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Message-ID: <63986ef9-10a4-bcef-369d-0bad28b192d1@huawei.com>
Date: Wed, 19 Jul 2023 22:44:47 +0800
From: Pu Lehui <pulehui@...wei.com>
To: Björn Töpel <bjorn@...nel.org>,
Pu Lehui <pulehui@...weicloud.com>, <bpf@...r.kernel.org>,
<linux-riscv@...ts.infradead.org>, <netdev@...r.kernel.org>,
<linux-kernel@...r.kernel.org>
CC: 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 <yhs@...com>,
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>,
Palmer Dabbelt <palmer@...belt.com>,
Guo Ren <guoren@...nel.org>, Song Shuai <suagrfillet@...il.com>
Subject: Re: [PATCH bpf] riscv, bpf: Adapt bpf trampoline to optimized riscv
ftrace framework
On 2023/7/19 4:06, Björn Töpel wrote:
> Pu Lehui <pulehui@...weicloud.com> writes:
>
>> From: Pu Lehui <pulehui@...wei.com>
>>
>> Commit 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to
>> half") optimizes the detour code size of kernel functions to half with
>> T0 register and the upcoming DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv
>> is based on this optimization, we need to adapt riscv bpf trampoline
>> based on this. One thing to do is to reduce detour code size of bpf
>> programs, and the second is to deal with the return address after the
>> execution of bpf trampoline. Meanwhile, add more comments and rename
>> some variables to make more sense. The related tests have passed.
>>
>> This adaptation needs to be merged before the upcoming
>> DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv, otherwise it will crash due
>> to a mismatch in the return address. So we target this modification to
>> bpf tree and add fixes tag for locating.
>
> Thank you for working on this!
>
>> Fixes: 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to half")
>
> This is not a fix. Nothing is broken. Only that this patch much come
> before or as part of the ftrace series.
Yep, it's really not a fix. I have no idea whether this patch target to
bpf-next tree can be ahead of the ftrace series of riscv tree?
> >> Signed-off-by: Pu Lehui <pulehui@...wei.com>
>> ---
>> arch/riscv/net/bpf_jit_comp64.c | 110 ++++++++++++++------------------
>> 1 file changed, 47 insertions(+), 63 deletions(-)
>>
>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>> index c648864c8cd1..ffc9aa42f918 100644
>> --- a/arch/riscv/net/bpf_jit_comp64.c
>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>> @@ -241,7 +241,7 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
>> if (!is_tail_call)
>> emit_mv(RV_REG_A0, RV_REG_A5, ctx);
>> emit_jalr(RV_REG_ZERO, is_tail_call ? RV_REG_T3 : RV_REG_RA,
>> - is_tail_call ? 20 : 0, /* skip reserved nops and TCC init */
>> + is_tail_call ? 12 : 0, /* skip reserved nops and TCC init */
>
> Maybe be explicit, and use the "DETOUR_INSNS" from below (and convert to
> bytes)?
>
>> ctx);
>> }
>>
>> @@ -618,32 +618,7 @@ static int add_exception_handler(const struct bpf_insn *insn,
>> return 0;
>> }
>>
>> -static int gen_call_or_nops(void *target, void *ip, u32 *insns)
>> -{
>> - s64 rvoff;
>> - int i, ret;
>> - struct rv_jit_context ctx;
>> -
>> - ctx.ninsns = 0;
>> - ctx.insns = (u16 *)insns;
>> -
>> - if (!target) {
>> - for (i = 0; i < 4; i++)
>> - emit(rv_nop(), &ctx);
>> - return 0;
>> - }
>> -
>> - rvoff = (s64)(target - (ip + 4));
>> - emit(rv_sd(RV_REG_SP, -8, RV_REG_RA), &ctx);
>> - ret = emit_jump_and_link(RV_REG_RA, rvoff, false, &ctx);
>> - if (ret)
>> - return ret;
>> - emit(rv_ld(RV_REG_RA, -8, RV_REG_SP), &ctx);
>> -
>> - return 0;
>> -}
>> -
>> -static int gen_jump_or_nops(void *target, void *ip, u32 *insns)
>> +static int gen_jump_or_nops(void *target, void *ip, u32 *insns, bool is_call)
>> {
>> s64 rvoff;
>> struct rv_jit_context ctx;
>> @@ -658,38 +633,38 @@ static int gen_jump_or_nops(void *target, void *ip, u32 *insns)
>> }
>>
>> rvoff = (s64)(target - ip);
>> - return emit_jump_and_link(RV_REG_ZERO, rvoff, false, &ctx);
>> + return emit_jump_and_link(is_call ? RV_REG_T0 : RV_REG_ZERO,
>> + rvoff, false, &ctx);
>
> Nit: Please use the full 100 char width.
