[LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event
Wangnan (F)
wangnan0 at huawei.com
Tue Aug 4 19:05:07 PDT 2015
Send again since llvmdev is moved to llvm-dev at lists.llvm.org
On 2015/8/5 9:58, Wangnan (F) wrote:
>
>
> On 2015/8/4 17:01, Wangnan (F) wrote:
>> For people who in llvmdev:
>>
>> This mail is belong to a thread in linux kernel mailing list, the
>> first message
>> can be retrived from:
>>
>> http://lkml.kernel.org/r/55B1535E.8090406@plumgrid.com
>>
>> 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.
>>
>> On 2015/8/4 3:44, Alexei Starovoitov wrote:
>>> On 7/31/15 3:18 AM, Wangnan (F) wrote:
>>>
>>
>> [SNIP]
>>
>>> didn't have time to look at it.
>>> from your llvm patches looks like you've got quite experienced
>>> with it already :)
>>>
>>>> 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("llvm.bpf.store.half");
>>> 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;
>>>>
>>>> test_func(__builtin_bpf_typeid(&__str_my_str));
>>>> test_func(__builtin_bpf_typeid(&__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.
>>
>
> Hi Alexei,
>
> Just found that llvm::Intrinsic::eh_typeid_for is function specific.
> ID of same type in
> different functions may be different. Here is an example:
>
> static int (*bpf_output_event)(unsigned long, void *buf, int size) =
> (void *) 0x1234;
>
> 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;
>
> int func(int *ctx)
> {
> struct my_str var_a;
> struct my_str2 var_b;
> bpf_output_event(__builtin_bpf_typeid(&__str_my_str), &var_a,
> sizeof(var_a));
> bpf_output_event(__builtin_bpf_typeid(&__str_my_str2), &var_b,
> sizeof(var_b));
> return 0;
> }
>
> int func2(int *ctx)
> {
> struct my_str var_a;
> struct my_str2 var_b;
>
> /* change order here */
> bpf_output_event(__builtin_bpf_typeid(&__str_my_str2), &var_b,
> sizeof(var_b));
> bpf_output_event(__builtin_bpf_typeid(&__str_my_str), &var_a,
> sizeof(var_a))
> return 0;
> }
>
> This program uses __builtin_bpf_typeid(llvm::Intrinsic::eh_typeid_for)
> in func and func2
> for same two types but in different order. We expect same type get
> same ID.
>
> Compiled with:
>
> $ clang -target bpf -S -O2 -c ./test_bpf_typeid.c
>
> The result is:
>
> .text
> .globl func
> .align 8
> func: # @func
> # BB#0: # %entry
> mov r2, r10
> addi r2, -8
> mov r1, 1
> mov r3, 8
> call 4660
> mov r2, r10
> addi r2, -24
> mov r1, 2
> mov r3, 12
> call 4660
> mov r0, 0
> ret
>
> .globl func2
> .align 8
> func2: # @func2
> # BB#0: # %entry
> mov r2, r10
> addi r2, -24
> mov r1, 1 <--- we want 2 here.
> mov r3, 12
> call 4660
> mov r2, r10
> addi r2, -8
> mov r1, 2 <--- we want 1 here.
> mov r3, 8
> call 4660
> mov r0, 0
> ret
>
> .comm __str_my_str,8,4 # @__str_my_str
> .comm __str_my_str2,12,4 # @__str_my_str2
>
>
> Conclusion: llvm::Intrinsic::eh_typeid_for is not on the right
> direction...
>
> Thank you.
More information about the llvm-dev
mailing list