foo (VER_NDX_GLOBAL) and foo@@v1 in one DSO

[Fangrui Song]

On Thu, Nov 10, 2022 at 7:27 PM Fangrui Song <i@maskray.me> wrote:
>
> Hello, I find a strange use of VER_NDX_GLOBAL and look forward to your thoughts.
>
> cat > a.s <<e
> .globl foo
> .symver foo, foo@@@v1
> foo:
>    nop
>    ret
>
> .globl foo_compat
> .symver foo_compat, foo@, remove
> foo_compat:
>    ret
> e
> cat > a.ver <<e
> v1 {};
> e
> cat > b.c <<e
> extern void foo(void);
> int main(void) { foo(); }
> e
>
> gcc -fuse-ld=bfd -shared -nostdlib -Wl,--version-script=a.ver a.s -o a.so
> gcc -fuse-ld=bfd -g b.c ./a.so -o b
>
> exit
>
> In a.so, we have a default version foo@@v1 as well as a VER_NDX_GLOBAL foo.
> The VER_NDX_GLOBAL foo is unusual.
>
> ```
> % readelf -W --dyn-s -V a.so
>
> Symbol table '.dynsym' contains 4 entries:
>     Num:    Value          Size Type    Bind   Vis      Ndx Name
>       0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
>       1: 0000000000001000     0 NOTYPE  GLOBAL DEFAULT    7 foo@@v1
>       2: 0000000000001002     0 NOTYPE  GLOBAL DEFAULT    7 foo
>       3: 0000000000000000     0 OBJECT  GLOBAL DEFAULT  ABS v1
>
> Version symbols section '.gnu.version' contains 4 entries:
>   Addr: 0x000000000000028e  Offset: 0x00028e  Link: 3 (.dynsym)
>    000:   0 (*local*)       2 (v1)            1h                2 (v1)
>
> Version definition section '.gnu.version_d' contains 2 entries:
>   Addr: 0x0000000000000298  Offset: 0x000298  Link: 4 (.dynstr)
>    000000: Rev: 1  Flags: BASE  Index: 1  Cnt: 1  Name: a.so
>    0x001c: Rev: 1  Flags: none  Index: 2  Cnt: 1  Name: v1
> ```
>
> There are two strange things:
>
> * foo is a VER_NDX_GLOBAL VERSYM_HIDDEN symbol (shown as `1h`). Is this an expected use case?
> * foo co-exist with foo@@v1. Is this an expected use case?
>
> In my understanding, when a stem symbol has a @ form, the unversioned VER_NDX_GLOBAL form cannot exist.
> In the example above, `.symver foo_compat, foo@v0, remove` would be fine.
>
> The above is reduced from a real world case of https://github.com/libfuse/libfuse/blob/fuse_2_9_bugfix/lib/fuse.c#L4955
> `FUSE_SYMVER(".symver fuse_new_compat2,fuse_new@");`
>
> % readelf -W --dyn-s /lib/x86_64-linux-gnu/libfuse.so.2 G fuse_new
>     200: 00000000000148b0    72 FUNC    GLOBAL DEFAULT   13 fuse_new@FUSE_2.5
>     201: 0000000000014870    16 FUNC    GLOBAL DEFAULT   13 fuse_new                      # this is VERSYM_HIDDEN
>     202: 0000000000014750     8 FUNC    GLOBAL DEFAULT   13 fuse_new@@FUSE_2.6
>     203: 0000000000014860    11 FUNC    GLOBAL DEFAULT   13 fuse_new@FUSE_2.2
>     216: 0000000000014880    35 FUNC    GLOBAL DEFAULT   13 fuse_new_compat1@@FUSE_2.2
>     218: 0000000000014870    16 FUNC    GLOBAL DEFAULT   13 fuse_new_compat2@@FUSE_2.2
>     228: 0000000000014860    11 FUNC    GLOBAL DEFAULT   13 fuse_new_compat22@@FUSE_2.5
>     230: 00000000000148b0    72 FUNC    GLOBAL DEFAULT   13 fuse_new_compat25@@FUSE_2.6

Ping about thoughts:)