bin/93317: ld-elf.so doesn't translate unresolved weak symbol into 0

[Qu Fuping]

>Number:         93317
>Category:       bin
>Synopsis:       ld-elf.so doesn't translate unresolved weak symbol into 0
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Feb 14 06:40:03 GMT 2006
>Closed-Date:
>Last-Modified:
>Originator:     Qu Fuping
>Release:        FreeBSD 6.0-RELEASE i386
>Organization:
IOS of CAS,  China
>Environment:
FreeBSD coolq.localdomain 6.0-RELEASE FreeBSD 6.0-RELEASE #0: Thu Nov  3 09:36:13 UTC 2005     root at x64.samsco.home:/usr/obj/usr/src/sys/GENERIC  i386

>Description:
              In ELF-spec, it quotes(page 69 of 106 from TLS-elf spec v1.2)
"When the link editor searches archive libraries, it extracts archive members that contain definitions of undefined global symbols. The member's definition may be either a global or a weak symbol. The link editor does not extract archive members to resolve undefined weak symbols. Unresolved weak symbols have a zero value."
    I meet this problem when I implement PIE support for FreeBSD. See http://mirror.vtx.ch/hlfs/view/unstable/glibc/chapter02/pie.html for information about PIE. 
    I modified kernel, libc, gcc, so it can generate PIE, and load it properly. But when ld-elf resolves symbols, things go wrong. The PIE contains a weak symbol named __register_frame_info, in gcc, it's called like this(in crtstuff.c)
    if(__register_frame_info)
        __register_frame_info(__EH_FRAME_BEGIN, &object);
    in rtld-elf/i386/reloc.c the "__register_frame_info" is resolved by reloc_non_plt
        ....
	    case R_386_GLOB_DAT:
		{
		    const Elf_Sym *def;
		    const Obj_Entry *defobj;

		    def = find_symdef(ELF_R_SYM(rel->r_info), obj, &defobj,
		      false, cache);
		    if (def == NULL)
			goto done;

		    *where = (Elf_Addr) (defobj->relocbase + def->st_value);
		}
		break;
         ....
    Since __register_frame_info is a weak symbol, it should resolve to 0 according to elf-spec. find_symdef DO returns a reference to "static sym_zero", def->st_value = 0, but for PIE, defobj->relocbase != 0, so this symbol is resolved to defobj->relocbase, and the app will jump to defobj->relocbase, then segmentation fault :(

>How-To-Repeat:
              The PIE contains lots of modifications, but I think the full description above is self-explained.

>Fix:
              if find_symdef returns sym_zero, it should be
        *where = (Elf_Addr)0;

>Release-Note:
>Audit-Trail:
>Unformatted: