[Exception Handling] Why does frame unwind label have static alignment 4?

[Joseph Faulls]

Hello all,

Small disclaimer of being new to EH, but I have come across a problem that seems quite fundamental to EH frame registry. I am targeting riscv64, but I believe the problem to be entirely platform agnostic.

When using EH frame registry, a label is put at the beginning of the frame unwind info to register and deregister frames:

libgcc/crtstuff.c:
> STATIC EH_FRAME_SECTION_CONST char __EH_FRAME_BEGIN__[]
>      __attribute__((section(__LIBGCC_EH_FRAME_SECTION_NAME__), aligned(4)))

This is then populated with struct dwarf_fde:

libgcc/unwind-dw2-fde.h:
> struct dwarf_fde
> {
>   uword length;
>   sword CIE_delta;
>   unsigned char pc_begin[];
> } __attribute__ ((packed, aligned (__alignof__ (void *))));

The trouble is that the __EH_FRAME_BEGIN__ has alignment 4, whereas the struct dwarf_fde has alignment of void*. When targeting 64-bit, these values are not the same.
It is this that is causing 4 bytes of padding. This is the disassembly of a simple program targeting riscv64 that throws an exception:
> 22954 000000000000e324 <__EH_FRAME_BEGIN__>:
> e324:       0000                    unimp
> e326:       0000                    unimp
> e328:       0010                    0x10
> e32a:       0000                    unimp
> e32c:       0000                    unimp
> e32e:       0000                    unimp
> e330:       00527a03                0x527a03

It is this that causes __register_frame_info_bases to fall flat on its face:

Libgcc/crtstuff.c in function frame_dummy:
> __register_frame_info (__EH_FRAME_BEGIN__, &object);

libgcc/unwind-dw2-fde.c
>   void
>   __register_frame_info_bases (const void *begin, struct object *ob,
>                                void *tbase, void *dbase)
>   {
>     /* If .eh_frame is empty, don't register at all.  */
>     if ((const uword *) begin == 0 || *(const uword *) begin == 0)
>       return;
> ...
> void
> __register_frame_info (const void *begin, struct object *ob)
> {
>   __register_frame_info_bases (begin, ob, 0, 0);
> }

This means no EH frames are ever registered, and so exceptions cannot be handled!

So, why does __EH_FRAME_BEGIN__ have alignment 4 and not of __alignof__ (void *) ? Is this a bug?

Thanks,
Joe