>
>> }
>>
>> +#define DETOUR_NINSNS 2
>
> Better name? Maybe call this patchable function entry something? Also,
How about RV_FENTRY_NINSNS?
> to catch future breaks like this -- would it make sense to have a
> static_assert() combined with something tied to
> -fpatchable-function-entry= from arch/riscv/Makefile?
It is very necessary, but it doesn't seem to be easy. I try to find GCC
related functions, something like __builtin_xxx, but I can't find it so
far. Also try to make it as a CONFIG_PATCHABLE_FUNCTION_ENTRY=4 in
Makefile and then static_assert, but obviously it shouldn't be done.
Maybe we can deal with this later when we have a solution?
>
>> +
>> int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
>> void *old_addr, void *new_addr)
>> {
>> - u32 old_insns[4], new_insns[4];
>> + u32 old_insns[DETOUR_NINSNS], new_insns[DETOUR_NINSNS];
>> bool is_call = poke_type == BPF_MOD_CALL;
>> - int (*gen_insns)(void *target, void *ip, u32 *insns);
>> - int ninsns = is_call ? 4 : 2;
>> int ret;
>>
>> - if (!is_bpf_text_address((unsigned long)ip))
>> + if (!is_kernel_text((unsigned long)ip) &&
>> + !is_bpf_text_address((unsigned long)ip))
>> return -ENOTSUPP;
>>
>> - gen_insns = is_call ? gen_call_or_nops : gen_jump_or_nops;
>> -
>> - ret = gen_insns(old_addr, ip, old_insns);
>> + ret = gen_jump_or_nops(old_addr, ip, old_insns, is_call);
>> if (ret)
>> return ret;
>>
>> - if (memcmp(ip, old_insns, ninsns * 4))
>> + if (memcmp(ip, old_insns, DETOUR_NINSNS * 4))
>> return -EFAULT;
>>
>> - ret = gen_insns(new_addr, ip, new_insns);
>> + ret = gen_jump_or_nops(new_addr, ip, new_insns, is_call);
>> if (ret)
>> return ret;
>>
>> cpus_read_lock();
>> mutex_lock(&text_mutex);
>> - if (memcmp(ip, new_insns, ninsns * 4))
>> - ret = patch_text(ip, new_insns, ninsns);
>> + if (memcmp(ip, new_insns, DETOUR_NINSNS * 4))
>> + ret = patch_text(ip, new_insns, DETOUR_NINSNS);
>> mutex_unlock(&text_mutex);
>> cpus_read_unlock();
>>
>> @@ -717,7 +692,7 @@ static void restore_args(int nregs, int args_off, struct rv_jit_context *ctx)
>> }
>>
>> static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_off,
>> - int run_ctx_off, bool save_ret, struct rv_jit_context *ctx)
>> + int run_ctx_off, bool save_retval, struct rv_jit_context *ctx)
>
> Why the save_retval name change? This churn is not needed IMO
> (especially since you keep using the _ret name below). Please keep the
> old name. >
>> {
>> int ret, branch_off;
>> struct bpf_prog *p = l->link.prog;
>> @@ -757,7 +732,7 @@ static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_of
>> if (ret)
>> return ret;
>>
>> - if (save_ret)
>> + if (save_retval)
>> emit_sd(RV_REG_FP, -retval_off, regmap[BPF_REG_0], ctx);
>>
>> /* update branch with beqz */
>> @@ -787,20 +762,19 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>> int i, ret, offset;
>> int *branches_off = NULL;
>> int stack_size = 0, nregs = m->nr_args;
>> - int retaddr_off, fp_off, retval_off, args_off;
>> - int nregs_off, ip_off, run_ctx_off, sreg_off;
>> + int fp_off, retval_off, args_off, nregs_off, ip_off, run_ctx_off, sreg_off;
>> struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
>> struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
>> struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
>> void *orig_call = func_addr;
>> - bool save_ret;
>> + bool save_retval, traced_ret;
>> u32 insn;
>>
>> /* Generated trampoline stack layout:
>> *
>> * FP - 8 [ RA of parent func ] return address of parent
>> * function
>> - * FP - retaddr_off [ RA of traced func ] return address of traced
>> + * FP - 16 [ RA of traced func ] return address of
>> traced
>
> BPF code uses frame pointers. Shouldn't the trampoline frame look like a
> regular frame [1], i.e. start with return address followed by previous
> frame pointer?
