Such early debug is bogus - do you have the preprocessed source for the corresponding TU?

A testcase is on pr96385 branch at:

https://gitlab.com/x86-gcc/gcc-bugs

On Thu, 30 Jul 2020, hjl.tools at gmail dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96385
> 
> H.J. Lu <hjl.tools at gmail dot com> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>              Status|WAITING                     |NEW
> 
> --- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
> A testcase is on pr96385 branch at:
> 
> https://gitlab.com/x86-gcc/gcc-bugs

I'd like to have the _source_ to whichever of the .o files
has the bogus relocation in the .gnulto.debug_info section.

The bug happens at the compile stage.

On Thu, 30 Jul 2020, rguenther at suse dot de wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96385
> 
> --- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> ---
> On Thu, 30 Jul 2020, hjl.tools at gmail dot com wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96385
> > 
> > H.J. Lu <hjl.tools at gmail dot com> changed:
> > 
> >            What    |Removed                     |Added
> > ----------------------------------------------------------------------------
> >              Status|WAITING                     |NEW
> > 
> > --- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
> > A testcase is on pr96385 branch at:
> > 
> > https://gitlab.com/x86-gcc/gcc-bugs
> 
> I'd like to have the _source_ to whichever of the .o files
> has the bogus relocation in the .gnulto.debug_info section.
> 
> The bug happens at the compile stage.

More specifically, .rela.gnu.debuglto_.debug_info may only contain
relocations to other .gnu.debuglto_.* sections and definitely
_not_ relocations to UNDEF symbols.

Please get users/hjl/pr26314/test branch at

https://gitlab.com/x86-binutils/binutils-gdb/-/tree/users/hjl/pr26314/test

and build binutils with GCC 10:

CC="/usr/gcc-10.1.1-x32/bin/gcc -flto -ffat-lto-objects -Wl,--as-needed -fcf-protection" CXX="/usr/gcc-10.1.1-x32/bin/g++ -flto -ffat-lto-objects -Wl,--as-needed -fcf-protection" /export/gnu/import/git/gitlab/x86-binutils/configure \
	--enable-shared \
	 \
	--enable-plugins --disable-gdb --disable-gdbserver --disable-libdecnumber --disable-readline --disable-sim --with-sysroot=/ --with-system-zlib \
	--prefix=/usr/local \
	--with-local-prefix=/usr/local

I got

[hjl@gnu-cfl-2 build-x86_64-linux]$ nm -D binutils/.libs/ar | grep lrealpath
0000000000000000 A lrealpath
[hjl@gnu-cfl-2 build-x86_64-linux]$

There is a small testcase in

https://sourceware.org/bugzilla/show_bug.cgi?id=26324

[hjl@gnu-cfl-2 pr26324]$ cat pr15146a.c
extern int xxx;

int
bar (void)
{
  return xxx;
}

int
main ()
{ 
  return 0;
}
[hjl@gnu-cfl-2 pr26324]$ cat pr15146b.c
int xxx = 3;
[hjl@gnu-cfl-2 pr26324]$ cat pr15146c.c
[hjl@gnu-cfl-2 pr26324]$ make
gcc -B./  -O2 -g -o x -Wl,-R,. \
  -Wl,--no-copy-dt-needed-entries -Wl,--no-as-needed pr15146a.o pr15146c.so
./ld: /tmp/ccHirkTe.debug.temp.o: undefined reference to symbol 'xxx'
./ld: ./pr15146b.so: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make: *** [Makefile:15: x] Error 1
[hjl@gnu-cfl-2 pr26324]$

The original pr26324a.o debug info contains reference to xxx.  But reference
to xxx has been removed by LTO.  simple_object_copy_lto_debug_sections fails
to remove the un-referenced symbol, xxx, from symbol table.

Created attachment 48975 [details]
A testcase

[hjl@gnu-cfl-2 pr26324]$ make
gcc -B./ -O2 -g -flto -ffat-lto-objects   -c -o pr26324a.o pr26324a.c
gcc -B./ -O2 -g -fPIC   -c -o pr15146c.o pr15146c.c
gcc -B./ -O2 -g -fPIC   -c -o pr15146b.o pr15146b.c
gcc -B./ -shared  -o pr15146b.so pr15146b.o
gcc -B./ -shared  -o pr15146c.so pr15146c.o pr15146b.so
gcc -B./  -O2 -g -o x -Wl,-R,. \
  -Wl,--no-copy-dt-needed-entries -Wl,--no-as-needed pr26324a.o pr15146c.so
./ld: /tmp/ccCGQCqp.debug.temp.o: undefined reference to symbol 'xxx'
./ld: ./pr15146b.so: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make: *** [Makefile:20: x] Error 1
[hjl@gnu-cfl-2 pr26324]$ 

-flto -ffat-lto-objects is the key.

(In reply to H.J. Lu from comment #8)
> The original pr26324a.o debug info contains reference to xxx.  But reference
> to xxx has been removed by LTO.  simple_object_copy_lto_debug_sections fails
> to remove the un-referenced symbol, xxx, from symbol table.

