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

[Y Song via cfe-dev]

On Mon, Jun 14, 2021 at 6:44 PM David Blaikie <dblaikie at gmail.com> wrote:
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> On Mon, Jun 14, 2021 at 4:54 PM David Rector <davrecthreads at gmail.com> wrote:
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>> On Jun 14, 2021, at 5:33 PM, Y Song via cfe-dev <cfe-dev at lists.llvm.org> wrote:
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>> On Mon, Jun 14, 2021 at 1:25 PM David Blaikie <dblaikie at gmail.com> wrote:
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>> On Mon, Jun 14, 2021 at 12:25 PM Y Song <ys114321 at gmail.com> wrote:
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>> On Fri, Jun 11, 2021 at 9:59 AM Alexei Starovoitov
>> <alexei.starovoitov at gmail.com> wrote:
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>> On Fri, Jun 11, 2021 at 07:17:32AM -0400, Aaron Ballman wrote:
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>> On Thu, Jun 10, 2021 at 8:47 PM Alexei Starovoitov
>> <alexei.starovoitov at gmail.com> wrote:
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>> On Thu, Jun 10, 2021 at 12:42 PM David Blaikie <dblaikie at gmail.com> wrote:
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>> Any suggestions/preferences for the spelling, Aaron?
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>> I don't know this domain particularly well, so takes these suggestions
>> with a giant grain of salt:
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>> 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.
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>> 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
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>> A bit more bike shedding colors...
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>> 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.
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>> 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.
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>> __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.
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>> Maybe we can name it as "bpf_tag" as it is a "tag" for "bpf" use case?
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>> David, in one of your early emails, you mentioned:
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>> ===
>> 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.
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>> Do you have any suggestions on what name to pick?
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>> Nah, not especially. bpf_tag sounds OK-ish to me if it suits you.
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>> The more generic the better IMO.  And, the less the need to parse string literals the better.
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>> Why not simply `__attribute__((debuginfo("arg1", "arg2", ...)))`, e.g.:
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>> ```
>> #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") { ... }
>> ```
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> Unless Paul & Adrian, etc chime in in agreement of a more general name, like 'debuginfo', I'm inclined to avoid that/go with something bpf specific until there's a broader use case/proposal, something we might be able to/want to encourage GCC to support too. Otherwise we're taking a pretty broad attribute name & choosing its behavior when we don't necessarily have a lot of leverage if GCC ends up using that name for something else.
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> & as for separate strings - maybe, but I'm not sure what that'll look like in the resulting DWARF, what sort of form would you propose using to encode that? (same question below \/)
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>> 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.
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>> 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.
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>> How do you want the above to be represented in dwarf?
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>> 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.
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> What DWARF form were you thinking of using for this? There isn't a built in form that provides encoding for multiple delimited/separated strings that I know of.

Actually I have not looked at the details on how to implement multiple
separated strings yet. Since you are mentioning there exists no such a
built-in form and the attribute is bpf specific, I will then just go
to one string only approach (e.g. "str1;str2" where ";" is the
delimiter). I just checked linux:include/linux/compiler_*.h, it is
possible "," may appear in some attributes, so I will use ";" as the
delimiter. Thanks for the clarification!

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>> 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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