Re: [ping2][PATCH 0/8][RFC] Support BTF decl_tag and type_tag annotations

[Yonghong Song <yhs@fb.com>]

On 5/11/22 11:44 AM, David Faust wrote:
> 
> 
> On 5/10/22 22:05, Yonghong Song wrote:
>>
>>
>> On 5/10/22 8:43 PM, Yonghong Song wrote:
>>>
>>>
>>> On 5/6/22 2:18 PM, David Faust wrote:
>>>>
>>>>
>>>> On 5/5/22 16:00, Yonghong Song wrote:
>>>>>
>>>>>
>>>>> On 5/4/22 10:03 AM, David Faust wrote:
>>>>>>
>>>>>>
>>>>>> On 5/3/22 15:32, Joseph Myers wrote:
>>>>>>> On Mon, 2 May 2022, David Faust via Gcc-patches wrote:
>>>>>>>
>>>>>>>> Consider the following example:
>>>>>>>>
>>>>>>>>       #define __typetag1 __attribute__((btf_type_tag("tag1")))
>>>>>>>>       #define __typetag2 __attribute__((btf_type_tag("tag2")))
>>>>>>>>       #define __typetag3 __attribute__((btf_type_tag("tag3")))
>>>>>>>>
>>>>>>>>       int __typetag1 * __typetag2 __typetag3 * g;
>>>>>>>>
>>>>>>>> The expected behavior is that 'g' is "a pointer with tags 'tag2' 
>>>>>>>> and
>>>>>>>> 'tag3',
>>>>>>>> to a pointer with tag 'tag1' to an int". i.e.:
>>>>>>>
>>>>>>> That's not a correct expectation for either GNU __attribute__ or
>>>>>>> C2x [[]]
>>>>>>> attribute syntax.  In either syntax, __typetag2 __typetag3 should
>>>>>>> apply to
>>>>>>> the type to which g points, not to g or its type, just as if you 
>>>>>>> had a
>>>>>>> type qualifier there.  You'd need to put the attributes (or 
>>>>>>> qualifier)
>>>>>>> after the *, not before, to make them apply to the pointer type.  
>>>>>>> See
>>>>>>> "Attribute Syntax" in the GCC manual for how the syntax is 
>>>>>>> defined for
>>>>>>> GNU
>>>>>>> attributes and deduce in turn, for each subsequence of the tokens
>>>>>>> matching
>>>>>>> the syntax for some kind of declarator, what the type for "T D1"
>>>>>>> would be
>>>>>>> as defined there and in the C standard, as deduced from the type for
>>>>>>> "T D"
>>>>>>> for a sub-declarator D.
>>>>>>>    >> But GCC's attribute parsing produces a variable 'g' which 
>>>>>>> is "a
>>>>>> pointer with
>>>>>>>> tag 'tag1' to a pointer with tags 'tag2' and 'tag3' to an int", 
>>>>>>>> i.e.
>>>>>>>
>>>>>>> In GNU syntax, __typetag1 applies to the declaration, whereas in C2x
>>>>>>> syntax it applies to int.  Again, if you wanted it to apply to the
>>>>>>> pointer
>>>>>>> type it would need to go after the * not before.
>>>>>>>
>>>>>>> If you are concerned with the fine details of what construct an
>>>>>>> attribute
>>>>>>> appertains to, I recommend using C2x syntax not GNU syntax.
>>>>>>>
>>>>>>
>>>>>> Joseph, thank you! This is very helpful. My understanding of the 
>>>>>> syntax
>>>>>> was not correct.
>>>>>>
>>>>>> (Actually, I made a bad mistake in paraphrasing this example from the
>>>>>> discussion of it in the series cover letter. But, the reason why 
>>>>>> it is
>>>>>> incorrect is the same.)
>>>>>>
>>>>>>
>>>>>> Yonghong, is the specific ordering an expectation in BPF programs or
>>>>>> other users of the tags?
>>>>>
>>>>> This is probably a language writing issue. We are saying tags only
>>>>> apply to pointer. We probably should say it only apply to pointee.
>>>>>
>>>>> $ cat t.c
>>>>> int const *ptr;
>>>>>
>>>>> the llvm ir debuginfo:
>>>>>
>>>>> !5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !6, size: 64)
>>>>> !6 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !7)
>>>>> !7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
>>>>>
>>>>> We could replace 'const' with a tag like below:
>>>>>
>>>>> int __attribute__((btf_type_tag("tag"))) *ptr;
>>>>>
>>>>> !5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !6, size: 64,
>>>>> annotations: !7)
>>>>> !6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
>>>>> !7 = !{!8}
>>>>> !8 = !{!"btf_type_tag", !"tag"}
>>>>>
>>>>> In the above IR, we generate annotations to pointer_type because