The alternative is to ask linker to handle it:

https://sourceware.org/pipermail/binutils/2020-July/112673.html

GCC 7.3.1 is OK:

[hjl@gnu-cfl-2 pr26324]$ make
/usr/gcc-7.3.1-x32/bin/gcc -B./ -O2 -g -flto -ffat-lto-objects   -c -o pr26324a.o pr26324a.c
/usr/gcc-7.3.1-x32/bin/gcc -B./ -O2 -g -fPIC   -c -o pr15146c.o pr15146c.c
/usr/gcc-7.3.1-x32/bin/gcc -B./ -O2 -g -fPIC   -c -o pr15146b.o pr15146b.c
/usr/gcc-7.3.1-x32/bin/gcc -B./ -shared  -o pr15146b.so pr15146b.o
/usr/gcc-7.3.1-x32/bin/gcc -B./ -shared  -o pr15146c.so pr15146c.o pr15146b.so
/usr/gcc-7.3.1-x32/bin/gcc -B./  -O2 -g -o x -Wl,-R,. \
  -Wl,--no-copy-dt-needed-entries -Wl,--no-as-needed pr26324a.o pr15146c.so
./x
PASS
[hjl@gnu-cfl-2 pr26324]$

(In reply to H.J. Lu from comment #10)
> (In reply to H.J. Lu from comment #8)
> > The original pr26324a.o debug info contains reference to xxx.  But reference
> > to xxx has been removed by LTO.  simple_object_copy_lto_debug_sections fails
> > to remove the un-referenced symbol, xxx, from symbol table.
> 
> The alternative is to ask linker to handle it:
> 
> https://sourceware.org/pipermail/binutils/2020-July/112673.html

The bug is we create the reference in the first place.

(In reply to Richard Biener from comment #12)
> (In reply to H.J. Lu from comment #10)
> > (In reply to H.J. Lu from comment #8)
> > > The original pr26324a.o debug info contains reference to xxx.  But reference
> > > to xxx has been removed by LTO.  simple_object_copy_lto_debug_sections fails
> > > to remove the un-referenced symbol, xxx, from symbol table.
> > 
> > The alternative is to ask linker to handle it:
> > 
> > https://sourceware.org/pipermail/binutils/2020-July/112673.html
> 
> The bug is we create the reference in the first place.

Oh, I see.  So we had multiple choices for "pruning" symbols from the symbol
table during simple_object_copy_lto_debug_sections and factoring in
HP-UX and Solaris (sic!...) emptying UND symbol names didn't work out.
So we do now

              if (discard)
                {
                  /* Make discarded symbols undefined and unnamed
                     in case it is local.  */
                  int bind = ELF_ST_BIND (*st_info);
                  int other = STV_DEFAULT;
                  if (bind == STB_LOCAL)
                    {
                      /* Make discarded local symbols unnamed and
                         defined in the first prevailing section.  */
                      ELF_SET_FIELD (type_functions, ei_class, Sym,
                                     ent, st_name, Elf_Word, 0);
                      ELF_SET_FIELD (type_functions, ei_class, Sym,
                                     ent, st_shndx, Elf_Half,
                                     sh_map[first_shndx]);
                    }
                  else
                    {
                      /* Make discarded global symbols hidden weak
                         undefined and sharing a name of a prevailing
                         symbol.  */
                      bind = STB_WEAK;
                      other = STV_HIDDEN;
                      ELF_SET_FIELD (type_functions, ei_class, Sym,
                                     ent, st_name, Elf_Word,
                                     prevailing_name_idx);
                      ELF_SET_FIELD (type_functions, ei_class, Sym,
                                     ent, st_shndx, Elf_Half, SHN_UNDEF);
                    }
                  *st_other = other;
                  *st_info = ELF_ST_INFO (bind, STT_NOTYPE);
                  ELF_SET_FIELD (type_functions, ei_class, Sym,
                                 ent, st_value, Elf_Addr, 0);
                  ELF_SET_FIELD (type_functions, ei_class, Sym,
                                 ent, st_size, Elf_Word, 0);
                }

but somehow the else path isn't triggered(?).  Note that all we try to
avoid is the need to rewrite relocation sections if we really would
prune symbols rather than NULL-ifying them somehow.  You can trace history
as to how the above code morphed with various bugs in the past...

Hmm, we don't treat SHN_UNDEF any special here - I guess we could at most
make all of them WEAK ... the following should address this.

diff --git a/libiberty/simple-object-elf.c b/libiberty/simple-object-elf.c
index c62d5bba551..7c9d492f6a4 100644
--- a/libiberty/simple-object-elf.c
+++ b/libiberty/simple-object-elf.c
@@ -1467,6 +1467,11 @@ simple_object_elf_copy_lto_debug_sections (simple_object_read *sobj,
                       && st_shndx < shnum
                       && pfnret[st_shndx - 1] == -1)
                discard = 1;
+             /* We also need to remove global UNDEFs which can
+                cause link fails later.  */
+             else if (st_shndx == SHN_UNDEF
+                      && ELF_ST_BIND (*st_info) == STB_GLOBAL)
+               discard = 1;
 
              if (discard)
                {

Note the symbol table entry should be "unused" (if there's anything like that).
At least ld usually treats it like so.  IIRC I left those dangling because
UNDEFs into the copied .debug_info section are bogus - and I did not want to
parse relocation sections to see where it is referenced from (otherwise I'd
assert for such UNDEFs).

