Re: [PAHOLE v4 2/3] dwarf_loader: Add support for DW_TAG_GNU_annotation

[Yonghong Song <yonghong.song@linux.dev>]

On 6/3/26 1:54 PM, Vineet Gupta wrote:
> Hi Yonghong,
>
> Thanks for reviewing the patches.
>
> On 6/3/26 1:08 PM, Yonghong Song wrote:
>>> [3] 
>>> https://lore.kernel.org/bpf/20260601183511.594100-2-vineet.gupta@linux.dev/
>>> [2] 
>>> https://lore.kernel.org/bpf/20260528223616.2035618-2-vineet.gupta@linux.dev/
>>> [1] 
>>> https://lore.kernel.org/bpf/20260526181818.4159927-2-vineet.gupta@linux.dev/
>> I download and build gcc-16.1 which will be used for gcc compilation.
>> I did some experiments with this patch set for both type tag and decl 
>> tag.
>> See below.
>
> FWIW, you definitely need a fix [1] on top of gcc 16.1.
> The patch explains the reasoning and did cause some missing entries in 
> type chain.
> I haven't checked if this changes anything specific to issues you 
> point below.
>
> [1] https://gcc.gnu.org/pipermail/bpf/2026-May/000147.html

Thanks for the pointer. The fix is related to type tag. So I suspect
typedef and struct decl_tag support is not done yet in gcc.

>
>
>>
>> For type tag
>> ============
>>
>> $ cat type_tag.c
>> /* btf_type_tag test cases.
>>    *
>>    * btf_type_tag annotates a *type* and the tag becomes part of the 
>> type
>>    * chain in BTF (TYPE_TAG kind), e.g. used by the kernel for __user 
>> / __rcu
>>    * / __percpu style annotations.
>>    *
>>    * Placement matters: the tag is written *before* the '*' so it 
>> annotates
>>    * the pointee type.  This produces  PTR -> TYPE_TAG 'x' -> <pointee>.
>>    *
>>    * Build:  clang -O2 -g -target bpf -c type_tag.c -o type_tag.o
>>    *         /home/yhs/work/gcc-build/opt/gcc-16.1/bin/gcc -O2 -gbtf 
>> -g -c type_tag.c -o type_tag.o
>>    * Dump:   bpftool btf dump file type_tag.o
>>    */
>>
>> #define __type_tag(x) __attribute__((btf_type_tag(x)))
>>
>> /* a single type tag on a pointer's pointee */
>> int __type_tag("user") *user_ptr_var;
>>
>> /* stacked type tags: the chain is preserved, outermost first */
>> int __type_tag("tag1") __type_tag("tag2") *stacked_var;
>>
>> /* type tags inside struct members */
>> struct bar {
>>           char __type_tag("rcu") *name;
>>           int  __type_tag("percpu") *counter;
>> };
>>
>> /* type tag on a function parameter's pointee */
>> int read_user(int __type_tag("user") *p)
>> {
>>           return *p;
>> }
>>
>> /* keep things referenced */
>> int use(struct bar *b)
>> {
>>           return read_user(user_ptr_var) + (stacked_var ? 
