On Wed, Aug 14, 2024 at 10:51:41AM +0200, Florian Rommel wrote: > Experimental implementation of Thomas' trampoline idea. > Link: https://lore.kernel.org/all/87wmkkpf5v.ffs@tglx/ > > Every breakpoint gets a trampoline entry containing the original > instruction (with a corrected displacement if necessary) and a jump > back into the function to the logically subsequent instruction. > With this, KGDB can skip a still present debug trap for a removed > breakpoint by routing the control flow through the trampoline. > > In this experimental implementation, the actual removal of the debug > trap instructions is completely disabled. So, all trap instructions > planted by KGDB currently remain in the code, and KGDB will skip all > these breakpoints through the trampolines when they are in the removed > state. There is not yet a dedicated breakpoint state for the > to-be-removed-but-still-present breakpoints. > > Inspect the trampolines via: > (gdb) x/16i kgdb_trampoline_page > > Signed-off-by: Florian Rommel <m...@florommel.de> > --- > I have been experimenting a bit with Thomas' idea of the trampoline > approach. It seems to work so far but of course has a lot of rough > edges.
Interesting. Perhaps a dumb question but is this trampoline code doing the same thing as the out-of-line single-step code in kprobes? > I am stuck for now, as I am not sure about the best way to implement > the safe context where KGDB finally removes the int3 instructions. I think this would actually fit really nicely into the debug core code. Firstly dbg_deactivate_sw_breakpoints() should strictly maintain BP_ACTIVE and BP_SET states (e.g. if the kgdb_arch_remove_breakpoint() fails then do not transition from BP_ACTIVE and BP_SET). There would be a little bit of extra logic to clean things up here (each equality check on BP_SET needs to be reviewed) but the resulting state tracking is more correct. Once we've done that we can add a new state BP_REMOVE_PENDING and arrange for this to be set by dbg_remove_sw_break() if the breakpoint is BP_ACTIVE. At this stage we can also arrange for dbg_deactivate_sw_breakpoints() to handle BP_REMOVE_PENDING to BP_REMOVE transitions by calling kgdb_arch_remove_breakpoint(). That's enough to eventuallyremove the int3 instructions but it relies on entering the debug trap handler and there's no limit on how long could take before that happens. For that reason I think the core should also attempt to transition BP_REMOVE_PENDING breakpoints to BP_REMOVE after kgdb_skipexception() returns true. That means if we keep trapping on a disabled breakpoint eventually we will hit a window where the text_mutex is free and clean things up. Daniel. _______________________________________________ Kgdb-bugreport mailing list Kgdb-bugreport@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kgdb-bugreport