I know it's all a bit awkward but originally this was supposed to be
a temporary measure until ld + plugin API understands how to only
partly claim an object and make ld link the original .gnu.debuglto_
prefixed debug-info sections unchanged into the final object ...

So you basically found a case where ld does _not_ treat the UNDEF as "unused".
In case that is not a bug in ld I am not really understanding how the bug
manifests which means I have a hard time creating a testcase for GCCs
LTO testsuite :/

Now the patch itself should be quite safe (we shouldn't be left with any
global UNDEFs).  But I'm curious if it resolves a "real" bug...

I also wonder if the result of the patch,

    34: 0000000000000000     0 NOTYPE  WEAK   HIDDEN     1 t.c.5958f17d
    35: 0000000000000000     0 NOTYPE  WEAK   HIDDEN   UND t.c.5958f17d
    36: 0000000000000000     0 NOTYPE  WEAK   HIDDEN   UND t.c.5958f17d
    37: 0000000000000000     0 NOTYPE  WEAK   HIDDEN   UND t.c.5958f17d

is in any way "safer" from an ELF perspective (not so much GNU ld since
we've dealt with Solaris and HP-UX linker issues in the past).

Alan, you fiddled with this earlier so any comments?

It looks fine to me, assuming you don't need to keep any of these undefined symbols.

The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:b32c5d0b72fda2588b4e170e75a9c64e4bf266c7

commit r11-2506-gb32c5d0b72fda2588b4e170e75a9c64e4bf266c7
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Aug 3 15:05:37 2020 +0200

    lto/96385 - avoid unused global UNDEFs in debug objects
    
    Unused global UNDEFs can have side-effects in some circumstances so
    the following patch avoids them by treating them the same as other
    to be discarded DEFs - make them local.
    
    2020-08-03  Richard Biener  <rguenther@suse.de>
    
            PR lto/96385
    libiberty/
            * simple-object-elf.c
            (simple_object_elf_copy_lto_debug_sections): Localize global
            UNDEFs and reuse the prevailing name.

So let's see whether HPUX or Solaris folks complain.

The releases/gcc-10 branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:63a2bdbfb42628800a6999e98804928855592ce7

commit r10-8746-g63a2bdbfb42628800a6999e98804928855592ce7
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Aug 3 15:05:37 2020 +0200

    lto/96385 - avoid unused global UNDEFs in debug objects
    
    Unused global UNDEFs can have side-effects in some circumstances so
    the following patch avoids them by treating them the same as other
    to be discarded DEFs - make them local.
    
    2020-08-03  Richard Biener  <rguenther@suse.de>
    
            PR lto/96385
    libiberty/
            * simple-object-elf.c
            (simple_object_elf_copy_lto_debug_sections): Localize global
            UNDEFs and reuse the prevailing name.
    
    (cherry picked from commit b32c5d0b72fda2588b4e170e75a9c64e4bf266c7)

The releases/gcc-9 branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:969be2412231d3bdd73300ac32684d416345a029

commit r9-9340-g969be2412231d3bdd73300ac32684d416345a029
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Aug 3 15:05:37 2020 +0200

    lto/96385 - avoid unused global UNDEFs in debug objects
    
    Unused global UNDEFs can have side-effects in some circumstances so
    the following patch avoids them by treating them the same as other
    to be discarded DEFs - make them local.
    
    2020-08-03  Richard Biener  <rguenther@suse.de>
    
            PR lto/96385
    libiberty/
            * simple-object-elf.c
            (simple_object_elf_copy_lto_debug_sections): Localize global
            UNDEFs and reuse the prevailing name.
    
    (cherry picked from commit b32c5d0b72fda2588b4e170e75a9c64e4bf266c7)

The releases/gcc-8 branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:e6a720bf5b1328ff018880063ae3d1e777e5b61d

commit r8-10922-ge6a720bf5b1328ff018880063ae3d1e777e5b61d
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Aug 3 15:05:37 2020 +0200

    lto/96385 - avoid unused global UNDEFs in debug objects
    
    Unused global UNDEFs can have side-effects in some circumstances so
    the following patch avoids them by treating them the same as other
    to be discarded DEFs - make them local.
    
    2020-08-03  Richard Biener  <rguenther@suse.de>
    
            PR lto/96385
    libiberty/
            * simple-object-elf.c
            (simple_object_elf_copy_lto_debug_sections): Localize global
            UNDEFs and reuse the prevailing name.
    
    (cherry picked from commit b32c5d0b72fda2588b4e170e75a9c64e4bf266c7)

Fixed.