On Thu, Jul 3, 2014 at 9:32 AM, Will Deacon <will.dea...@arm.com> wrote: > On Thu, Jul 03, 2014 at 05:13:50PM +0100, Andy Lutomirski wrote: >> On Thu, Jul 3, 2014 at 9:11 AM, Will Deacon <will.dea...@arm.com> wrote: >> > On Thu, Jul 03, 2014 at 04:39:21PM +0100, Andy Lutomirski wrote: >> >> On Thu, Jul 3, 2014 at 3:24 AM, Will Deacon <will.dea...@arm.com> wrote: >> >> > On Thu, Jul 03, 2014 at 08:43:07AM +0100, AKASHI Takahiro wrote: >> >> >> So it seems to me that we can't change a system call by ptrace(). >> >> >> Do I misunderstand anything? >> >> > >> >> > No, it looks like you have a point here. I don't think userspace has any >> >> > business with orig_x0, but changing syscallno is certainly useful. I can >> >> > think of two ways to fix this: >> >> > >> >> > (1) Updating syscallno based on w8, but this ties us to the current >> >> > ABI >> >> > and could get messy if this register changes in the future. >> >> > >> >> > (2) Adding a PTRACE_SET_SYSCALL request, like we have for arch/arm/, >> >> > but that means adding arch-specific stuff to arch_ptrace (which >> >> > currently goes straight to ptrace_request on arm64). >> >> > >> >> > It looks like x86 uses orig_ax, which I *think* means we would go with >> >> > (1) above if we followed their lead. >> >> >> >> w8 is a real register, right? On x86, at least orig_ax isn't a real >> >> register, so it's quite unlikely to conflict with hardware stuff. >> > >> > Yeah, w8 is the hardware register which the Linux ABI uses for the system >> > call number. I was thinking We could allow the debugger/tracer to update >> > the syscall number by updating that register, or do you see an issue with >> > that? (other than tying us to the current ABI). >> >> Not immediately, but I'm not super-familiar with ptrace. >> >> Is w8 clobbered or otherwise changed by syscalls? Using w8 for this >> has the odd effect that tracers can't force a return with a specific >> value of w8 without executing the corresponding syscall. If that's a >> meaningful limitation, then presumably some other channel should be >> used. > > Hmm, that's true. Currently w8 is preserved across a syscall by the kernel, > so it would be pretty bizarre for somebody to try and modify it but I guess > they could do it if they wanted to. However, they could just as easily > modify it on the syscall return path and have the same effect... > > Furthermore, glibc unconditionally emits a mov into w8 prior to the svc > instruction, so from a user's perspective that register always contains > the system call number.
That means that, if someone uses a seccomp trace action to skip or emulate a syscall by writing -1 to w8, then user code will see an unexpected -1 in w8. I don't know how much this matters. > > FWIW: the role of w8 in the PCS is `Indirect result location register', so > I'd expect it to be saved across the syscall anyway. > > Will -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