>> *stacked_var : 0) +
>>                  (b->counter ? *b->counter : 0);
>> }
>>
>> $ /home/yhs/work/gcc-build/opt/gcc-16.1/bin/gcc -O2 -gbtf -g -c 
>> type_tag.c -o type_tag.o
>> $ bpftool btf dump file type_tag.o
>> [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
>> [2] PTR '(anon)' type_id=15
>> [3] PTR '(anon)' type_id=17
>> [4] STRUCT 'bar' size=16 vlen=2
>>           'name' type_id=6 bits_offset=0
>>           'counter' type_id=7 bits_offset=64
>> [5] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
>> [6] PTR '(anon)' type_id=18
>> [7] PTR '(anon)' type_id=19
>> [8] FUNC_PROTO '(anon)' ret_type_id=1 vlen=1
>>           'b' type_id=9
>> [9] PTR '(anon)' type_id=4
>> [10] FUNC_PROTO '(anon)' ret_type_id=1 vlen=1
>>           'p' type_id=2
>> [11] VAR 'stacked_var' type_id=3, linkage=global
>> [12] VAR 'user_ptr_var' type_id=2, linkage=global
>> [13] FUNC 'use' type_id=8 linkage=global
>> [14] FUNC 'read_user' type_id=10 linkage=global
>> [15] TYPE_TAG 'user' type_id=1
>> [16] TYPE_TAG 'tag1' type_id=1
>> [17] TYPE_TAG 'tag2' type_id=16
>> [18] TYPE_TAG 'rcu' type_id=5
>> [19] TYPE_TAG 'percpu' type_id=1
>> [20] DATASEC '.bss' size=0 vlen=2
>>           type_id=11 offset=0 size=8 (VAR 'stacked_var')
>>           type_id=12 offset=0 size=8 (VAR 'user_ptr_var')
>> $
>>
>> $ clang -O2 -g -target bpf -c type_tag.c -o type_tag.o
>> $ bpftool btf dump file type_tag.o
>> [1] TYPE_TAG 'user' type_id=3
>> [2] PTR '(anon)' type_id=1
>> [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
>> [4] FUNC_PROTO '(anon)' ret_type_id=3 vlen=1
>>           'p' type_id=2
>> [5] FUNC 'read_user' type_id=4 linkage=global
>> [6] PTR '(anon)' type_id=7
>> [7] STRUCT 'bar' size=16 vlen=2
>>           'name' type_id=9 bits_offset=0
>>           'counter' type_id=12 bits_offset=64
>> [8] TYPE_TAG 'rcu' type_id=10
>> [9] PTR '(anon)' type_id=8
>> [10] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
>> [11] TYPE_TAG 'percpu' type_id=3
>> [12] PTR '(anon)' type_id=11
>> [13] FUNC_PROTO '(anon)' ret_type_id=3 vlen=1
>>           'b' type_id=6
>> [14] FUNC 'use' type_id=13 linkage=global
>> [15] VAR 'user_ptr_var' type_id=2, linkage=global
>> [16] TYPE_TAG 'tag1' type_id=3
>> [17] TYPE_TAG 'tag2' type_id=16
>> [18] PTR '(anon)' type_id=17
>> [19] VAR 'stacked_var' type_id=18, linkage=global
>> [20] DATASEC '.bss' size=0 vlen=2
>>           type_id=15 offset=0 size=8 (VAR 'user_ptr_var')
>>           type_id=19 offset=0 size=8 (VAR 'stacked_var')
>> $
>>
>> So type tag matches between clang and gcc.
>
> Nice, thanks for cross-checking and confirming.
>
>
>> For decl tag
>> ============
>>
>> $ cat decl_tag.c
>> /* btf_decl_tag test cases.
>>    *
>>    * btf_decl_tag can be attached to:
>>    *   - global (incl. static) variables
>>    *   - functions
>>    *   - function parameters
>>    *   - struct/union types and their members
>>    *   - typedefs
>>    *
>>    * Build:  clang -O2 -target bpf -g -c decl_tag.c -o decl_tag.o
>>    *         /home/yhs/work/gcc-build/opt/gcc-16.1/bin/gcc -O2 -gbtf 
>> -g -c decl_tag.c -o decl_tag.o
>>    * Dump:   bpftool btf dump file decl_tag.o
>>    */
>>
>> #define __tag(x) __attribute__((btf_decl_tag(x)))
>>
>> /* tag on a global variable */
>> int global_var __tag("global_var_tag");
>>
>> /* tag on a static variable */
