Re: kernel sparse annotations vs. compiler attributes and debug_annotate_{type, decl} WAS: Re: [PATCH 0/9] Add debug_annotate attributes

[Yonghong Song <yhs@fb.com>]

On 6/21/22 9:12 AM, Jose E. Marchesi wrote:
> 
>> On 6/17/22 10:18 AM, Jose E. Marchesi wrote:
>>> Hi Yonghong.
>>>
>>>> On 6/15/22 1:57 PM, David Faust wrote:
>>>>>
>>>>> On 6/14/22 22:53, Yonghong Song wrote:
>>>>>>
>>>>>>
>>>>>> On 6/7/22 2:43 PM, David Faust wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> This patch series adds support for:
>>>>>>>
>>>>>>> - Two new C-language-level attributes that allow to associate (to "annotate" or
>>>>>>>       to "tag") particular declarations and types with arbitrary strings. As
>>>>>>>       explained below, this is intended to be used to, for example, characterize
>>>>>>>       certain pointer types.
>>>>>>>
>>>>>>> - The conveyance of that information in the DWARF output in the form of a new
>>>>>>>       DIE: DW_TAG_GNU_annotation.
>>>>>>>
>>>>>>> - The conveyance of that information in the BTF output in the form of two new
>>>>>>>       kinds of BTF objects: BTF_KIND_DECL_TAG and BTF_KIND_TYPE_TAG.
>>>>>>>
>>>>>>> All of these facilities are being added to the eBPF ecosystem, and support for
>>>>>>> them exists in some form in LLVM.
>>>>>>>
>>>>>>> Purpose
>>>>>>> =======
>>>>>>>
>>>>>>> 1)  Addition of C-family language constructs (attributes) to specify free-text
>>>>>>>         tags on certain language elements, such as struct fields.
>>>>>>>
>>>>>>>         The purpose of these annotations is to provide additional information about
>>>>>>>         types, variables, and function parameters of interest to the kernel. A
>>>>>>>         driving use case is to tag pointer types within the linux kernel and eBPF
>>>>>>>         programs with additional semantic information, such as '__user' or '__rcu'.
>>>>>>>
>>>>>>>         For example, consider the linux kernel function do_execve with the
>>>>>>>         following declaration:
>>>>>>>
>>>>>>>           static int do_execve(struct filename *filename,
>>>>>>>              const char __user *const __user *__argv,
>>>>>>>              const char __user *const __user *__envp);
>>>>>>>
>>>>>>>         Here, __user could be defined with these annotations to record semantic
>>>>>>>         information about the pointer parameters (e.g., they are user-provided) in
>>>>>>>         DWARF and BTF information. Other kernel facilites such as the eBPF verifier
>>>>>>>         can read the tags and make use of the information.
>>>>>>>
>>>>>>> 2)  Conveying the tags in the generated DWARF debug info.
>>>>>>>
>>>>>>>         The main motivation for emitting the tags in DWARF is that the Linux kernel
>>>>>>>         generates its BTF information via pahole, using DWARF as a source:
>>>>>>>
>>>>>>>             +--------+  BTF                  BTF   +----------+
>>>>>>>             | pahole |-------> vmlinux.btf ------->| verifier |
>>>>>>>             +--------+                             +----------+
>>>>>>>                 ^                                        ^
>>>>>>>                 |                                        |
>>>>>>>           DWARF |                                    BTF |
>>>>>>>                 |                                        |
>>>>>>>              vmlinux                              +-------------+
>>>>>>>              module1.ko                           | BPF program |
>>>>>>>              module2.ko                           +-------------+
>>>>>>>                ...
>>>>>>>
>>>>>>>         This is because:
>>>>>>>
>>>>>>>         a)  Unlike GCC, LLVM will only generate BTF for BPF programs.
>>>>>>>
>>>>>>>         b)  GCC can generate BTF for whatever target with -gbtf, but there is no
>>>>>>>             support for linking/deduplicating BTF in the linker.
