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

[Fangrui Song]

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