On Thu, Sep 11, 2014 at 3:29 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: > On Thu, Sep 11, 2014 at 2:54 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>> >>> the verifier log contains full trace. Last unsafe instruction + error >>> in many cases is useless. What we found empirically from using >>> it over last 2 years is that developers have different learning curve >>> to adjust to 'safe' style of C. Pretty much everyone couldn't >>> figure out why program is rejected based on last error. Therefore >>> verifier emits full log. From the 1st insn all the way till the last >>> 'unsafe' instruction. So the log is multiline output. >>> 'Understanding eBPF verifier messages' section of >>> Documentation/networking/filter.txt provides few trivial >>> examples of these multiline messages. >>> Like for the program: >>> BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), >>> BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), >>> BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), >>> BPF_LD_MAP_FD(BPF_REG_1, 0), >>> BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem), >>> BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1), >>> BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0), >>> BPF_EXIT_INSN(), >>> the verifier log_buf is: >>> 0: (7a) *(u64 *)(r10 -8) = 0 >>> 1: (bf) r2 = r10 >>> 2: (07) r2 += -8 >>> 3: (b7) r1 = 0 >>> 4: (85) call 1 >>> 5: (15) if r0 == 0x0 goto pc+1 >>> R0=map_ptr R10=fp >>> 6: (7a) *(u64 *)(r0 +4) = 0 >>> misaligned access off 4 size 8 >>> >>> It will surely change over time as verifier becomes smarter, >>> supports new types, optimizations and so on. >>> So this log is not an ABI. It's for humans to read. >>> The log explains _how_ verifier came to conclusion >>> that the program is unsafe. >> >> Given that you've already arranged (I think) for the verifier to be >> compilable in the kernel and in userspace, would it make more sense to >> have the kernel version just say yes or no and to make it easy for >> user code to retry verification in userspace if they want a full >> explanation? > > Good memory :) Long ago I had a hack where I compiled > verifier.o for kernel and linked it with userspace wrappers to > have the same verifier for userspace. It was very fragile. > and maps were not separate objects and there were no fds. > It's not feasible anymore, since different subsystems > will configure different bpf_context and helper functions and > verifier output is dynamic based on maps that were created. > For example, if user's samples/bpf/sock_example.c does > bpf_create_map(HASH, sizeof(key) * 2, ...); > instead of > bpf_create_map(HASH, sizeof(key), ...); > the same program will be rejected in first case and will be > accepted in the second, because map sizes and ebpf > program expectations are mismatching. Hmm. This actually furthers my thought that the relocations should be a real relocation table. Then you could encode the types of the referenced objects in the table, and a program could be verified without looking up the fds. The only extra step would be to confirm that the actual types referenced match those in the table. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
