On Mon, Oct 15, 2012 at 01:44:55PM +0200, Rabin Vincent wrote: > 2012/10/15 Dave Martin <dave.mar...@linaro.org>: > > On Sun, Oct 14, 2012 at 09:23:13PM +0200, Rabin Vincent wrote: > >> Add basic uprobes support for ARM. > >> > >> perf probe --exec and SystemTap's userspace probing work. The ARM > >> kprobes test code has also been run in a userspace harness to test the > >> uprobe instruction decoding. > > > > The assumption that the target code is ARM appears to be buried all over > > the place. > > Right, as stated: > > >> Caveats: > >> - Thumb is not supported > > > Certainly this code as currently written must depend on !THUMB2_KERNEL. > > Why? It currently works for ARM userspace even if the kernel is > Thumb-2.
My bad, I misread what was happening in the Makefile changes. My concern is about whether we can build the ARM and Thumb-2 kprobes code into the same kernel. If so, no problem, but I believe this is not a tested configuration for kprobes itself. If you've not already done so, it should be possible to test this by adding CONFIG_THUMB2_KERNEL=y to your config, providing your hardware is Thumb-2 capable. > > However, there's an underlying problem here which we'd need to solve. > > The kprobes code can take advantage of the fact that the kernel is all > > ARM or (almost) all Thumb code. So there is no support for kprobes > > supporting ARM and Thumb at the same time. > > > > With userspace, we don't have this luxury. With Debian armhf, Ubuntu > > and Linaro building Thumb-2 userspaces, it may be an increasingly common > > configuration independently of whether the kernel is built in Thumb-2 > > or not. > > > > Furthermode, there is no such thing as a pure Thumb-2 system in practice: > > PLTs are always ARM, for example. For uprobes to work well in userspace, > > both should be supported together. > > Right. I don't think it's difficult to support both of them together, > my thought was just to have userspace tell us if they want to probe a > Thumb instruction via the usual 0th-bit set convention, and then take it > from there. I didn't do the Thumb handling currently because I didn't > really look into modifying the kprobes Thumb instruction decoding and > the IT handling for uprobes. Having looked at the code a little more closely, it looks more closely aligned with this goal than I thought at first. I agree with your suggestion to require the caller to specify explicitly whether they want to insert an ARM or Thumb probe (indeed, I think there's no other way to do it, since guessing the target instruction set can't be done reliably). The code for setting a probe can then switch trivially on that low bit. Additional undef_hooks would be needed for the Thumb case, but this looks relatively straightforward, as you say. General question which I'm not sure I understand yet: is is possible to combine uprobes/kprobes decode more completely? It's not obvious to me whether the uprobes-specific decoding only relates to features which architecturally execute differently in user mode versus privileged mode. Some explanation somewhere could be helpful. Cheers ---Dave -- 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/
