
 #define SYSX(f, f3264, f32)	.long	f32
 #endif
 #define SYSCALL_SPU(func)	SYSCALL(func)

arch/powerpc/kernel/systbl_chk.c

···
 #ifdef CONFIG_PPC64
 #define OLDSYS(func)		-1
 #define SYS32ONLY(func)		-1
+#define PPC64ONLY(func)		__NR_##func
 #else
 #define OLDSYS(func)		__NR_old##func
 #define SYS32ONLY(func)		__NR_##func
+#define PPC64ONLY(func)		-1
 #endif
 #define SYSX(f, f3264, f32)	-1
 

arch/powerpc/platforms/cell/spu_callbacks.c

···
 #define PPC_SYS(func)		sys_ni_syscall,
 #define OLDSYS(func)		sys_ni_syscall,
 #define SYS32ONLY(func)		sys_ni_syscall,
+#define PPC64ONLY(func)		sys_ni_syscall,
 #define SYSX(f, f3264, f32)	sys_ni_syscall,
 
 #define SYSCALL_SPU(func)	sys_##func,