Re: [PATCH 0/9] Add debug_annotate attributes

[Yonghong Song <yhs@fb.com>]

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

> 
> 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

Maybe you can also show what dwarf debug_info looks like?

> 
> 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')
> 
> 
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