On Mon, Jan 17, 2022 at 9:24 AM Joseph Faulls <joseph.fau...@imgtec.com> wrote:
>
> 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
This is:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=27576
--
H.J.
