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Message-ID: <a7238521c6f2caee4d76b58fafa60f133cf08041.camel@intel.com>
Date: Wed, 13 Sep 2023 22:05:27 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Mehta, Sohil" <sohil.mehta@...el.com>,
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Subject: Re: [PATCH 2/2] arch: Reserve map_shadow_stack() syscall number for
all architectures
On Wed, 2023-09-13 at 12:18 -0700, Sohil Mehta wrote:
> 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_st
> > > ack
> >
> > 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
It might have something to do with that powerpc's COND_SYSCALL()
implementation only defines the struct pt_regs variety, but TBH I get a
bit lost when I get to the inline assembly symbol definitions parts and
how it all ties together.
Doing powerpc's version as sys_ni_syscall seems to be consistent at
least, and makes sense with respect to the code you quoted.
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