Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

For people who in llvmdev:

This mail is belong to a thread in linux kernel mailing list, the first message
can be retrived from:

  [RFC PATCH v4 0/3] Make eBPF programs output data to perf event

Our goal is to fild a way to make BPF program get an unique ID for each type
so it can pass the ID to other part of kernel, then we can retrive the type and
decode the structure using DWARF information. Currently we have two problem
needs to solve:

1. Dwarf information generated by BPF backend lost all DW_AT_name field;

2. How to get typeid from local variable? I tried llvm.eh_typeid_for
    but it support global variable only.

Following is my response to Alexei.


didn't have time to look at it.
from your llvm patches looks like you've got quite experienced
with it already :slight_smile:

I'll post 2 LLVM patches by replying this mail. Please have a look and
help me
send them to LLVM if you think my code is correct.

patch 1:
I don't quite understand the purpose of builtin_dwarf_cfa
returning R11. It's a special register seen inside llvm codegen
only. It doesn't have kernel meaning.

Kernel side verifier allows us to do arithmetic computation using two local variable
address or local variable address and R11. Therefore, we can compute the location
of a local variable using:

   mark = &my_var_a - __builtin_frame_address(0);

If the stack allocation is fixed (if the location is never reused), the above 'mark'
can be uniquely identify a local variable. That's why I'm interesting in it. However
I'm not sure whether the prerequestion is hold.

patch 2:
do we really need to hack clang?
Can you just define a function that aliases to intrinsic,
like we do for ld_abs/ld_ind ?
void bpf_store_half(void *skb, u64 off, u64 val) asm("");
then no extra patches necessary.

struct my_str {
         int x;
         int y;
struct my_str __str_my_str;

struct my_str2 {
         int x;
         int y;
         int z;
struct my_str2 __str_my_str2;

         mov r1, 1
         call 4660
         mov r1, 2
         call 4660

this part looks good. I think it's usable.

> 1. llvm.eh_typeid_for can be used on global variables only. So for each
> output
> structure we have to define a global varable.

why? I think it should work with local pointers as well.

It is defined by LLVM, in lib/CodeGen/Analysis.cpp:

/// ExtractTypeInfo - Returns the type info, possibly bitcast, encoded in V.
GlobalValue *llvm::ExtractTypeInfo(Value *V) {
   assert((GV || isa<ConstantPointerNull>(V)) &&
          "TypeInfo must be a global variable or NULL"); <-- we can use only constant pointers
   return GV;

So from llvm::Intrinsic::eh_typeid_for we can get type of global variables only.

We may need a new intrinsic for that.

> 2. We still need to find a way to connect the fetchd typeid with DWARF
> info.
> Inserting that ID into DWARF may workable?

hmm, that id should be the same id we're seeing in dwarf, right?

There's no 'typeid' field in dwarf originally. I'm still looking for a way
to inject this ID into dwarf infromation.

I think it's used in exception handling which is reusing some of
the dwarf stuff for this, so the must be a way to connect that id
to actual type info. Though last time I looked at EH was
during g++ hacking days. No idea how llvm does it exactly, but
I'm assuming the logic for rtti should be similar.

I'm not sure whether RTTI use dwarf to deduce type information. I think not,
because dwarf infos can be stripped out.