>
oops, will fix it. Also we need to consider two types of trampoline
stack layout, that is:
* 1. trampoline called from function entry
* --------------------------------------
* FP + 8 [ RA of parent func ] return address of parent
* function
* FP + 0 [ FP ]
*
* FP - 8 [ RA of traced func ] return address of traced
* function
* FP - 16 [ FP ]
* --------------------------------------
*
* 2. trampoline called directly
* --------------------------------------
* FP - 8 [ RA of caller func ] return address of caller
* function
* FP - 16 [ FP ]
* --------------------------------------
>> * function
>> * FP - fp_off [ FP of parent func ]
>> *
>> @@ -833,17 +807,20 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>> if (nregs > 8)
>> return -ENOTSUPP;
>>
>> - /* room for parent function return address */
>> + /* room for return address of parent function */
>> stack_size += 8;
>>
>> - stack_size += 8;
>> - retaddr_off = stack_size;
>> + /* whether return to return address of traced function after bpf trampoline */
>> + traced_ret = func_addr && !(flags & BPF_TRAMP_F_SKIP_FRAME);
>> + /* room for return address of traced function */
>> + if (traced_ret)
>> + stack_size += 8;
>>
>> stack_size += 8;
>> fp_off = stack_size;
>>
>> - save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
>> - if (save_ret) {
>> + save_retval = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
>> + if (save_retval) {
>> stack_size += 8;
>> retval_off = stack_size;
>> }
>> @@ -869,7 +846,11 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>>
>> emit_addi(RV_REG_SP, RV_REG_SP, -stack_size, ctx);
>>
>> - emit_sd(RV_REG_SP, stack_size - retaddr_off, RV_REG_RA, ctx);
>> + /* store return address of parent function */
>> + emit_sd(RV_REG_SP, stack_size - 8, RV_REG_RA, ctx);
>> + /* store return address of traced function */
>> + if (traced_ret)
>> + emit_sd(RV_REG_SP, stack_size - 16, RV_REG_T0, ctx);
>> emit_sd(RV_REG_SP, stack_size - fp_off, RV_REG_FP, ctx);
>>
>> emit_addi(RV_REG_FP, RV_REG_SP, stack_size, ctx);
>> @@ -890,7 +871,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>>
>> /* skip to actual body of traced function */
>> if (flags & BPF_TRAMP_F_SKIP_FRAME)
>> - orig_call += 16;
>> + orig_call += 8;
>
> Use the define above so it's obvious what you're skipping.
>
>>
>> if (flags & BPF_TRAMP_F_CALL_ORIG) {
>> emit_imm(RV_REG_A0, (const s64)im, ctx);
>> @@ -962,22 +943,25 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>> if (flags & BPF_TRAMP_F_RESTORE_REGS)
>> restore_args(nregs, args_off, ctx);
>>
>> - if (save_ret)
>> + if (save_retval)
>> emit_ld(RV_REG_A0, -retval_off, RV_REG_FP, ctx);
>>
>> emit_ld(RV_REG_S1, -sreg_off, RV_REG_FP, ctx);
>>
>> - if (flags & BPF_TRAMP_F_SKIP_FRAME)
>> - /* return address of parent function */
>> + if (traced_ret) {
>> + /* restore return address of parent function */
>> emit_ld(RV_REG_RA, stack_size - 8, RV_REG_SP, ctx);
>> - else
>> - /* return address of traced function */
>> - emit_ld(RV_REG_RA, stack_size - retaddr_off, RV_REG_SP, ctx);
>> + /* restore return address of traced function */
>> + emit_ld(RV_REG_T0, stack_size - 16, RV_REG_SP, ctx);
>> + } else {
>> + /* restore return address of parent function */
>> + emit_ld(RV_REG_T0, stack_size - 8, RV_REG_SP, ctx);
>> + }
>>
>> emit_ld(RV_REG_FP, stack_size - fp_off, RV_REG_SP, ctx);
>> emit_addi(RV_REG_SP, RV_REG_SP, stack_size, ctx);
>>
>> - emit_jalr(RV_REG_ZERO, RV_REG_RA, 0, ctx);
>> + emit_jalr(RV_REG_ZERO, RV_REG_T0, 0, ctx);
>>
>> ret = ctx->ninsns;
>> out:
>> @@ -1664,7 +1648,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>>
>> void bpf_jit_build_prologue(struct rv_jit_context *ctx)
>> {
>> - int i, stack_adjust = 0, store_offset, bpf_stack_adjust;
>> + int stack_adjust = 0, store_offset, bpf_stack_adjust;
>>
>> bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, 16);
>> if (bpf_stack_adjust)
>> @@ -1691,9 +1675,9 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx)
>>
>> store_offset = stack_adjust - 8;
>>
>> - /* reserve 4 nop insns */
>> - for (i = 0; i < 4; i++)
>> - emit(rv_nop(), ctx);
>> + /* 2 nops reserved for auipc+jalr pair */
>> + emit(rv_nop(), ctx);
>> + emit(rv_nop(), ctx);
>
> Use the define above, instead of hardcoding two nops.
>
>
> Thanks,
> Björn
>
> [1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#frame-pointer-convention
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