Messages in this thread | | | Date | Thu, 21 Sep 2023 21:53:23 +0800 | Subject | Re: [PATCH bpf-next v2 1/1] bpf, arm64: support exceptions | From | Xu Kuohai <> |
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On 9/21/2023 9:16 PM, Puranjay Mohan wrote: > Xu Kuohai <xukuohai@huaweicloud.com> writes: > >> On 9/17/2023 8:00 AM, 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@gmail.com> >>> --- >>> arch/arm64/net/bpf_jit_comp.c | 98 ++++++++++++++++---- >>> tools/testing/selftests/bpf/DENYLIST.aarch64 | 1 - >>> 2 files changed, 79 insertions(+), 20 deletions(-) >>> >>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c >>> index 7d4af64e3982..fcc55e558863 100644 >>> --- a/arch/arm64/net/bpf_jit_comp.c >>> +++ b/arch/arm64/net/bpf_jit_comp.c >>> @@ -21,6 +21,7 @@ >>> #include <asm/insn.h> >>> #include <asm/patching.h> >>> #include <asm/set_memory.h> >>> +#include <asm/stacktrace.h> >>> >>> #include "bpf_jit.h" >>> >>> @@ -285,7 +286,7 @@ static bool is_lsi_offset(int offset, int scale) >>> /* Tail call offset to jump into */ >>> #define PROLOGUE_OFFSET (BTI_INSNS + 2 + PAC_INSNS + 8) >>> >>> -static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) >>> +static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf, bool is_exception_cb) >>> { >>> const struct bpf_prog *prog = ctx->prog; >>> const bool is_main_prog = !bpf_is_subprog(prog); >>> @@ -333,19 +334,28 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) >>> emit(A64_MOV(1, A64_R(9), A64_LR), ctx); >>> emit(A64_NOP, ctx); >>> >>> - /* Sign lr */ >>> - if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)) >>> - emit(A64_PACIASP, ctx); >>> - >>> - /* Save FP and LR registers to stay align with ARM64 AAPCS */ >>> - emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx); >>> - emit(A64_MOV(1, A64_FP, A64_SP), ctx); >>> - >>> - /* Save callee-saved registers */ >>> - emit(A64_PUSH(r6, r7, A64_SP), ctx); >>> - emit(A64_PUSH(r8, r9, A64_SP), ctx); >>> - emit(A64_PUSH(fp, tcc, A64_SP), ctx); >>> - emit(A64_PUSH(fpb, A64_R(28), A64_SP), ctx); >>> + if (!is_exception_cb) { >>> + /* Sign lr */ >>> + if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)) >>> + emit(A64_PACIASP, ctx); >>> + /* Save FP and LR registers to stay align with ARM64 AAPCS */ >>> + emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx); >>> + emit(A64_MOV(1, A64_FP, A64_SP), ctx); >>> + >>> + /* Save callee-saved registers */ >>> + emit(A64_PUSH(r6, r7, A64_SP), ctx); >>> + emit(A64_PUSH(r8, r9, A64_SP), ctx); >>> + emit(A64_PUSH(fp, tcc, A64_SP), ctx); >>> + emit(A64_PUSH(fpb, A64_R(28), A64_SP), ctx); >>> + } else { >>> + /* Exception callback receives FP of Main Program as third parameter */ >>> + emit(A64_MOV(1, A64_FP, A64_R(2)), ctx); >>> + /* >>> + * Main Program already pushed the frame record and the callee-saved registers. The >>> + * exception callback will not push anything and re-use the main program's stack. >>> + */ >>> + emit(A64_SUB_I(1, A64_SP, A64_FP, 80), ctx); /* 10 registers are on the stack */ >> >> To ensure th calculated A6_SP is always correct, add an assertion >> to ensure the distance between A64_FP and A64_SP is 80 after all >> callee-registers are pushed to the stack? >> > > I agree that this should be done. Can you give an example how this > should be implemented? >
IIUC, bpf_throw is essentially a tail call to the exception boundary prog, so can we reset the SP to the PROLOGUE_OFFSET position? If so, we can rely on the assertion of PROLOGUE_OFFSET in build_prologue, or we can add a simliar assertion.
