Re: [PATCH v6 net-next 1/6] net: filter: add "load 64-bit immediate" eBPF instruction

On Mon, Aug 25, 2014 at 6:53 PM, Alexei Starovoitov [off-list ref] wrote:

On Mon, Aug 25, 2014 at 6:38 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Mon, Aug 25, 2014 at 6:35 PM, Alexei Starovoitov [off-list ref] wrote:

quoted

On Mon, Aug 25, 2014 at 6:06 PM, David Miller [off-list ref] wrote:

quoted

From: Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org>
Date: Mon, 25 Aug 2014 18:00:53 -0700

quoted

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 :(

quoted

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.

quoted

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