>>>>> we didn't invent a new DI type for encode btf_type_tag. But it is
>>>>> totally okay to have IR looks like
>>>>>
>>>>> !5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !11, size: 64)
>>>>> !11 = !DIBtfTypeTagType(..., baseType: !6, name: !"Tag")
>>>>> !6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
>>>>>
>>>> OK, thanks.
>>>>
>>>> There is still the question of why the DWARF generated for this case
>>>> that I have been concerned about:
>>>>
>>>>     int __typetag1 * __typetag2 __typetag3 * g;
>>>>
>>>> differs between GCC (with this series) and clang. After studying it,
>>>> GCC is doing with the attributes exactly as is described in the
>>>> Attribute Syntax portion of the GCC manual where the GNU syntax is
>>>> described. I do not think there is any problem here.
>>>>
>>>> So the difference in DWARF suggests to me that clang is not handling
>>>> the GNU attribute syntax in this particular case correctly, since it
>>>> seems to be associating __typetag2 and __typetag3 to g's type rather
>>>> than the type to which it points.
>>>>
>>>> I am not sure whether for the use purposes of the tags this difference
>>>> is very important, but it is worth noting.
>>>>
>>>>
>>>> As Joseph suggested, it may be better to encourage users of these tags
>>>> to use the C2x attribute syntax if they are concerned with precisely
>>>> which construct the tag applies.
>>>>
>>>> This would also be a way around any issues in handling the attributes
>>>> due to the GNU syntax.
>>>>
>>>> I tried a few test cases using C2x syntax BTF type tags with a
>>>> clang-15 build, but ran into some issues (in particular, some of the
>>>> tag attributes being ignored altogether). I couldn't find confirmation
>>>> whether C2x attribute syntax is fully supported in clang yet, so maybe
>>>> this isn't expected to work. Do you know whether the C2x syntax is
>>>> fully supported in clang yet?
>>>
>>> Actually, I don't know either. But since the btf decl_tag and type_tag
>>> are also used to compile linux kernel and the minimum compiler version
>>> to compile kernel is gcc5.1 and clang11. I am not sure whether gcc5.1
>>> supports c2x or not, I guess probably not. So I think we most likely
>>> cannot use c2x syntax.
>>
>> Okay, I think we can guard btf_tag's with newer compiler versions.
>> What kind of c2x syntax you intend to use? I can help compile kernel
>> with that syntax and llvm15 to see what is the issue and may help
>> fix it in clang if possible.
> 
> 
> I am thinking to use the [[]] C2x standard attribute syntax. The syntax 
> makes it quite clear to which entity each attribute applies, and in my 
> opinion is a little more intuitive/less surprising too.
> 
> It's documented here (PDF):
>    
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2731.pdf 
> 
> See sections 6.7.11 for the syntax and 6.7.6 for declarations. Section 
> 6.7.6.1 specifically describes using the attribute syntax with pointer 
> declarators.
> 
> The attribute syntax itself for BTF tags is:
>    [[clang::btf_type_tag("tag1")]]
> or
>    [[gnu::btf_type_tag("tag1")]]
> 
> 
> I am also looking into whether, with the C2x syntax, we really need two
> separate attributes (type_tag and decl_tag) at the language level. It 
> might be possible with C2x syntax to use just one language attribute 
> (e.g. just btf_tag).
> 
> 
> A simple declaration for a tagged pointer to an int:
> 
>    int * [[gnu::btf_type_tag("tag1")]] x;
> 
> And for the example from this thread:
> 
>    #define __typetag1 [[gnu::btf_type_tag("type-tag-1")]]
>    #define __typetag2 [[gnu::btf_type_tag("type-tag-2")]]
>    #define __typetag3 [[gnu::btf_type_tag("type-tag-3")]]
> 
>    int * __typetag1 * __typetag2 __typetag3 g;
> 
> Here each tag applies to the preceding pointer, so the result is 
> unsurprising.
> 
