[PATCH 1/2] powerpc: Add a proper syscall for switching endianness

[Michael Ellerman]

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 <mpe at ellerman.id.au>

fixup
---
 arch/powerpc/include/asm/systbl.h      |  1 +
 arch/powerpc/include/asm/unistd.h      |  2 +-
 arch/powerpc/include/uapi/asm/unistd.h |  1 +
 arch/powerpc/kernel/entry_64.S         |  5 +++++
 arch/powerpc/kernel/syscalls.c         | 17 +++++++++++++++++
 5 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/systbl.h b/arch/powerpc/include/asm/systbl.h
index 91062eef582f..c3ee21a1d9cf 100644
--- a/arch/powerpc/include/asm/systbl.h
+++ b/arch/powerpc/include/asm/systbl.h
@@ -367,3 +367,4 @@ SYSCALL_SPU(getrandom)
 SYSCALL_SPU(memfd_create)
 SYSCALL_SPU(bpf)
 COMPAT_SYS(execveat)
+PPC_SYS(switch_endian)
diff --git a/arch/powerpc/include/asm/unistd.h b/arch/powerpc/include/asm/unistd.h
index 36b79c31eedd..f4f8b667d75b 100644
--- a/arch/powerpc/include/asm/unistd.h
+++ b/arch/powerpc/include/asm/unistd.h
@@ -12,7 +12,7 @@
 #include <uapi/asm/unistd.h>
 
 
-#define __NR_syscalls		363
+#define __NR_syscalls		364
 
 #define __NR__exit __NR_exit
 #define NR_syscalls	__NR_syscalls
diff --git a/arch/powerpc/include/uapi/asm/unistd.h b/arch/powerpc/include/uapi/asm/unistd.h
index ef5b5b1f3123..e4aa173dae62 100644
--- a/arch/powerpc/include/uapi/asm/unistd.h
+++ b/arch/powerpc/include/uapi/asm/unistd.h
@@ -385,5 +385,6 @@
 #define __NR_memfd_create	360
 #define __NR_bpf		361
 #define __NR_execveat		362
+#define __NR_switch_endian	363
 
 #endif /* _UAPI_ASM_POWERPC_UNISTD_H_ */
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index d180caf2d6de..afbc20019c2e 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -356,6 +356,11 @@ _GLOBAL(ppc64_swapcontext)
 	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)
diff --git a/arch/powerpc/kernel/syscalls.c b/arch/powerpc/kernel/syscalls.c
index b2702e87db0d..5fa92706444b 100644
--- a/arch/powerpc/kernel/syscalls.c
+++ b/arch/powerpc/kernel/syscalls.c
@@ -121,3 +121,20 @@ long ppc_fadvise64_64(int fd, int advice, u32 offset_high, u32 offset_low,
 	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;
+}
-- 
2.1.0