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Date: Mon, 18 Sep 2023 22:15:58 +0800
From: Xu Kuohai <xukuohai@...weicloud.com>
To: Puranjay Mohan <puranjay12@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Zi Shen Lim <zlim.lnx@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>, bpf@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH bpf-next v2 1/1] bpf, arm64: support exceptions
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@...il.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?
> + }
>
> /* 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.
> +
> 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?
> + * 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
> + 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.
> + break;
> + }
And it's better to simplify the if-while(1)-if to:
while (!unwind_next_frame_record(&state)) {
...
}
> +}
> 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
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