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? --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