>>>>>>>
>>>>>>>         In the scenario above, the verifier needs access to the pointer tags of
>>>>>>>         both the kernel types/declarations (conveyed in the DWARF and translated
>>>>>>>         to BTF by pahole) and those of the BPF program (available directly in BTF).
>>>>>>>
>>>>>>>         Another motivation for having the tag information in DWARF, unrelated to
>>>>>>>         BPF and BTF, is that the drgn project (another DWARF consumer) also wants
>>>>>>>         to benefit from these tags in order to differentiate between different
>>>>>>>         kinds of pointers in the kernel.
>>>>>>>
>>>>>>> 3)  Conveying the tags in the generated BTF debug info.
>>>>>>>
>>>>>>>         This is easy: the main purpose of having this info in BTF is for the
>>>>>>>         compiled eBPF programs. The kernel verifier can then access the tags
>>>>>>>         of pointers used by the eBPF programs.
>>>>>>>
>>>>>>>
>>>>>>> For more information about these tags and the motivation behind them, please
>>>>>>> refer to the following linux kernel discussions:
>>>>>>>
>>>>>>>       https://lore.kernel.org/bpf/20210914223004.244411-1-yhs@fb.com/
>>>>>>>       https://lore.kernel.org/bpf/20211012164838.3345699-1-yhs@fb.com/
>>>>>>>       https://lore.kernel.org/bpf/20211112012604.1504583-1-yhs@fb.com/
>>>>>>>
>>>>>>>
>>>>>>> Implementation Overview
>>>>>>> =======================
>>>>>>>
>>>>>>> To enable these annotations, two new C language attributes are added:
>>>>>>> __attribute__((debug_annotate_decl("foo"))) and
>>>>>>> __attribute__((debug_annotate_type("bar"))). Both attributes accept a single
>>>>>>> arbitrary string constant argument, which will be recorded in the generated
>>>>>>> DWARF and/or BTF debug information. They have no effect on code generation.
>>>>>>>
>>>>>>> Note that we are not using the same attribute names as LLVM (btf_decl_tag and
>>>>>>> btf_type_tag, respectively). While these attributes are functionally very
>>>>>>> similar, they have grown beyond purely BTF-specific uses, so inclusion of "btf"
>>>>>>> in the attribute name seems misleading.
>>>>>>>
>>>>>>> DWARF support is enabled via a new DW_TAG_GNU_annotation. When generating DWARF,
>>>>>>> declarations and types will be checked for the corresponding attributes. If
>>>>>>> present, a DW_TAG_GNU_annotation DIE will be created as a child of the DIE for
>>>>>>> the annotated type or declaration, one for each tag. These DIEs link the
>>>>>>> arbitrary tag value to the item they annotate.
>>>>>>>
>>>>>>> For example, the following variable declaration:
>>>>>>>
>>>>>>>       #define __typetag1 __attribute__((debug_annotate_type ("typetag1")))
>>>>>>>
>>>>>>>       #define __decltag1 __attribute__((debug_annotate_decl ("decltag1")))
>>>>>>>       #define __decltag2 __attribute__((debug_annotate_decl ("decltag2")))
>>>>>>>
>>>>>>>       int * __typetag1 x __decltag1 __decltag2;
>>>>>>
>>>>>> Based on the above example
>>>>>>             static int do_execve(struct filename *filename,
>>>>>>               const char __user *const __user *__argv,
>>>>>>               const char __user *const __user *__envp);
>>>>>>
>>>>>> Should the above example should be the below?
>>>>>>         int __typetag1 * x __decltag1 __decltag2
>>>>>>
>>>>> This example is not related to the one above. It is just meant to
>>>>> show the behavior of both attributes. My apologies for not making
>>>>> that clear.
>>>>
>>>> Okay, it should be fine if the dwarf debug_info is shown.
