linux-kernel - Re: pt_regs->ax == -ENOSYS

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Date:   Tue, 27 Apr 2021 17:05:16 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>
Subject: Re: pt_regs->ax == -ENOSYS

On 4/27/21 4:23 PM, Andy Lutomirski wrote:
> 
> I much prefer the model of saying that the bits that make sense for
> the syscall type (all 64 for 64-bit SYSCALL and the low 32 for
> everything else) are all valid.  This way there are no weird reserved
> bits, no weird ptrace() interactions, etc.  I'm a tiny bit concerned
> that this would result in a backwards compatibility issue, but not
> very.  This would involve changing syscall_get_nr(), but that doesn't
> seem so bad.  The biggest problem is that seccomp hardcoded syscall
> nrs to 32 bit.
> 
> An alternative would be to declare that we always truncate to 32 bits,
> except that 64-bit SYSCALL with high bits set is an error and results
> in ENOSYS. The ptrace interaction there is potentially nasty.
> 
> Basically, all choices here kind of suck, and I haven't done a real
> analysis of all the issues...
> 

OK, I really don't understand this. The *current* way of doing it causes 
a bunch of ugly corner conditions, including in ptrace, which this would 
get rid of. It isn't any different than passing any other argument which 
is an int -- in fact we have this whole machinery to deal with that subcase.

If it makes you feel better, we could even sign-extend the value in 
orig_ax, but that seems unnecessary and a bit broken to me.

	-hpa