Re: [RFC PATCH RESEND bpf-next 3/4] riscv, bpf: Add bpf_arch_text_poke support for RV64

[Björn Töpel]

Pu Lehui <pulehui@xxxxxxxxxxxxxxx> writes:

> From: Pu Lehui <pulehui@xxxxxxxxxx>
>
> Implement bpf_arch_text_poke for RV64. For call scenario,
> ftrace framework reserve 4 nops for RV64 kernel function
> as function entry, and use auipc+jalr instructions to call
> kernel or module functions. However, since the auipc+jalr
> call instructions is non-atomic operation, we need to use
> stop-machine to make sure instruction patching in atomic
> context. As for jump scenario, since we only jump inside
> the trampoline, a jal instruction is sufficient.

Hmm, is that really true? More below!

>
> Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
> ---
>  arch/riscv/net/bpf_jit.h        |   5 ++
>  arch/riscv/net/bpf_jit_comp64.c | 131 +++++++++++++++++++++++++++++++-
>  2 files changed, 134 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> index d926e0f7ef57..bf9802a63061 100644
> --- a/arch/riscv/net/bpf_jit.h
> +++ b/arch/riscv/net/bpf_jit.h
> @@ -573,6 +573,11 @@ static inline u32 rv_fence(u8 pred, u8 succ)
>  	return rv_i_insn(imm11_0, 0, 0, 0, 0xf);
>  }
>  
> +static inline u32 rv_nop(void)
> +{
> +	return rv_i_insn(0, 0, 0, 0, 0x13);
> +}
> +
>  /* RVC instrutions. */
>  
>  static inline u16 rvc_addi4spn(u8 rd, u32 imm10)
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index bf4721a99a09..fa8b03c52463 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -8,6 +8,8 @@
>  #include <linux/bitfield.h>
>  #include <linux/bpf.h>
>  #include <linux/filter.h>
> +#include <linux/memory.h>
> +#include <linux/stop_machine.h>
>  #include "bpf_jit.h"
>  
>  #define RV_REG_TCC RV_REG_A6
> @@ -238,7 +240,7 @@ 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 ? 4 : 0, /* skip TCC init */
> +		  is_tail_call ? 20 : 0, /* skip reserved nops and TCC init */
>  		  ctx);
>  }
>  
> @@ -615,6 +617,127 @@ static int add_exception_handler(const struct bpf_insn *insn,
>  	return 0;
>  }
>  
> +struct text_poke_args {
> +	void *addr;
> +	const void *insns;
> +	size_t len;
> +	atomic_t cpu_count;
> +};
> +
> +static int do_text_poke(void *data)
> +{
> +	int ret = 0;
> +	struct text_poke_args *patch = data;
> +
> +	if (atomic_inc_return(&patch->cpu_count) == num_online_cpus()) {
> +		ret = patch_text_nosync(patch->addr, patch->insns, patch->len);
> +		atomic_inc(&patch->cpu_count);
> +	} else {
> +		while (atomic_read(&patch->cpu_count) <= num_online_cpus())
> +			cpu_relax();
> +		smp_mb();
> +	}
> +
> +	return ret;
> +}
> +
> +static int bpf_text_poke_stop_machine(void *addr, const void *insns, size_t len)
> +{
> +	struct text_poke_args patch = {
> +		.addr = addr,
> +		.insns = insns,
> +		.len = len,
> +		.cpu_count = ATOMIC_INIT(0),
> +	};
> +
> +	return stop_machine(do_text_poke, &patch, cpu_online_mask);
> +}
> +
> +static int gen_call_or_nops(void *target, void *ip, u32 *insns)
> +{
> +	int i, ret;
> +	s64 rvoff;
> +	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);
> +	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 bpf_text_poke_call(void *ip, void *old_addr, void *new_addr)
> +{
> +	int ret;
> +	u32 old_insns[4], new_insns[4];
> +
> +	ret = gen_call_or_nops(old_addr, ip + 4, old_insns);
> +	if (ret)
> +		return ret;
> +
> +	ret = gen_call_or_nops(new_addr, ip + 4, new_insns);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&text_mutex);
> +	if (memcmp(ip, old_insns, sizeof(old_insns))) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +
> +	if (memcmp(ip, new_insns, sizeof(new_insns)))
> +		ret = bpf_text_poke_stop_machine(ip, new_insns,
> sizeof(new_insns));

I'd rather see that you added a patch_text variant to
arch/riscv/kernel/patch.c (something like your
bpf_text_poke_stop_machine()), and use that here. Might be other users
of that as well -- Andy's ftrace patch maybe? :-)

> +out:
> +	mutex_unlock(&text_mutex);
> +	return ret;
> +}
> +
> +static int bpf_text_poke_jump(void *ip, void *old_addr, void *new_addr)
> +{
> +	int ret;
> +	u32 old_insn, new_insn;
> +
> +	old_insn = old_addr ? rv_jal(RV_REG_ZERO, (s64)(old_addr - ip) >> 1) : rv_nop();
> +	new_insn = new_addr ? rv_jal(RV_REG_ZERO, (s64)(new_addr - ip) >> 1) : rv_nop();
> +
> +	mutex_lock(&text_mutex);
> +	if (memcmp(ip, &old_insn, sizeof(old_insn))) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +
> +	if (memcmp(ip, &new_insn, sizeof(new_insn)))
> +		ret = patch_text_nosync(ip, &new_insn, sizeof(new_insn));
> +out:
> +	mutex_unlock(&text_mutex);
> +	return ret;
> +}
> +
> +int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
> +		       void *old_addr, void *new_addr)

AFAIU there's nothing in the bpf_arch_text_poke() API that say that
BPF_MOD_JUMP is jumps within the trampoline. That is one usage, but not
the only one. In general, the jal might not have enough reach.

I believe that this needs to be an auipc/jalr pair similar to
BPF_MOD_CALL (w/o linked register). 


And again, thanks for working on the RV trampoline!
Björn