On Mon, Nov 07, 2022 at 03:35:07PM +0000, Will Deacon wrote: > On Mon, Nov 07, 2022 at 03:32:24PM +0000, Mark Rutland wrote: > > On Mon, Nov 07, 2022 at 02:49:31PM +0000, Will Deacon wrote: > > > [+Mark R] > > > > > > On Thu, Jul 28, 2022 at 02:02:50AM +0000, Jianlin Lv wrote: > > > > This is the arm64 version of ftrace-based kprobes to avoid the overhead > > > > with regular kprobes, by using the ftrace infrastructure. > > > > > > > > Signed-off-by: Jianlin Lv <iecedge@xxxxxxxxx> > > > > --- > > > > .../debug/kprobes-on-ftrace/arch-support.txt | 2 +- > > > > arch/arm64/Kconfig | 1 + > > > > arch/arm64/kernel/probes/Makefile | 1 + > > > > arch/arm64/kernel/probes/kprobes-ftrace.c | 81 +++++++++++++++++++ > > > > include/linux/kprobes.h | 2 + > > > > kernel/kprobes.c | 4 +- > > > > 6 files changed, 88 insertions(+), 3 deletions(-) > > > > create mode 100644 arch/arm64/kernel/probes/kprobes-ftrace.c > > > > > > Sorry for the slow reply on this, but I think this deserved to be split > > > into two patches: the first one reworking the core check_ftrace_location() > > > logic to work properly with branch-and-link style architectures, and the > > > second one adding support for arm64. > > > > I'd prefer we don't do this at all; there a bunch of issues with kprobes *not* > > taking an exception, since we get a dodgy not-quite-real pt_regs, and to clean > > up the existing issues the plan is: > > > > 1) Move ftrace over to ftrace_regs > > 2) Implement fprobes using ftrace_regs > > 3) Remove kretprobes > > > > ... and regular kprobes will need to take an exception (via BRK) to get a real > > pt_regs, so that can't be optimized to use ftrace. > > OKey doke. Does that mean that other architectures will follow the same > approach of taking an exception, I think once everyone has FPROBE, KPROBES_ON_FTRACE becomes redundant, and could be removed (leaving kprobes to always follow a take-an-exception flow on all architectures). > or do they somehow work by magic? Some architectures don't need to take an exception to be able to create a full pt_regs (e.g. x86's flags are accessible in a way arm64's PSTATE isn't), but that needs to be generated / restored differently to exception entry/return, and so even where it's possible it can be painful to maintain (and slower than using ftrace_regs), so I suspect KPROBES_ON_FTRACE would be removed. So different constaints more than magic. Thanks, Mark.
