[Bug target/111464] New: Using DW_EH_PE_udata4 for amd64 non-pic breaks linking in some situations

["john at feith dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111464

            Bug ID: 111464
           Summary: Using DW_EH_PE_udata4 for amd64 non-pic breaks linking
                    in some situations
           Product: gcc
           Version: 9.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john at feith dot com
  Target Milestone: ---

Created attachment 55924
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55924&action=edit
Patch for amd64 R_X86_64_32 relocation issue

While the following was observed using gcc 9.5.0 configured to use gas / ld
from binutils 2.39 the code in question seems to be unchanged in this respect
for the current gcc 13.2.0 release.

I bootstrapped gcc 9.5.0 on FreeBSD 13.2 (amd64) and used it to build various
other programs without issue.  However building gdb 11.2 results in:

  ada-exp.o: relocation R_X86_64_32 against symbol
`__gxx_personality_v0@@CXXABI_1.3' can not be used when making a PDE object;

Looking at the 64 bit assembly language generate from ada-exp.c shows:

  .Lframe1:
        .long   .LECIE1-.LSCIE1 # Length of Common Information Entry
  .LSCIE1:
        ...
        .byte   0x3     # Personality (udata4)
        .long   __gxx_personality_v0

gas generates a R_X86_64_32 relocation for __gxx_personality_v0 and then ld
throws the error when attempting to link the various object along with the
shared libraries into the executable fails.  The ld error comes from bfd
elf64-x86-64.c:

  case R_X86_64_32:
    if (!ABI_64_P (abfd))
      goto pointer;
    /* Fall through.  */
    ...
    /* Check relocation overflow as these relocs may lead to
       run-time relocation overflow.  Don't error out for
       sections we don't care about, such as debug sections or
       when relocation overflow check is disabled.  */

Note also the FreeBSD rtld-elf also doesn't support R_X86_64_32 so it's just
as well that the binutils ld throw the error.

In gcc i386.c asm_preferred_eh_data_format has:

  ...
  if (ix86_cmodel == CM_SMALL
      || (ix86_cmodel == CM_MEDIUM && code))
    return DW_EH_PE_udata4;
  return DW_EH_PE_absptr;

The intent is that for 64 bit small and (in the code case) medium model to
save some space by encoding the address using 4 bytes instead of the full
8 byte pointer.  That may be fine in some cases (i.e. full static linking),
however break certain other mixes (i.e. the forementioned non-PIE gdb build
using a shared C++ library).

The attached change simply changes the non-PIC case to always encode the
full pointer.  Note this should be an essentially no-op change for 32 bits.

The resulting 64 bit assembly language generate from ada-exp.c shows:

  .Lframe1:
        .long   .LECIE1-.LSCIE1 # Length of Common Information Entry
  .LSCIE1:
        ...
        .byte   0       # Personality (absolute)
        .quad   __gxx_personality_v0

and allows the gdb build to produce a working executable.

This doesn't seem to be a FreeBSD platform specific issue per-say as the
binutils code that checks for R_X86_64_32 isn't platform specific.

ChangeLog:

Fri Sep 15 01:50:44 EDT 2023  John Wehle  (john@feith.com)

        * i386.c (asm_preferred_eh_data_format): Always use DW_EH_PE_absptr
        when non-pic.

-- John Wehle