>>>>
>>>>>
>>>>>>>
>>>>>>> Produces the following DWARF information:
>>>>>>>
>>>>>>>      <1><1e>: Abbrev Number: 3 (DW_TAG_variable)
>>>>>>>         <1f>   DW_AT_name        : x
>>>>>>>         <21>   DW_AT_decl_file   : 1
>>>>>>>         <22>   DW_AT_decl_line   : 7
>>>>>>>         <23>   DW_AT_decl_column : 18
>>>>>>>         <24>   DW_AT_type        : <0x49>
>>>>>>>         <28>   DW_AT_external    : 1
>>>>>>>         <28>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0 	(DW_OP_addr: 0)
>>>>>>>         <32>   DW_AT_sibling     : <0x49>
>>>>>>>      <2><36>: Abbrev Number: 1 (User TAG value: 0x6000)
>>>>>>>         <37>   DW_AT_name        : (indirect string, offset: 0xd6): debug_annotate_decl
>>>>>>>         <3b>   DW_AT_const_value : (indirect string, offset: 0xcd): decltag2
>>>>>>>      <2><3f>: Abbrev Number: 1 (User TAG value: 0x6000)
>>>>>>>         <40>   DW_AT_name        : (indirect string, offset: 0xd6): debug_annotate_decl
>>>>>>>         <44>   DW_AT_const_value : (indirect string, offset: 0x0): decltag1
>>>>>>>      <2><48>: Abbrev Number: 0
>>>>>>>      <1><49>: Abbrev Number: 4 (DW_TAG_pointer_type)
>>>>>>>         <4a>   DW_AT_byte_size   : 8
>>>>>>>         <4b>   DW_AT_type        : <0x5d>
>>>>>>>         <4f>   DW_AT_sibling     : <0x5d>
>>>>>>>      <2><53>: Abbrev Number: 1 (User TAG value: 0x6000)
>>>>>>>         <54>   DW_AT_name        : (indirect string, offset: 0x9): debug_annotate_type
>>>>>>>         <58>   DW_AT_const_value : (indirect string, offset: 0x1d): typetag1
>>>>>>>      <2><5c>: Abbrev Number: 0
>>>>>>>      <1><5d>: Abbrev Number: 5 (DW_TAG_base_type)
>>>>>>>         <5e>   DW_AT_byte_size   : 4
>>>>>>>         <5f>   DW_AT_encoding    : 5	(signed)
>>>>>>>         <60>   DW_AT_name        : int
>>>>>>>      <1><64>: Abbrev Number: 0
>>>>
>>>> This shows the info in .debug_abbrev. What I mean is to
>>>> show the related info in .debug_info section which seems more useful to
>>>> understand the relationships between different tags. Maybe this is due
>>>> to that I am not fully understanding what <1>/<2> means in <1><49> and
>>>> <2><53> etc.
>>> I think that dump actually shows .debug_info, with the abbrevs
>>> expanded...
>>> Anyway, it seems to us that the root of this problem is the fact the
>>> kernel sparse annotations, such as address_space(__user), are:
>>> 1) To be processed by an external kernel-specific tool (
>>>      https://sparse.docs.kernel.org/en/latest/annotations.html) and not a
>>>      C compiler, and therefore,
>>> 2) Not quite the same than compiler attributes (despite the way they
>>>      look.)  In particular, they seem to assume an ordering different than
>>>      of GNU attributes: in some cases given the same written order, they
>>>      refer to different things!.  Which is quite unfortunate :(
>>
>> Yes, currently __user/__kernel macros (implemented with address_space
>> attribute) are processed by macros.
>>
>>> Now, if I understood properly, you plan to change the definition of
>>> __user and __kernel in the kernel sources in order to generate the tag
>>> compiler attributes, correct?
>>
>> Right. The original __user definition likes:
>>    # define __user         __attribute__((noderef, address_space(__user)))
>>
>> The new attribute looks like
>>    # define BTF_TYPE_TAG(value) __attribute__((btf_type_tag(#value)))
>>    #  define __user        BTF_TYPE_TAG(user)
> 
> Ok I see.  So the kernel will stop using sparse attributes to implement
> __user and __kernel and start using compiler attributes for tags
> instead.
