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. " To rephrase it differently. BPF_LD_MAP_FD(BPF_REG_1, process_local_map_fd), is very much what you suggesting by "load immediate relocated pointer" Right? > have the JIT do it? That might be easier to understand than having > the verifier do it, and it'll avoid committing to ABIs before we need > them. that part I don't understand. The patch that handles pseudo_with_map_fd -> -> normal_with_kernel_pointer conversion is only 147 lines: https://git.kernel.org/cgit/linux/kernel/git/ast/bpf.git/commit/?id=d82d3daa20465dfdc6b2a0094ad27de9edbb328b Cannot think of shorter version. -- 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
