[llvm-dev] [cfe-dev] put "str" in __attribute__((annotate("str"))) to dwarf

[David Rector via llvm-dev]

> On Jun 14, 2021, at 5:33 PM, Y Song via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> On Mon, Jun 14, 2021 at 1:25 PM David Blaikie <dblaikie at gmail.com> wrote:
>> 
>> 
>> 
>> On Mon, Jun 14, 2021 at 12:25 PM Y Song <ys114321 at gmail.com> wrote:
>>> 
>>> On Fri, Jun 11, 2021 at 9:59 AM Alexei Starovoitov
>>> <alexei.starovoitov at gmail.com> wrote:
>>>> 
>>>> On Fri, Jun 11, 2021 at 07:17:32AM -0400, Aaron Ballman wrote:
>>>>> On Thu, Jun 10, 2021 at 8:47 PM Alexei Starovoitov
>>>>> <alexei.starovoitov at gmail.com> wrote:
>>>>>> 
>>>>>> On Thu, Jun 10, 2021 at 12:42 PM David Blaikie <dblaikie at gmail.com> wrote:
>>>>>>> 
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> Any suggestions/preferences for the spelling, Aaron?
>>>>>>>> 
>>>>>>>> I don't know this domain particularly well, so takes these suggestions
>>>>>>>> with a giant grain of salt:
>>>>>>>> 
>>>>>>>> If the concept is specific to DWARF and you don't think it'll need to
>>>>>>>> extend into other debug formats, you could go with `dwarf_annotate`.
>>>>>>>> If it's not really a DWARF thing but is more about B[P|T]F, then
>>>>>>>> `btf_annotate`  or `bpf_annotate` could work, but those may be a bit
>>>>>>>> mysterious to folks outside of the domain. If it's a generic debug
>>>>>>>> info concept, probably `debug_info_annotate` or something.
>>>>>>> 
>>>>>>> 
>>>>>>> Arguably it can/could be a generic debug info or dwarf thing, but for now we don't have any use for it other than to squirrel info along to BTF/BPF so I'm on the fence about which prefix to use exactly
>>>>>>> 
>>>>>> 
>>>>>> A bit more bike shedding colors...
>>>>>> 
>>>>>> The __rcu and __user annations might be used by the clang itself eventually.
>>>>>> Currently the "sparse" tool is doing this analysis and warns users
>>>>>> when __rcu pointer is incorrectly accessed in the kernel C code.
>>>>>> If clang can do that directly that could be a huge selling point
>>>>>> for folks to switch from gcc to clang for kernel builds.
>>>>>> The front-end would treat such annotations as arbitrary string, but
>>>>>> special "building-linux-kernel-pass" would interpret the semantical context.
>>>>> 
>>>>> Are __rcu and __user annotations notionally distinct things from bpf
>>>>> (and perhaps each other as well)? Distinct enough that it would make
>>>>> sense to use a different attribute name for user source for each need?
>>>>> I suspect the answer is yes given that the existing annotations have
>>>>> their own names which are distinct, but I don't know this domain
>>>>> enough to be sure.
>>>> 
>>>> __rcu and __user don't overlap. __rcu is not a single annotation though.
>>>> It's a combination of annotations in pointers, functions, macros.
>>>> Some functions have:
>>>> __acquires(rcu)
>>>> another function might have:
>>>> __acquires(rcu_bh)
>>>> There are several flavors of the RCU in the kernel.
>>>> So single __attribute__((rcu_annotate("foo"))) won't work even within RCU scope.
>>>> But if we do:
>>>> struct foo {
>>>>  void * __attribute__((tag("ptr.rcu_bh")) ptr;
>>>> };
>>>> int bar(int) __attribute__((tag("acquires.rcu_bh")) { ... }
>>>> int baz(int) __attribute__((tag("releases.rcu_bh")) { ... }
>>>> int qux(int) __attribute__((tag("acquires.rcu_sched")) { ... }
>>>> ...
>>>> The clang pass can parse these strings and correlate one tag to another.
>>>> RCU flavors come and go, so clang cannot hard code the names.
>>> 
>>> Maybe we can name it as "bpf_tag" as it is a "tag" for "bpf" use case?
>>> 
>>> David, in one of your early emails, you mentioned:
>>> 
>>> ===
>>> Arguably it can/could be a generic debug info or dwarf thing, but for
>>> now we don't have any use for it other than to squirrel info along to
>>> BTF/BPF so I'm on the fence about which prefix to use exactly
>>> ===
>>> 
>>> and suggests since it might be used in the future for non-bpf things,
>>> maybe the name could be a little more generic then bpf-specific.
>>> 
>>> Do you have any suggestions on what name to pick?
>> 
>> 
>> Nah, not especially. bpf_tag sounds OK-ish to me if it suits you.
> 

The more generic the better IMO.  And, the less the need to parse string literals the better.  

Why not simply `__attribute__((debuginfo("arg1", "arg2", ...)))`, e.g.:

```
#define BPF_TAG(...) __attribute__((debuginfo("bpf", __VA_ARGS__)))
struct foo {
 void * BPF_TAG("ptr","rcu","bh") ptr;
};
#define BPF_RCU_TAG(PFX, ...) BPF(PFX, "rcu", __VA_ARGS__)
int bar(int) BPF_RCU_TAG("acquires","bh") { ... }
int baz(int) BPF_RCU_TAG("releases","bh") { ... }
int qux(int) BPF_RCU_TAG("acquires","sched") { ... }
```

> Sounds good. I will use "bpf_tag" as the starting point now.
> Also, it is possible "bpf_tag" may appear multiple times for the same
> function, declaration etc.
> 
> For example,
>  #define __bpf_tag(s) __attribute__((bpf_tag(s)))
>  int g __bpf_tag("str1") __bpf_tag("str2");
> Let us say we introduced a LLVM vendor tag DWARF_AT_LLVM_bpf_tag.
> 
> How do you want the above to be represented in dwarf?
> 
> My current scheme is to put all bpf_tag's in a string, separated by ",".
> This will make things simpler. So the final output will be
>     DWARF_AT_LLVM_bpf_tag  "str1,str2"
> I may need to do a discussion with the kernel folks to use a different
> delimiter than ",", but we still represent all tags with ONE string.
> 
> But alternatively, it could be represented as a list of strings like
>   DWARF_AT_LLVM_bpf_tag
>             "str1"
>             "str2"
> is similar to DWARF_AT_location.
> 
> The first internal representation
>   DWARF_AT_LLVM_bpf_tag  "str1,str2"
> should be easier for IR/bitcode read/write and dwarf parsing.
> 
> What do you think?
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