> 
>>> Is that the reason why LLVM implements what we assume to be the
>>> sparse
>>> ordering, and not the correct GNU attributes ordering, for the tag
>>> attributes?
>>
>> Note that __user attributes apply to pointee's and not pointers.
>> Just like
>>     const int *p;
>> the 'const' is not applied to pointer 'p', but the pointee of 'p'.
>>
>> What current llvm dwarf generation with
>>     pointer
>>       <--- btf_type_tag
>> is just ONE implementation. As I said earlier, I am okay to
>> have dwarf implementation like
>>     p->btf_type_tag->const->int.
>> If you can propose an implementation like this in dwarf. I can propose
>> to change implementation in llvm.
> 
> I think we are miscommunicating.
> 
> Looks like there is a divergence on what attributes apply to what
> language entities between the sparse compiler and GCC/LLVM.  How to
> represent that in DWARF is a different matter.
> 
> For this example:
> 
>    int __typetag1 * __typetag2 __typetag3 * g;
> 
> a) GCC associates __typetag1 with the pointer-to-pointer-to-int.
> b) LLVM associates __typetag1 to pointer-to-int.
> 
> Where:
> 
> a) Is the expected behavior of a compiler attributes, as documented in
>     the GCC manual.
> 
> b) Is presumably what the sparse compiler expects, but _not_ the
>     ordering expected for a compiler GNU attribute.
> 
> So, if the kernel source __user and __kernel annotations (which
> currently expand to sparse attributes) follow the sparse ordering, and
> you want to implement __user and __kernel in terms of compiler
> attributes instead (the annotation attributes) then you will have to:
> 
> 1) Fix LLVM to implement the usual ordering for these attributes and
> 2) fix the kernel sources to use that ordering
> 
> [Incidentally, the same applies to another "ex-sparse" attribute you
>   have in the kernel and also implemented in LLVM with a weird ordering:
>   the address_space attribute.]
> 
> For 2), it may be possible to write a coccinnelle script to generate the
> patch...

I don't think (2) (to change kernel source for different attr ordering)
will work. So the only thing we can do is in compiler/pahole except 
macro replacement in kernel.

> 
> Does this make sense?
> 
>>> If that is so, we have quite a problem here: I don't think we can
>>> change
>>> the way GCC handles GNU-like attributes just because the kernel sources
>>> want to hook on these __user/__kernel sparse annotations to generate the
>>> compiler tags, even if we could mayhaps get GCC to handle
>>> debug_annotate_type and debug_annotate_decl differently.  Some would say
>>> doing so would perpetuate the mistake instead of fixing it...
>>> Is my understanding correct?
>>
>> Let us just say that the btf_type_tag attribute applies to pointees.
>> Does this help?
>>
>>>
>>>>>>
>>>>>> Maybe you can also show what dwarf debug_info looks like
>>>>> I am not sure what you mean. This is the .debug_info section as output
>>>>> by readelf -w. I did trim some information not relevant to the discussion
>>>>> such as the DW_TAG_compile_unit DIE, for brevity.
>>>>>
>>>>>>
>>>>>>>
>>>>>>> In the case of BTF, the annotations are recorded in two type kinds recently
>>>>>>> added to the BTF specification: BTF_KIND_DECL_TAG and BTF_KIND_TYPE_TAG.
>>>>>>> The above example declaration prodcues the following BTF information:
>>>>>>>
>>>>>>> [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
>>>>>>> [2] PTR '(anon)' type_id=3
>>>>>>> [3] TYPE_TAG 'typetag1' type_id=1
>>>>>>> [4] DECL_TAG 'decltag1' type_id=6 component_idx=-1
>>>>>>> [5] DECL_TAG 'decltag2' type_id=6 component_idx=-1
>>>>>>> [6] VAR 'x' type_id=2, linkage=global
>>>>>>> [7] DATASEC '.bss' size=0 vlen=1
>>>>>>> 	type_id=6 offset=0 size=8 (VAR 'x')
>>>>>>>
>>>>>>>
>>>>>> [...]