Messages in this thread | | | Date | Mon, 18 Nov 2013 21:23:35 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] Add a text_poke syscall |
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On 11/18/2013 04:27 PM, Andi Kleen wrote: > > Proposed man page: > > NAME > text_poke - Safely modify running instructions (x86) > > SYNOPSYS > int text_poke(void *addr, const void *opcode, size_t len, > void (*handler)(void), int timeout); > > DESCRIPTION > The text_poke system allows to safely modify code that may > be currently executing in parallel on other threads. > Patch the instruction at addr with the new instructions > at opcode of length len. The target instruction will temporarily > be patched with a break point, before it is replaced > with the final replacement instruction. When the break point > hits the code handler will be called in the context > of the thread. The handler does not save any registers > and cannot return. Typically it would consist of the > original instruction and then a jump to after the original > instruction. The handler is only needed during the > patching process and can be overwritten once the syscall > returns. timeout defines an optional timout to indicate > to the kernel how long the patching could be delayed. > Right now it has to be 0. >
I think I would prefer an interface which took a list of patch points, or implemented only the aspects which are impossible to do in user space.
All we really need in the kernel is the IPI broadcasts - the rest can be done in user space, including intercepting SIGTRAP. For userspace it is probably the best to just put a thread to sleep until the patching is done, which can be done with a futex.
One advantage with doing this in userspace is that the kernel doesn't have to be responsible avoiding holding a thread due to a slightly different SIGTRAP -- it will all come out after the signal handler is restored, anyway.
That being said, the user space code would really need to be librarized.
-hpa
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