[RFC PATCH RESEND bpf-next 3/4] riscv, bpf: Add bpf_arch_text_poke support for RV64
Pu Lehui
pulehui at huaweicloud.com
Wed Dec 21 19:01:56 PST 2022
On 2022/12/20 11:00, Xu Kuohai wrote:
> On 12/20/2022 10:13 AM, Pu Lehui wrote:
>> From: Pu Lehui <pulehui at huawei.com>
>>
>> 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.
>>
>> Signed-off-by: Pu Lehui <pulehui at huawei.com>
>> ---
>> 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()) {
>
> seems this sync is not needed, why not calling stop machine like this:
>
> stop_machine(do_text_poke, &patch, NULL);
>
I think both version should be ok. While the NULL version uses first cpu
as master to patch_text and the others touch nmi watchdog, this version
attempts to let other cpus enter cpu_relax to calm down, and let master
cpu owns system resources as possible.
>> + 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));
>
> since there are 4 instructions being replaced, it is possible that
> serveral old instructions were already executed before stop machine.
>
Yes, there are unpredictable problems after stop machine in that
situation. In addition, probing stop machine sub-functions may lead to
loop nesting problems. And this is the current problem with riscv ftrace.
But the surprise was that Andy's work [1] was about to solve these
problems. We can synchronize changes in the future.
[1]
https://lore.kernel.org/linux-riscv/20220913094252.3555240-1-andy.chiu@sifive.com
>> +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)
>> +{
>> + if (!is_kernel_text((unsigned long)ip) &&
>> + !is_bpf_text_address((unsigned long)ip))
>> + return -ENOTSUPP;
>> +
>> + return poke_type == BPF_MOD_CALL ?
>> + bpf_text_poke_call(ip, old_addr, new_addr) :
>> + bpf_text_poke_jump(ip, old_addr, new_addr);
>> +}
>> +
>> int bpf_jit_emit_insn(const struct bpf_insn *insn, struct
>> rv_jit_context *ctx,
>> bool extra_pass)
>> {
>> @@ -1266,7 +1389,7 @@ 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 stack_adjust = 0, store_offset, bpf_stack_adjust;
>> + int i, stack_adjust = 0, store_offset, bpf_stack_adjust;
>> bool is_main_prog = ctx->prog->aux->func_idx == 0;
>> bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, 16);
>> @@ -1294,6 +1417,10 @@ 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);
>> +
>> /* First instruction is always setting the tail-call-counter
>> * (TCC) register. This instruction is skipped for tail calls.
>> * Force using a 4-byte (non-compressed) instruction.
More information about the linux-riscv
mailing list