> Actually, this is where I found something that looks like an issue with 
> the C2x attribute syntax in clang. The tags 2 and 3 go missing, but with 
> no warning nor other indication.
> 
> Compiling this example with gcc:
> 
> $ ~/toolchains/bpf/bin/bpf-unknown-none-gcc -c -gbtf -gdwarf c2x.c -o 
> c2x.o --std=c2x
> $ ~/toolchains/llvm/bin/llvm-dwarfdump c2x.o
> 
> 0x0000000c: DW_TAG_compile_unit
>                DW_AT_producer    ("GNU C2X 12.0.1 20220401 
> (experimental) -gbtf -gdwarf -std=c2x")
>                DW_AT_language    (DW_LANG_C11)
>                DW_AT_name    ("c2x.c")
>                DW_AT_comp_dir    ("/home/dfaust/playpen/btf/tags")
>                DW_AT_stmt_list    (0x00000000)
> 
> 0x0000001e:   DW_TAG_variable
>                  DW_AT_name    ("g")
>                  DW_AT_decl_file    ("/home/dfaust/playpen/btf/tags/c2x.c")
>                  DW_AT_decl_line    (16)
>                  DW_AT_decl_column    (0x2a)
>                  DW_AT_type    (0x00000032 "int **")
>                  DW_AT_external    (true)
>                  DW_AT_location    (DW_OP_addr 0x0)
> 
> 0x00000032:   DW_TAG_pointer_type
>                  DW_AT_byte_size    (8)
>                  DW_AT_type    (0x0000004e "int *")
>                  DW_AT_sibling    (0x0000004e)
> 
> 0x0000003b:     DW_TAG_LLVM_annotation
>                    DW_AT_name    ("btf_type_tag")
>                    DW_AT_const_value    ("type-tag-3")
> 
> 0x00000044:     DW_TAG_LLVM_annotation
>                    DW_AT_name    ("btf_type_tag")
>                    DW_AT_const_value    ("type-tag-2")
> 
> 0x0000004d:     NULL
> 
> 0x0000004e:   DW_TAG_pointer_type
>                  DW_AT_byte_size    (8)
>                  DW_AT_type    (0x00000061 "int")
>                  DW_AT_sibling    (0x00000061)
> 
> 0x00000057:     DW_TAG_LLVM_annotation
>                    DW_AT_name    ("btf_type_tag")
>                    DW_AT_const_value    ("type-tag-1")
> 
> 0x00000060:     NULL
> 
> 0x00000061:   DW_TAG_base_type
>                  DW_AT_byte_size    (0x04)
>                  DW_AT_encoding    (DW_ATE_signed)
>                  DW_AT_name    ("int")
> 
> 0x00000068:   NULL
> 
> 
> and with clang (changing the attribute prefix to clang:: appropriately):
> 
> $ ~/toolchains/llvm/bin/clang -target bpf -g -c c2x.c -o c2x.o.ll --std=c2x
> $ ~/toolchains/llvm/bin/llvm-dwarfdump c2x.o.ll
> 
> 0x0000000c: DW_TAG_compile_unit
>                DW_AT_producer    ("clang version 15.0.0 
> (https://github.com/llvm/llvm-project.git 
> f80e369f61ebd33dd9377bb42fcab64d17072b18)")
>                DW_AT_language    (DW_LANG_C99)
>                DW_AT_name    ("c2x.c")
>                DW_AT_str_offsets_base    (0x00000008)
>                DW_AT_stmt_list    (0x00000000)
>                DW_AT_comp_dir    ("/home/dfaust/playpen/btf/tags")
>                DW_AT_addr_base    (0x00000008)
> 
> 0x0000001e:   DW_TAG_variable
>                  DW_AT_name    ("g")
>                  DW_AT_type    (0x00000029 "int **")
>                  DW_AT_external    (true)
>                  DW_AT_decl_file    ("/home/dfaust/playpen/btf/tags/c2x.c")
>                  DW_AT_decl_line    (12)
>                  DW_AT_location    (DW_OP_addrx 0x0)
> 
> 0x00000029:   DW_TAG_pointer_type
>                  DW_AT_type    (0x00000032 "int *")
> 
> 0x0000002e:     DW_TAG_LLVM_annotation
>                    DW_AT_name    ("btf_type_tag")
>                    DW_AT_const_value    ("type-tag-1")
> 
> 0x00000031:     NULL
> 
> 0x00000032:   DW_TAG_pointer_type
>                  DW_AT_type    (0x00000037 "int")
> 
> 0x00000037:   DW_TAG_base_type
>                  DW_AT_name    ("int")
>                  DW_AT_encoding    (DW_ATE_signed)
>                  DW_AT_byte_size    (0x04)
> 
> 0x0000003b:   NULL

