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Wed, 01 Jul 2026 11:41:26 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Wed, 01 Jul 2026 11:41:26 -0700 Message-Id: Cc: "Mathieu Desnoyers" , , , , Subject: Re: [RFC PATCH 2/4] tracing/probes: Compile all fetchargs into a single BPF program per event From: "Alexei Starovoitov" To: "Masami Hiramatsu (Google)" , "Steven Rostedt" , "Shuah Khan" X-Mailer: aerc References: <178291352217.1566898.14481561093843379745.stgit@devnote2> <178291354144.1566898.14374948740441958770.stgit@devnote2> In-Reply-To: <178291354144.1566898.14374948740441958770.stgit@devnote2> On Wed Jul 1, 2026 at 6:45 AM PDT, Masami Hiramatsu (Google) wrote: > +static bool trace_probe_can_compile_bpf(struct trace_probe *tp) > +{ > + int i; > + > + if (tp->nr_args =3D=3D 0) > + return false; > + > + for (i =3D 0; i < tp->nr_args; i++) { > + struct probe_arg *parg =3D &tp->args[i]; > + struct fetch_insn *code =3D parg->code; > + > + while (code->op !=3D FETCH_OP_END) { > + switch (code->op) { > + case FETCH_OP_REG: > + case FETCH_OP_IMM: > + case FETCH_OP_DEREF: > + case FETCH_OP_ST_RAW: > + case FETCH_OP_ST_MEM: > + break; > + case FETCH_OP_ARG: > + if (regs_get_kernel_argument_offset(code->param) < 0) > + return false; > + break; > + default: > + return false; > + } > + code++; > + } > + } > + return true; > +} > + > +static void trace_probe_compile_bpf(struct trace_probe *tp) > +{ > + struct bpf_insn *insns; > + int i =3D 0; > + struct bpf_prog *prog; > + int err, idx; > + > + if (!trace_probe_can_compile_bpf(tp)) > + return; > + > + insns =3D kmalloc_array(512, sizeof(struct bpf_insn), GFP_KERNEL); > + if (!insns) > + return; > + > + /* Prologue: R6 =3D ctx */ > + insns[i++] =3D BPF_MOV64_REG(BPF_REG_6, BPF_REG_1); > + /* R7 =3D ctx->rec */ > + insns[i++] =3D BPF_LDX_MEM(BPF_DW, BPF_REG_7, BPF_REG_6, > + offsetof(struct fetch_bpf_ctx, rec)); > + /* R8 =3D ctx->data */ > + insns[i++] =3D BPF_LDX_MEM(BPF_DW, BPF_REG_8, BPF_REG_6, > + offsetof(struct fetch_bpf_ctx, data)); > + /* R9 =3D total size (0) */ > + insns[i++] =3D BPF_MOV64_IMM(BPF_REG_9, 0); > + > + for (idx =3D 0; idx < tp->nr_args; idx++) { > + struct probe_arg *parg =3D &tp->args[idx]; > + struct fetch_insn *code =3D parg->code; > + > + while (code->op !=3D FETCH_OP_END && i < 500) { > + switch (code->op) { > + case FETCH_OP_REG: > + /* R0 =3D *(unsigned long *)(R7 + code->param) */ > + insns[i++] =3D BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_7, code->param= ); > + break; > + case FETCH_OP_ARG: { > + int offset =3D regs_get_kernel_argument_offset(code->param); > + /* R0 =3D *(unsigned long *)(R7 + offset) */ > + insns[i++] =3D BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_7, offset); > + break; > + } > + case FETCH_OP_IMM: > + insns[i++] =3D BPF_LD_IMM64(BPF_REG_0, code->immediate); > + break; > + case FETCH_OP_DEREF: > + /* Add offset: R3 =3D R0 + code->offset (src) */ > + insns[i++] =3D BPF_MOV64_REG(BPF_REG_2, BPF_REG_0); > + if (code->offset) > + insns[i++] =3D BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, > + code->offset); > + /* R1 =3D dst (R10 - 8 on stack) */ > + insns[i++] =3D BPF_MOV64_REG(BPF_REG_1, BPF_REG_10); > + insns[i++] =3D BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, -8); > + /* R3 =3D size */ > + insns[i++] =3D BPF_MOV64_IMM(BPF_REG_3, sizeof(unsigned long)); > + /* Call copy_from_kernel_nofault(dst, src, size) */ > + insns[i++] =3D BPF_EMIT_CALL(copy_from_kernel_nofault); > + /* if (R0 < 0) return R0; */ > + insns[i++] =3D BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1); > + insns[i++] =3D BPF_EXIT_INSN(); > + /* R0 =3D *(unsigned long *)(R10 - 8) */ > + insns[i++] =3D BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8); > + break; > + case FETCH_OP_ST_RAW: > + /* Store R0 into R8 (data) + parg->offset based on size */ > + switch (code->size) { > + case 1: > + insns[i++] =3D BPF_STX_MEM(BPF_B, BPF_REG_8, BPF_REG_0, > + parg->offset); > + break; > + case 2: > + insns[i++] =3D BPF_STX_MEM(BPF_H, BPF_REG_8, BPF_REG_0, > + parg->offset); > + break; > + case 4: > + insns[i++] =3D BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_0, > + parg->offset); > + break; > + case 8: > + insns[i++] =3D BPF_STX_MEM(BPF_DW, BPF_REG_8, BPF_REG_0, > + parg->offset); > + break; > + } > + break; > + case FETCH_OP_ST_MEM: > + /* Add offset: R2 =3D R0 + code->offset (src) */ > + insns[i++] =3D BPF_MOV64_REG(BPF_REG_2, BPF_REG_0); > + if (code->offset) > + insns[i++] =3D BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, > + code->offset); > + /* R1 =3D dst (R8 + parg->offset) */ > + insns[i++] =3D BPF_MOV64_REG(BPF_REG_1, BPF_REG_8); > + if (parg->offset) > + insns[i++] =3D BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, > + parg->offset); > + /* R3 =3D size */ > + insns[i++] =3D BPF_MOV64_IMM(BPF_REG_3, code->size); > + /* Call copy_from_kernel_nofault(dst, src, size) */ > + insns[i++] =3D BPF_EMIT_CALL(copy_from_kernel_nofault); > + /* if (R0 < 0) return R0; */ > + insns[i++] =3D BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1); > + insns[i++] =3D BPF_EXIT_INSN(); > + break; > + default: > + goto out; > + } > + code++; > + } > + } Nack. I really don't like it. There were days in the past when the kernel generating bpf directly was app= ealing. These days are gone. Performance improvements for fetchargs is not a good r= eason to add all this complexity and bypass verifier checks. bpf insns should come from user space.