On Mon, Nov 06, 2023 at 10:04:09AM +0100, Puranjay Mohan wrote: > Hi Mark, > > On Thu, Nov 2, 2023 at 5:59 PM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > > On Sun, Sep 17, 2023 at 12:00:45AM +0000, Puranjay Mohan wrote: > > > Implement arch_bpf_stack_walk() for the ARM64 JIT. This will be used > > > by bpf_throw() to unwind till the program marked as exception boundary and > > > run the callback with the stack of the main program. > > > > > > The prologue generation code has been modified to make the callback > > > program use the stack of the program marked as exception boundary where > > > callee-saved registers are already pushed. > > > > > > As the bpf_throw function never returns, if it clobbers any callee-saved > > > registers, they would remain clobbered. So, the prologue of the > > > exception-boundary program is modified to push R23 and R24 as well, > > > which the callback will then recover in its epilogue. > > > > > > The Procedure Call Standard for the Arm 64-bit Architecture[1] states > > > that registers r19 to r28 should be saved by the callee. BPF programs on > > > ARM64 already save all callee-saved registers except r23 and r24. This > > > patch adds an instruction in prologue of the program to save these > > > two registers and another instruction in the epilogue to recover them. > > > > > > These extra instructions are only added if bpf_throw() used. Otherwise > > > the emitted prologue/epilogue remains unchanged. > > > > > > [1] https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst > > > > > > Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx> > > > --- > > > > [...] > > > > > +void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie) > > > +{ > > > + struct stack_info stacks[] = { > > > + stackinfo_get_task(current), > > > + }; > > > > Can bpf_throw() only be used by BPF programs that run in task context, or is it > > possible e.g. for those to run within an IRQ handler (or otherwise on the IRQ > > stack)? > > I will get back on this with more information. > > > > > > + > > > + struct unwind_state state = { > > > + .stacks = stacks, > > > + .nr_stacks = ARRAY_SIZE(stacks), > > > + }; > > > + unwind_init_common(&state, current); > > > + state.fp = (unsigned long)__builtin_frame_address(1); > > > + state.pc = (unsigned long)__builtin_return_address(0); > > > + > > > + if (unwind_next_frame_record(&state)) > > > + return; > > > + while (1) { > > > + /* We only use the fp in the exception callback. Pass 0 for sp as it's unavailable*/ > > > + if (!consume_fn(cookie, (u64)state.pc, 0, (u64)state.fp)) > > > + break; > > > + if (unwind_next_frame_record(&state)) > > > + break; > > > + } > > > +} > > > > IIUC you're not using arch_stack_walk() because you need the FP in addition to > > the PC. > > Yes, > > > Is there any other reason you need to open-code this? > > No, > > > > > If not, I'd rather rework the common unwinder so that it's possible to get at > > the FP. I had patches for that a while back: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/stacktrace/metadata > > > > ... and I'm happy to rebase that and pull out the minimum necessary to make > > that possible. > > It would be great if you can rebase and push the code, I can rebase this on > your work and not open code this implementation. I've rebased the core of that atop v6.6, and pushed that out to my arm64/stacktrace/kunwind branch: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/stacktrace/kunwind Once v6.7-rc1 is out, I'll rebase that and post it out (possibly with some of the other patches atop). With that I think you can implement arch_bpf_stack_walk() in stacktrace.c using kunwind_stack_walk() in a similar way to how arch_stack_walk() is implemented in that branch. If BPF only needs a single consume_fn, that can probably be even simpler as you won't need a struct to hold the consume_fn and cookie value. Mark.