>>> + } >>> >>> /* Set up BPF prog stack base register */ >>> emit(A64_MOV(1, fp, A64_SP), ctx); >>> @@ -365,6 +375,13 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf) >>> emit_bti(A64_BTI_J, ctx); >>> } >>> >>> + /* >>> + * Program acting as exception boundary should save all ARM64 Callee-saved registers as the >>> + * exception callback needs to recover all ARM64 Callee-saved registers in its epilogue. >>> + */ >>> + if (prog->aux->exception_boundary) >>> + emit(A64_PUSH(A64_R(23), A64_R(24), A64_SP), ctx); >> >> Blindly storing x23/x24 to BPF_FP -8/16 is incorrect, as the stack >> space below BPF_FP might be written with other values by the bpf >> prog. >> > > Thanks for pointing this out. I will set fp = A64_SP - 16 so to allocate > space for saving x23/x24. And I will take care while poping back in the epilogue. > >>> + >>> emit(A64_SUB_I(1, fpb, fp, ctx->fpb_offset), ctx); >>> >>> /* Stack must be multiples of 16B */ >>> @@ -653,7 +670,7 @@ static void build_plt(struct jit_ctx *ctx) >>> plt->target = (u64)&dummy_tramp; >>> } >>> >>> -static void build_epilogue(struct jit_ctx *ctx) >>> +static void build_epilogue(struct jit_ctx *ctx, bool is_exception_cb) >>> { >>> const u8 r0 = bpf2a64[BPF_REG_0]; >>> const u8 r6 = bpf2a64[BPF_REG_6]; >>> @@ -666,6 +683,14 @@ static void build_epilogue(struct jit_ctx *ctx) >>> /* We're done with BPF stack */ >>> emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_size), ctx); >>> >>> + /* >>> + * Program acting as exception boundary pushes R23 and R24 in addition to BPF callee-saved >>> + * registers. Exception callback uses the boundary program's stack frame, so recover these >> >> Keep the line width within 80 characters? > > bdc48fa11e46 ("checkpatch/coding-style: deprecate 80-column warning") > removed the warning so I started using 100 character lines. >
80-column is not a hard limit, but it's still preferred, right? And I don't think it's a good idea to include two different line width styles in a single file.
>> >>> + * extra registers in the above two cases. >>> + */ >>> + if (ctx->prog->aux->exception_boundary || is_exception_cb) >>> + emit(A64_POP(A64_R(23), A64_R(24), A64_SP), ctx); >>> + >>> /* Restore x27 and x28 */ >>> emit(A64_POP(fpb, A64_R(28), A64_SP), ctx); >>> /* Restore fs (x25) and x26 */ >>> @@ -1575,7 +1600,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog) >>> * BPF line info needs ctx->offset[i] to be the offset of >>> * instruction[i] in jited image, so build prologue first. >>> */ >>> - if (build_prologue(&ctx, was_classic)) { >>> + if (build_prologue(&ctx, was_classic, prog->aux->exception_cb)) { >>> prog = orig_prog; >>> goto out_off; >>> } >>> @@ -1586,7 +1611,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog) >>> } >>> >>> ctx.epilogue_offset = ctx.idx; >>> - build_epilogue(&ctx); >>> + build_epilogue(&ctx, prog->aux->exception_cb); >>> build_plt(&ctx); >>> >>> extable_align = __alignof__(struct exception_table_entry); >>> @@ -1614,7 +1639,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog) >>> ctx.idx = 0; >>> ctx.exentry_idx = 0; >>> >>> - build_prologue(&ctx, was_classic); >>> + build_prologue(&ctx, was_classic, prog->aux->exception_cb); >>> >>> if (build_body(&ctx, extra_pass)) { >>> bpf_jit_binary_free(header); >>> @@ -1622,7 +1647,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog) >>> goto out_off; >>> } >>> >>> - build_epilogue(&ctx); >>> + build_epilogue(&ctx, prog->aux->exception_cb); >>> build_plt(&ctx); >>> >>> /* 3. Extra pass to validate JITed code. */ >>> @@ -2286,3 +2311,38 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type, >>> >>> return ret; >>> } >>> + >>> +bool bpf_jit_supports_exceptions(void) >>> +{ >>> + /* We unwind through both kernel frames (starting from within bpf_throw call) and >>> + * BPF frames. Therefore we require FP unwinder to be enabled to walk kernel frames and >>> + * reach BPF frames in the stack trace. >>> + * ARM64 kernel is aways compiled with CONFIG_FRAME_POINTER=y >>> + */ >>> + return true; >>> +} >>> + >>> +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), >>> + }; >>> + >> >> Seems there is no need to define "stacks" as an array > > Sure, will change in next version. > >> >>> + 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)) >> >> When PTR_AUTH is implemented, lr is encoded before being pushed to >> the stack, but unwind_next_frame_record() does not decode state.pc >> when fetching it from the stack. > > Thanks for pointing this out. I will fix this in the next version. > >>> + break; >>> + } >> >> And it's better to simplify the if-while(1)-if to: >> >> while (!unwind_next_frame_record(&state)) { >> ... >> } > > Sure, > Will use this method in the next version. > >> >>> +} >>> diff --git a/tools/testing/selftests/bpf/DENYLIST.aarch64 b/tools/testing/selftests/bpf/DENYLIST.aarch64 >>> index f5065576cae9..7f768d335698 100644 >>> --- a/tools/testing/selftests/bpf/DENYLIST.aarch64 >>> +++ b/tools/testing/selftests/bpf/DENYLIST.aarch64 >>> @@ -1,6 +1,5 @@ >>> bpf_cookie/multi_kprobe_attach_api # kprobe_multi_link_api_subtest:FAIL:fentry_raw_skel_load unexpected error: -3 >>> bpf_cookie/multi_kprobe_link_api # kprobe_multi_link_api_subtest:FAIL:fentry_raw_skel_load unexpected error: -3 >>> -exceptions # JIT does not support calling kfunc bpf_throw: -524 >>> fexit_sleep # The test never returns. The remaining tests cannot start. >>> kprobe_multi_bench_attach # bpf_program__attach_kprobe_multi_opts unexpected error: -95 >>> kprobe_multi_test/attach_api_addrs # bpf_program__attach_kprobe_multi_opts unexpected error: -95 > > > Thanks, > Puranjay
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