Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 27 Apr 2021 17:11:11 -0700 | Subject | Re: pt_regs->ax == -ENOSYS |
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On Tue, Apr 27, 2021 at 5:05 PM H. Peter Anvin <hpa@zytor.com> 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
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