Re: pt_regs->ax == -ENOSYS

[Andy Lutomirski]

On Tue, Apr 27, 2021 at 5:05 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> 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.
>

Let's suppose we decide to truncate the syscall nr.  What would the
actual semantics be?  Would ptrace see the truncated value in orig_ax?
 How about syscall user dispatch?  What happens if ptrace writes a
value with high bits set to orig_ax?  Do we truncate it again?  Or do
we say that ptrace *can't* write too large a value?

For better for worse, RAX is 64 bits, orig_ax is a 64-bit field, and
it currently has nonsensical semantics.  Redefining orig_ax as a
32-bit field is surely possible, but doing so cleanly is not
necessarily any easier than any other approach.  If it weren't for
seccomp, I would say that the obviously correct answer is to just
treat it everywhere as a 64-bit number.

--Andy