Re: [PATCH 2/2] arch: Reserve map_shadow_stack() syscall number for all architectures

Sohil Mehta
2023-09-13T19:19:01Z

On 9/11/2023 2:10 PM, Edgecombe, Rick P wrote:
> On Mon, 2023-09-11 at 18:02 +0000, Sohil Mehta wrote:
>> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl
>> b/arch/powerpc/kernel/syscalls/syscall.tbl
>> index 20e50586e8a2..2767b8a42636 100644
>> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
>> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
>> @@ -539,3 +539,4 @@
>>  450    nospu   set_mempolicy_home_node         sys_set_mempolicy_hom
>> e_node
>>  451    common  cachestat                       sys_cachestat
>>  452    common  fchmodat2                       sys_fchmodat2
>> +453    common  map_shadow_stack                sys_map_shadow_stack
>
> I noticed in powerpc, the not implemented syscalls are manually mapped
> to sys_ni_syscall. It also has some special extra sys_ni_syscall()
> implementation bits to handle both ARCH_HAS_SYSCALL_WRAPPER and
> !ARCH_HAS_SYSCALL_WRAPPER. So wondering if it might need special
> treatment. Did you see those parts?
>

Thanks for pointing this out. Powerpc seems to be unique in their
handling of not implemented syscalls. Maybe it's because of their
special casing of the ARCH_HAS_SYSCALL_WRAPPER.

The code below in arch/powerpc/include/asm/syscalls.h suggests to me
that it should be safe to map map_shadow_stack() to sys_ni_syscall() and
the special handling will be taken care of.

#ifndef CONFIG_ARCH_HAS_SYSCALL_WRAPPER
long sys_ni_syscall(void);
#else
long sys_ni_syscall(const struct pt_regs *regs);
#endif

I don't quite understand the underlying reasoning for it though. Do you
have any additional insight into how we should handle this?

I am thinking of doing the following in the next iteration unless
someone chimes in and says otherwise.

--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -539,4 +539,4 @@
450 nospu set_mempolicy_home_node sys_set_mempolicy_home_node
451 common cachestat sys_cachestat
452 common fchmodat2 sys_fchmodat2
-453 common map_shadow_stack sys_map_shadow_stack
+453 common map_shadow_stack sys_ni_syscall