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 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.
To avoid confusion, here is the full example (from the cover letter).
The difference in the results is clear in the DWARF.
Consider the following example:
#define __typetag1 __attribute__((btf_type_tag("type-tag-1")))
#define __typetag2 __attribute__((btf_type_tag("type-tag-2")))
#define __typetag3 __attribute__((btf_type_tag("type-tag-3")))
int __typetag1 * __typetag2 __typetag3 * g;
<var_decl 0x7ffff7547090 g
type <pointer_type 0x7ffff7548000
type <pointer_type 0x7ffff75097e0 type <integer_type 0x7ffff74495e8 int>
asm_written unsigned DI
size <integer_cst 0x7ffff743c450 constant 64>
unit-size <integer_cst 0x7ffff743c468 constant 8>
align:64 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff7450888
attributes <tree_list 0x7ffff75275c8
purpose <identifier_node 0x7ffff753a1e0 btf_type_tag>
value <tree_list 0x7ffff7527550
value <string_cst 0x7ffff75292e0 type <array_type
0x7ffff7509738>
readonly constant static "type-tag-3\000">>
chain <tree_list 0x7ffff75275a0 purpose <identifier_node
0x7ffff753a1e0 btf_type_tag>
value <tree_list 0x7ffff75274d8
value <string_cst 0x7ffff75292c0 type <array_type
0x7ffff7509738>
readonly constant static "type-tag-2\000">>>>
pointer_to_this <pointer_type 0x7ffff7509888>>
asm_written unsigned DI size <integer_cst 0x7ffff743c450 64> unit-size
<integer_cst 0x7ffff743c468 8>
align:64 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff7509930
attributes <tree_list 0x7ffff75275f0 purpose <identifier_node
0x7ffff753a1e0 btf_type_tag>
value <tree_list 0x7ffff7527438
value <string_cst 0x7ffff75292a0 type <array_type
0x7ffff7509738>
readonly constant static "type-tag-1\000">>>>
public static unsigned DI defer-output /home/dfaust/playpen/btf/annotate.c:29:42 size
<integer_cst 0x7ffff743c450 64> unit-size <integer_cst 0x7ffff743c468 8>
align:64 warn_if_not_align:0>
The current implementation produces the following DWARF:
<1><1e>: Abbrev Number: 4 (DW_TAG_variable)
<1f> DW_AT_name : g
<21> DW_AT_decl_file : 1
<22> DW_AT_decl_line : 6
<23> DW_AT_decl_column : 42
<24> DW_AT_type : <0x32>
<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)
<1><32>: Abbrev Number: 2 (DW_TAG_pointer_type)
<33> DW_AT_byte_size : 8
<33> DW_AT_type : <0x45>
<37> DW_AT_sibling : <0x45>
<2><3b>: Abbrev Number: 1 (User TAG value: 0x6000)
<3c> DW_AT_name : (indirect string, offset: 0x18): btf_type_tag
<40> DW_AT_const_value : (indirect string, offset: 0xc7): type-tag-1
<2><44>: Abbrev Number: 0
<1><45>: Abbrev Number: 2 (DW_TAG_pointer_type)
<46> DW_AT_byte_size : 8
<46> DW_AT_type : <0x61>
<4a> DW_AT_sibling : <0x61>
<2><4e>: Abbrev Number: 1 (User TAG value: 0x6000)
<4f> DW_AT_name : (indirect string, offset: 0x18): btf_type_tag
<53> DW_AT_const_value : (indirect string, offset: 0xd): type-tag-3
<2><57>: Abbrev Number: 1 (User TAG value: 0x6000)
<58> DW_AT_name : (indirect string, offset: 0x18): btf_type_tag
<5c> DW_AT_const_value : (indirect string, offset: 0xd2): type-tag-2
<2><60>: Abbrev Number: 0
<1><61>: Abbrev Number: 5 (DW_TAG_base_type)
<62> DW_AT_byte_size : 4
<63> DW_AT_encoding : 5 (signed)
<64> DW_AT_name : int
<1><68>: Abbrev Number: 0
This does not agree with the DWARF produced by LLVM/clang for the same case:
(clang 15.0.0 git 142501117a78080d2615074d3986fa42aa6a0734)
<1><1e>: Abbrev Number: 2 (DW_TAG_variable)
<1f> DW_AT_name : (indexed string: 0x3): g
<20> DW_AT_type : <0x29>
<24> DW_AT_external : 1
<24> DW_AT_decl_file : 0
<25> DW_AT_decl_line : 6
<26> DW_AT_location : 2 byte block: a1 0 ((Unknown location op
0xa1))
<1><29>: Abbrev Number: 3 (DW_TAG_pointer_type)
<2a> DW_AT_type : <0x35>
<2><2e>: Abbrev Number: 4 (User TAG value: 0x6000)
<2f> DW_AT_name : (indexed string: 0x5): btf_type_tag
<30> DW_AT_const_value : (indexed string: 0x7): type-tag-2
<2><31>: Abbrev Number: 4 (User TAG value: 0x6000)
<32> DW_AT_name : (indexed string: 0x5): btf_type_tag
<33> DW_AT_const_value : (indexed string: 0x8): type-tag-3
<2><34>: Abbrev Number: 0
<1><35>: Abbrev Number: 3 (DW_TAG_pointer_type)
<36> DW_AT_type : <0x3e>
<2><3a>: Abbrev Number: 4 (User TAG value: 0x6000)
<3b> DW_AT_name : (indexed string: 0x5): btf_type_tag
<3c> DW_AT_const_value : (indexed string: 0x6): type-tag-1
<2><3d>: Abbrev Number: 0
<1><3e>: Abbrev Number: 5 (DW_TAG_base_type)
<3f> DW_AT_name : (indexed string: 0x4): int
<40> DW_AT_encoding : 5 (signed)
<41> DW_AT_byte_size : 4
<1><42>: Abbrev Number: 0
Because GCC produces BTF from the internal DWARF DIE tree, the BTF also differs.
This can be seen most obviously in the BTF type reference chains:
GCC
VAR (g) -> ptr -> tag1 -> ptr -> tag3 -> tag2 -> int
LLVM
VAR (g) -> ptr -> tag3 -> tag2 -> ptr -> tag1 -> int