>> static int static_var __tag("static_var_tag");
>>
>> /* multiple tags on one declaration */
>> int multi_tag_var __tag("tag_a") __tag("tag_b");
>>
>> /* tag on struct type and its members */
>> struct foo {
>>           int a __tag("member_a_tag");
>>           int b __tag("member_b_tag");
>> } __tag("struct_foo_tag");
>>
>> /* tag on a typedef */
>> typedef struct foo foo_t1 __tag("typedef_foo_tag");
>> typedef struct {int foo2;} foo_t2 __tag("typedef_foo2_tag");
>>
>> /* tag on a function and its parameters */
>> __tag("func_add_tag")
>> int add(int x __tag("param_x_tag"), int y __tag("param_y_tag"))
>> {
>>           return x + y;
>> }
>>
>> /* keep the globals/types alive so they land in BTF */
>> int use(foo_t1 *f, foo_t2 *g)
>> {
>>           return add(global_var + static_var + multi_tag_var, f->a + 
>> g->foo2);
>> }
>>
>> $ /home/yhs/work/gcc-build/opt/gcc-16.1/bin/gcc -O2 -gbtf -g -c 
>> decl_tag.c -o decl_tag.o
>> decl_tag.c:30:1: warning: ‘btf_decl_tag’ attribute does not apply to 
>> types [-Wattributes]
>>      30 | } __tag("struct_foo_tag");
>>         | ^
>>
>> $ bpftool btf dump file decl_tag.o
>> [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
>> [2] STRUCT 'foo' size=8 vlen=2
>>           'a' type_id=1 bits_offset=0
>>           'b' type_id=1 bits_offset=32
>> [3] TYPEDEF 'foo_t1' type_id=2
>> [4] STRUCT '(anon)' size=4 vlen=1
>>           'foo2' type_id=1 bits_offset=0
>> [5] TYPEDEF 'foo_t2' type_id=4
>> [6] FUNC_PROTO '(anon)' ret_type_id=1 vlen=2
>>           'f' type_id=7
>>           'g' type_id=8
>> [7] PTR '(anon)' type_id=3
>> [8] PTR '(anon)' type_id=5
>> [9] FUNC_PROTO '(anon)' ret_type_id=1 vlen=2
>>           'x' type_id=1
>>           'y' type_id=1
>> [10] VAR 'multi_tag_var' type_id=1, linkage=global
>> [11] VAR 'global_var' type_id=1, linkage=global
>> [12] FUNC 'use' type_id=6 linkage=global
>> [13] FUNC 'add' type_id=9 linkage=global
>> [14] DECL_TAG 'global_var_tag' type_id=11 component_idx=-1
>> [15] DECL_TAG 'static_var_tag' type_id=0 component_idx=-1
>> [16] DECL_TAG 'tag_b' type_id=10 component_idx=-1
>> [17] DECL_TAG 'tag_a' type_id=10 component_idx=-1
>> [18] DECL_TAG 'member_a_tag' type_id=2 component_idx=0
>> [19] DECL_TAG 'member_b_tag' type_id=2 component_idx=1
>> [20] DECL_TAG 'param_x_tag' type_id=13 component_idx=0
>> [21] DECL_TAG 'param_y_tag' type_id=13 component_idx=1
>> [22] DECL_TAG 'func_add_tag' type_id=13 component_idx=-1
>> [23] DATASEC '.bss' size=0 vlen=2
>>           type_id=10 offset=0 size=4 (VAR 'multi_tag_var')
>>           type_id=11 offset=0 size=4 (VAR 'global_var')
>> $ bpftool btf dump file decl_tag.o | grep DECL_TAG
>> [14] DECL_TAG 'global_var_tag' type_id=11 component_idx=-1
>> [15] DECL_TAG 'static_var_tag' type_id=0 component_idx=-1
>> [16] DECL_TAG 'tag_b' type_id=10 component_idx=-1
>> [17] DECL_TAG 'tag_a' type_id=10 component_idx=-1
>> [18] DECL_TAG 'member_a_tag' type_id=2 component_idx=0
>> [19] DECL_TAG 'member_b_tag' type_id=2 component_idx=1
>> [20] DECL_TAG 'param_x_tag' type_id=13 component_idx=0
>> [21] DECL_TAG 'param_y_tag' type_id=13 component_idx=1
>> [22] DECL_TAG 'func_add_tag' type_id=13 component_idx=-1
>> $
>>
>> Three decl tags (struct_foo_tag, typedef_foo_tag and typedef_foo2_tag)
>> are missing here:
>
> I'll double check, once I'm back from PTO.
