On Tue, 2015-03-10 at 18:36 +1100, Michael Ellerman wrote:
> 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 syscalls 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.
You can set _TIF_RESTOREALL to force a restore of all the registers on
the way back which should do the job.
Cheers,
Ben.
>
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
> ---
> 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/syscalls.c | 7 +++++++
> 4 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/systbl.h
> b/arch/powerpc/include/asm/systbl.h
> index 91062eef582f..524bf5dff9f9 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)
> +SYSCALL(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/syscalls.c b/arch/powerpc/kernel/syscalls.c
> index b2702e87db0d..0ea01957990e 100644
> --- a/arch/powerpc/kernel/syscalls.c
> +++ b/arch/powerpc/kernel/syscalls.c
> @@ -121,3 +121,10 @@ 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)
> +{
> + current->thread.regs->msr ^= MSR_LE;
> +
> + return 0;
> +}
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