Re: [PATCH bpf-next 1/2] libbpf: add option to force-anonymize nested structs for BTF dump

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2025-12-17 at 16:39 -0800, Alexei Starovoitov wrote:
> On Wed, Dec 17, 2025 at 4:19 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > On Wed, 2025-12-17 at 15:34 -0800, Alexei Starovoitov wrote:
> > > On Wed, Dec 17, 2025 at 2:47 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > > > 
> > > > >   $ cat ms-ext-test2.c
> > > > >   struct foo {
> > > > >     int a;
> > > > >   } __attribute__((preserve_access_index));
> > > > > 
> > > > >   struct bar {
> > > > >     struct foo;
> > > > >   } __attribute__((preserve_access_index));
> > > > > 
> > > > >   int buz(struct bar *bar) {
> > > > >     return bar->a;
> > > > >   }
> > > > > 
> > > > >   $ clang -O2 -g -fms-extensions --target=bpf -c ms-ext-test2.c
> > > > >   ms-ext-test2.c:6:3: warning: anonymous structs are a Microsoft extension [-Wmicrosoft-anon-tag]
> > > > >       6 |   struct foo;
> > > > >         |   ^~~~~~~~~~
> > > > >   1 warning generated.
> > > > > 
> > > > >   $ llvm-objdump -Sdr ms-ext-test2.o
> > > > > 
> > > > >   ms-ext-test2.o: file format elf64-bpf
> > > > > 
> > > > >   Disassembly of section .text:
> > > > > 
> > > > >   0000000000000000 <buz>:
> > > > >   ;   return bar->a;
> > > > >          0:       61 10 00 00 00 00 00 00 w0 = *(u32 *)(r1 + 0x0)
> > > > >                   0000000000000000:  CO-RE <byte_off> [2] struct bar::<anon 0>.a (0:0:0)
> > > > >          1:       95 00 00 00 00 00 00 00 exit
> > > > > 
> > > > > Note the "<anon 0>" in the relocation.
> > > > > It appears that we loose no information if structures are unrolled.
> > > 
> > > Forgot to mention the CORE concern earlier...
> > > Does the above work with current logic in relo_core.c ?
> > > If not, we should definitely unconditionally unroll
> > > to avoid fixing CORE.
> > 
> > I think there might be an issue with CO-RE.
> > Here is an example:
> > 
> >   struct foo {
> >     int a;
> >   } __attribute__((preserve_access_index));
> > 
> >   struct bar {
> >   #ifdef USE_MS
> >     struct foo;
> >   #else
> >     struct { int a; };
> >   #endif
> >   } __attribute__((preserve_access_index));
> > 
> >   int buz(struct bar *bar) {
> >     return bar->a;
> >   }
> > 
> > Here is what I get with USE_MS:
> > 
> >   $ llvm-objdump -Sdr ms-ext-test2.o
> > 
> >   ms-ext-test2.o: file format elf64-bpf
> > 
> >   Disassembly of section .text:
> > 
> >   0000000000000000 <buz>:
> >   ;   return bar->a;
> >          0:       61 10 00 00 00 00 00 00 w0 = *(u32 *)(r1 + 0x0)
> >                   0000000000000000:  CO-RE <byte_off> [2] struct bar::<anon 0>.a (0:0:0)
> >          1:       95 00 00 00 00 00 00 00 exit
> > 
> >   $ bpftool btf dump file ms-ext-test2.o
> >   [1] PTR '(anon)' type_id=2
> >   [2] STRUCT 'bar' size=4 vlen=1
> >           '(anon)' type_id=3 bits_offset=0
> >   [3] STRUCT 'foo' size=4 vlen=1
> >           'a' type_id=4 bits_offset=0
> >   [4] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> >   [5] FUNC_PROTO '(anon)' ret_type_id=4 vlen=1
> >           'bar' type_id=1
> >   [6] FUNC 'buz' type_id=5 linkage=global
> > 
> > And here is without USE_MS:
> > 
> >   $ llvm-objdump -Sdr ms-ext-test2.o
> > 
> >   ms-ext-test2.o: file format elf64-bpf
> > 
> >   Disassembly of section .text:
> > 
> >   0000000000000000 <buz>:
> >   ;   return bar->a;
> >          0:       61 10 00 00 00 00 00 00 w0 = *(u32 *)(r1 + 0x0)
> >                   0000000000000000:  CO-RE <byte_off> [2] struct bar::<anon 0>.a (0:0:0)
> >          1:       95 00 00 00 00 00 00 00 exit
> > 
> >   $ bpftool btf dump file ms-ext-test2.o
> >   [1] PTR '(anon)' type_id=2
> >   [2] STRUCT 'bar' size=4 vlen=1
> >           '(anon)' type_id=3 bits_offset=0
> >   [3] STRUCT '(anon)' size=4 vlen=1
> >           'a' type_id=4 bits_offset=0
> >   [4] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> >   [5] FUNC_PROTO '(anon)' ret_type_id=4 vlen=1
> >           'bar' type_id=1
> >   [6] FUNC 'buz' type_id=5 linkage=global
> > 
> > So, with USE_MS the relocation captures the offset inside 'struct foo'.
> > And this is important for CO-RE offsets resolution.
> > So unrolling structures is actually a problem.
> 
> Sounds like we should silence the warning the way Song proposed and
> tell all users to add -fms-extension to their builds.

Sounds like it.