>
>> struct foo {
>>           int a __tag("member_a_tag");
>>           int b __tag("member_b_tag");
>> } __tag("struct_foo_tag");
>>
>> /* tag on a typedef */
>> typedef struct foo foo_t1 __tag("typedef_foo_tag");
>> typedef struct {int foo2;} foo_t2 __tag("typedef_foo2_tag");
>>
>> $ clang -O2 -g -target bpf -c decl_tag.c -o decl_tag.o
>> $ bpftool btf dump file decl_tag.o
>> [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
>> [2] FUNC_PROTO '(anon)' ret_type_id=1 vlen=2
>>           'x' type_id=1
>>           'y' type_id=1
>> [3] FUNC 'add' type_id=2 linkage=global
>> [4] DECL_TAG 'param_x_tag' type_id=3 component_idx=0
>> [5] DECL_TAG 'param_y_tag' type_id=3 component_idx=1
>> [6] DECL_TAG 'func_add_tag' type_id=3 component_idx=-1
>> [7] PTR '(anon)' type_id=8
>> [8] TYPEDEF 'foo_t1' type_id=10
>> [9] DECL_TAG 'typedef_foo_tag' type_id=8 component_idx=-1
>> [10] STRUCT 'foo' size=8 vlen=2
>>           'a' type_id=1 bits_offset=0
>>           'b' type_id=1 bits_offset=32
>> [11] DECL_TAG 'struct_foo_tag' type_id=10 component_idx=-1
>> [12] DECL_TAG 'member_a_tag' type_id=10 component_idx=0
>> [13] DECL_TAG 'member_b_tag' type_id=10 component_idx=1
>> [14] PTR '(anon)' type_id=15
>> [15] TYPEDEF 'foo_t2' type_id=17
>> [16] DECL_TAG 'typedef_foo2_tag' type_id=15 component_idx=-1
>> [17] STRUCT '(anon)' size=4 vlen=1
>>           'foo2' type_id=1 bits_offset=0
>> [18] FUNC_PROTO '(anon)' ret_type_id=1 vlen=2
>>           'f' type_id=7
>>           'g' type_id=14
>> [19] FUNC 'use' type_id=18 linkage=global
>> [20] VAR 'global_var' type_id=1, linkage=global
>> [21] DECL_TAG 'global_var_tag' type_id=20 component_idx=-1
>> [22] VAR 'multi_tag_var' type_id=1, linkage=global
>> [23] DECL_TAG 'tag_a' type_id=22 component_idx=-1
>> [24] DECL_TAG 'tag_b' type_id=22 component_idx=-1
>> [25] DATASEC '.bss' size=0 vlen=2
>>           type_id=20 offset=0 size=4 (VAR 'global_var')
>>           type_id=22 offset=0 size=4 (VAR 'multi_tag_var')
>> $ bpftool btf dump file decl_tag.o | grep DECL_TAG
>> [4] DECL_TAG 'param_x_tag' type_id=3 component_idx=0
>> [5] DECL_TAG 'param_y_tag' type_id=3 component_idx=1
>> [6] DECL_TAG 'func_add_tag' type_id=3 component_idx=-1
>> [9] DECL_TAG 'typedef_foo_tag' type_id=8 component_idx=-1
>> [11] DECL_TAG 'struct_foo_tag' type_id=10 component_idx=-1
>> [12] DECL_TAG 'member_a_tag' type_id=10 component_idx=0
>> [13] DECL_TAG 'member_b_tag' type_id=10 component_idx=1
>> [16] DECL_TAG 'typedef_foo2_tag' type_id=15 component_idx=-1
>> [21] DECL_TAG 'global_var_tag' type_id=20 component_idx=-1
>> [23] DECL_TAG 'tag_a' type_id=22 component_idx=-1
>> [24] DECL_TAG 'tag_b' type_id=22 component_idx=-1
>>
>> DECL_TAG 'static_var_tag' type_id=0 component_idx=-1 is missing in llvm.
>> This is execpted for llvm since 'static_var' is 'inlined' since it is
>> value is 0 and the compiler optimization removed it in function use().
>> In llvm Dwarf->BTF conversion is very late and we only emit survived
>> globals.
>>
>> gcc emits 'static_var_tag' probably in frontend. Emitting
>> 'static_var_tag' is okay, just not used.
>>
>> I think gcc should support
>>    - declaration tag for typedef, and
>>    - declaration tag for the whole struct (like above 'struct_foo_tag')
>> to be compatible with llvm.
>
> OK we will look into this.

Thanks!

>
> Thx,
> -Vineet
>