Re: [PATCH v6 net-next 1/6] net: filter: add "load 64-bit immediate" eBPF instruction
From: Alexei Starovoitov
Date: Mon Aug 25 2014 - 22:03:01 EST
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
>>>> 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:
>> 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),
>> 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:
yes, it will be rejected, because type of R1 will not match
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.
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