Re: [PATCH bpf-next v2 1/1] bpf, arm64: support exceptions

[Puranjay Mohan <puranjay12@gmail.com>]

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? 

>> +	}
>>   
>>   	/* 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.

>
>> +	 * 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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