Symbol table '.dynsym' contains entries with LOCAL visibility and type SECTION

[William Tambe]

On Sat, Mar 28, 2020 at 7:41 PM Fangrui Song <i@maskray.me> wrote:
>
> On 2020-03-26, William Tambe via Binutils wrote:
> >On Thu, Mar 26, 2020 at 8:51 PM William Tambe <tambewilliam@gmail.com> wrote:
> >>
> >> What could be the reason that entries with LOCAL visibility and type
> >> SECTION are showing in the symbol table '.dynsym' as shown below ?
> >> What could be done to prevent that ?
> >
> >Please, note that this is a custom binutils port for which we are
> >implementing shared-library support.
> >We are missing something that is causing entries with LOCAL visibility
> >and type SECTION to show in the symbol table '.dynsym' .
> >
> >What could be done to prevent such behavior ?
> >
> >>
> >> Symbol table '.dynsym' contains 14 entries:
> >>    Num:    Value  Size Type    Bind   Vis      Ndx Name
> >>      0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
> >>      1: 000001f4     0 SECTION LOCAL  DEFAULT    5
> >>      2: 00000216     0 SECTION LOCAL  DEFAULT    6
> >>      3: 000003cc     0 SECTION LOCAL  DEFAULT    7
> >>      4: 000013e8     0 SECTION LOCAL  DEFAULT    8
> >>      5: 000013ec     0 SECTION LOCAL  DEFAULT    9
> >>      6: 000013f4     0 SECTION LOCAL  DEFAULT   10
> >>      7: 00001484     0 SECTION LOCAL  DEFAULT   12
> >>      8: 0000149c     0 SECTION LOCAL  DEFAULT   14
> >>      9: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND foo2
> >>     10: 000001f4     0 FUNC    GLOBAL DEFAULT    5 _init
> >>     11: 000003cc     0 FUNC    GLOBAL DEFAULT    7 _fini
> >>     12: 00000364    42 FUNC    GLOBAL DEFAULT    6 func
> >>     13: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND foo
>
> A STB_LOCAL .dynsym entry may be created if a dynamic relocation
> references it.
>
> // a.c
> char y;
> struct{char _; const char *x; char *y;} __attribute__((packed)) _ = {'_', "a", &y};
> void _start() {}
>
> % powerpc64-linux-gnu-gcc -shared -nostdlib a.c -o a.so
> % readelf -Wr a.so
>
> Relocation section '.rela.dyn' at offset 0x288 contains 4 entries:
>      Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
> 000000000001fee8  0000000000000016 R_PPC64_RELATIVE                          2e8
> 000000000001fef0  0000000000000016 R_PPC64_RELATIVE                          27f00
> 0000000000020001  000000010000002b R_PPC64_UADDR64        00000000000002e8 .text + 28
> 0000000000020009  000000040000002b R_PPC64_UADDR64        0000000000020011 y + 0
>
> R_PPC64_RELATIVE cannot be used because 0x20001 is misaligned.
>
> sh fdpic also needs it: http://git.musl-libc.org/cgit/musl/commit/?id=7a9669e977e5f750cf72ccbd2614f8b72ce02c4c
>
>
> But if you see a lot of STT_SECTION in your custom port, I guess you triggered some bugs.

elf_backend_finish_dynamic_section() is the last place we thought we
could iterate through all symbols and remove unwanted symbols, but we
can't see the STT_SECTION symbols there; that means they are added
later.

Any pointers on how we could further debug this issue ?