[PATCH v6 19/25] powerpc: Remove high-order word clearing on compat syscall entry

[Michael Ellerman]

Rohan McLure <rmclure at linux.ibm.com> writes:
> Remove explicit clearing of the high order-word of user parameters when
> handling compatibility syscalls in system_call_exception. The
> COMPAT_SYSCALL_DEFINEx macros handle this clearing through an
> explicit cast to the signature type of the target handler.

Unfortunately this doesn't work.

We don't have compat handlers for everything, so we end up with 64-bit
values getting passsed in and things break.

Our hugetlb_vs_thp selftest hits it, as seen in ftrace:

  12848 mmap(0xffffffffa0000000, 16777216, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xf6fb0000
  12848 munmap(0xffffffffa0000000, 16777216) = -1 EINVAL (Invalid argument)

But in the source the only mmap()/munmap() is of 0xa0000000.

Looking at x86 they send all 32-bit syscalls via a wrapper that does the
truncation (SC_IA32_REGS_TO_ARGS). So I think we could do something
similar eventually and get rid of this explicit clearing.

But I don't want to predicate this whole series on that, so I've dropped
this patch for now, and reworked some of the following patches to keep
the register clearing.

cheers

> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index 9875486f6168..15af0ed019a7 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -157,14 +157,6 @@ notrace long system_call_exception(long r3, long r4, long r5,
>  
>  	if (unlikely(is_compat_task())) {
>  		f = (void *)compat_sys_call_table[r0];
> -
> -		r3 &= 0x00000000ffffffffULL;
> -		r4 &= 0x00000000ffffffffULL;
> -		r5 &= 0x00000000ffffffffULL;
> -		r6 &= 0x00000000ffffffffULL;
> -		r7 &= 0x00000000ffffffffULL;
> -		r8 &= 0x00000000ffffffffULL;
> -
>  	} else {
>  		f = (void *)sys_call_table[r0];
>  	}
> -- 
> 2.34.1