On Mon, Aug 25, 2014 at 6:54 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Mon, Aug 25, 2014 at 6:53 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: >> On Mon, Aug 25, 2014 at 6:38 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>> On Mon, Aug 25, 2014 at 6:35 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: >>>> On Mon, Aug 25, 2014 at 6:06 PM, David Miller <davem@xxxxxxxxxxxxx> wrote: >>>>> From: Alexei Starovoitov <ast@xxxxxxxxxxxx> >>>>> Date: Mon, 25 Aug 2014 18:00:53 -0700 >>>>> >>>>>> add BPF_LD_IMM64 instruction to load 64-bit immediate value into a register. >>>>> >>>>> I think you need to rethink this. >>>>> >>>>> I understand that you want to be able to compile arbitrary C code into >>>>> eBPF, but you have to restrict strongly what data the eBPF code can get >>>>> to. >>>> >>>> I believe verifier already does restrict it. I don't see any holes in >>>> the architecture. I'm probably not explaining it clearly though :( >>>> >>>>> Arbitrary pointer loads is asking for trouble. >>>> >>>> Of course. >>>> There is no arbitrary pointer from user space. >>>> Verifier checks all pointers. >>>> I guess this commit log description is confusing. >>>> It says: >>>> BPF_LD_IMM64(R1, const_imm_map_ptr) >>>> that's what appears in the program _after_ it goes through verifier. >>>> User space cannot pass a pointer into the kernel. >>> >>> If you don't intend for userspace to load a program that contains this >>> instruction, then why does it need to be an instruction that the >>> verifier rewrites? Why not have an instruction "load immediate >> >> user space use _pseudo_ bpf_ld_imm64 instruction. >> _pseudo_ stands for using 'map_fd' as imm instead of pointer. >> >>> relocated pointer" that contains a reference to a relocation table and >> >> Andy, I guess you missed explanation in: >> https://lkml.org/lkml/2014/8/13/111 >> " >> Obviously user space doesn't know what kernel map pointer is associated >> with process-local map-FD. >> So it's using pseudo BPF_LD_IMM64 instruction. >> BPF_LD_IMM64 with src_reg == 0 -> generic move 64-bit immediate into dst_reg >> BPF_LD_IMM64 with src_reg == BPF_PSEUDO_MAP_FD -> mov map_fd into dst_reg >> Other values are reserved for now. (They will be used to implement >> global variables, strings and other constants and per-cpu areas in the future) >> So the programs look like: >> BPF_LD_MAP_FD(BPF_REG_1, process_local_map_fd), >> BPF_CALL(BPF_FUNC_map_lookup_elem), >> eBPF verifier scans the program for such pseudo instructions, converts >> process_local_map_fd -> in-kernel map pointer >> and drops 'pseudo' flag of BPF_LD_IMM64 instruction. >> " > > Will a program that uses BPF_LD_IMM64 w/o the FPG_REG_1 thing be accepted? If you mean the program like: BPF_LD_IMM64(BPF_REG_1, 0xdead), BPF_CALL(BPF_FUNC_map_lookup_elem), yes, it will be rejected, because type of R1 will not match map_lookup() argument constraints. See check_ld_imm() in verifier.c where it assigns the type during verification. There are 5 tests in verifier testsuite that test things around bpf_ld_imm64 and 2 tests around _pseudo_ bpf_ld_imm64. -- 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
