Messages in this thread | | | Subject | Re: pt_regs->ax == -ENOSYS | From | "H. Peter Anvin" <> | Date | Tue, 27 Apr 2021 17:20:55 -0700 |
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On 4/27/21 5:11 PM, Andy Lutomirski wrote: > 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. >
We *used* to truncate the system call number; that was unsigned. It causes massive headache to ptrace if a 32-bit ptrace wants to write -1, which is a bit hacky.
I would personally like to see orig_ax to be the register passed in and for the truncation to happen by syscall_get_nr().
I also note that kernel/seccomp.c and the tracing infrastructure all expect a signed int as the system call number. Yes, orig_ax is a 64-bit field, but so are the other register fields which doesn't necessarily directly reflect the value of an argument -- like, say, %rdi in the case of sys_write - it is an int argument so it gets sign extended; this is *not* reflected in ptrace.
-hpa
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