Thanks. I checked with current clang. The generated code looks
like above. Basically, for code like below

    #define __typetag1 [[clang::btf_type_tag("type-tag-1")]]
    #define __typetag2 [[clang::btf_type_tag("type-tag-2")]]
    #define __typetag3 [[clang::btf_type_tag("type-tag-3")]]

    int * __typetag1 * __typetag2 __typetag3 g;

The IR type looks like
    __typetag3 -> __typetag2 -> * (ptr1) -> __typetag1 -> * (ptr2) -> int

The IR is similar to what we did if using 
__attribute__((btf_type_tag(""))), but their
semantic interpretation is quite different.
For example, with c2x format,
    __typetag1 applies to ptr2
with __attribute__ format, it applies pointee of ptr1.

But more importantly, c2x format is incompatible with
the usage of linux kernel. The following are a bunch of kernel
__user usages. Here, __user intends to be replaced with a btf_type_tag.

vfio_pci_core.h:        ssize_t (*rw)(struct vfio_pci_core_device *vdev, 
char __user *buf,
vfio_pci_core.h:                                  char __user *buf, 
size_t count,
vfio_pci_core.h:extern ssize_t vfio_pci_bar_rw(struct 
vfio_pci_core_device *vdev, char __user *buf,
vfio_pci_core.h:extern ssize_t vfio_pci_vga_rw(struct 
vfio_pci_core_device *vdev, char __user *buf,
vfio_pci_core.h:                                      char __user *buf, 
size_t count,
vfio_pci_core.h:                                void __user *arg, size_t 
argsz);
vfio_pci_core.h:ssize_t vfio_pci_core_read(struct vfio_device 
*core_vdev, char __user *buf,
vfio_pci_core.h:ssize_t vfio_pci_core_write(struct vfio_device 
*core_vdev, const char __user *buf,
vringh.h:                    vring_desc_t __user *desc,
vringh.h:                    vring_avail_t __user *avail,
vringh.h:                    vring_used_t __user *used);
vt_kern.h:int con_set_cmap(unsigned char __user *cmap);
vt_kern.h:int con_get_cmap(unsigned char __user *cmap);
vt_kern.h:int con_set_trans_old(unsigned char __user * table);
vt_kern.h:int con_get_trans_old(unsigned char __user * table);
vt_kern.h:int con_set_trans_new(unsigned short __user * table);
vt_kern.h:int con_get_trans_new(unsigned short __user * table);

You can see, we will not able to simply replace __user
with [[clang::btf_type_tag("user")]] because it won't work
according to c2x expectations.

> 
> 
>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>> This example comes from my testing against clang to check that the 
>>>>>> BTF
>>>>>> generated by both toolchains is compatible. In this case we get
>>>>>> different results when using the GNU attribute syntax.
>>>>>>
>> [...]