Bug 31447

This is the current "workaround" I use when working with individual ET_REL DWARF files (also works for any other ELF type):

 static char *debuginfo_path = NULL;
 static const Dwfl_Callbacks dwfl_callbacks =
    {
      .find_elf = dwfl_build_id_find_elf,
      .find_debuginfo = dwfl_standard_find_debuginfo,
      .section_address = dwfl_offline_section_address,
      .debuginfo_path = &debuginfo_path,
    };

 int main (int argc, char **argv)
 {
    Dwfl *dwfl = dwfl_begin (&dwfl_callbacks);
    Dwfl_Module *module = dwfl_report_elf (dwfl, argv[1], argv[1], -1, 0, false);
    if (module == NULL)
      printf ("%s\n", dwfl_errmsg (-1));
    dwfl_report_end (dwfl, NULL, NULL);

    Dwarf_Addr bias;
    Dwarf *dwarf = dwfl_module_getdwarf (module, &bias);
    if (dwarf == NULL)
      printf ("%s\n", dwfl_errmsg (-1));

    /* ... do something with dwarf ... */

    dwfl_end (dwfl); // Also cleans up dwarf.
 }

But this is obviously a little cumbersome if not really using any other libdwfl features.

Hey @mjw

```
$ eu-readelf -a qq.o | grep Type
  Type: REL (Relocatable file)

$ eu-readelf -S qq.o | grep REL
[ 2] .rela.text           RELA         0000000000000000 000004b8 00000048 24 I     19   1  8
[ 6] .rela.debug_info     RELA         0000000000000000 00000500 000000f0 24 I     19   5  8
[ 9] .rela.debug_aranges  RELA         0000000000000000 000005f0 00000030 24 I     19   8  8
[11] .rela.debug_line     RELA         0000000000000000 00000620 00000060 24 I     19  10  8
[18] .rela.eh_frame       RELA         0000000000000000 00000680 00000030 24 I     19  17  8

```


When applying relocation on a relocatable file (the above qq.o) with the new function
dwelf_relocation_debug_sections (Elf *elf), it only needs to apply relocation on debug sections (which are 
[ 6] .rela.debug_info, 
[ 9] .rela.debug_aranges,
[11] .rela.debug_line
), am I right?

Yes, just process the .rela.debug_* sections.
Note that you don't need to depend on the exact name.
You can follow the section header sh_info field, which points to the section (number) to which the relocations apply.
If that section starts with .debug you can apply the relocations.
Or better use ebl_debugscn_p, which checks against the list in libebl/eblopenbackend.c (default_debugscn_p).

See also in strip.c, where it says:

          /* Make sure that this relocation section points to a
             section to relocate with contents, that isn't
             allocated and that is a debug section.  */

Or libdwfl/relocate.c (relocate_section). Which contains similar logic.

@mjw

I have function dwelf_relocation_debug_sections() added in libdwelf.a.

```
$ pwd
/home/dichen/elfutils
$ eu-readelf -s libdwelf/libdwelf.a | grep dwelf_relocation_debug_sections
libdwelf/libdwelf.a(dwelf_relocation_debug_sections.o):
    1: 0000000000000000      0 FILE    LOCAL  DEFAULT      ABS dwelf_relocation_debug_sections.c
  290: 000000000000077c   3783 FUNC    GLOBAL DEFAULT      126 dwelf_relocation_debug_sections

```

And I can call the new function in a test program:

```
#include <fcntl.h>
#include <gelf.h>
#include <libelf.h>
#include <stdio.h>
#include <unistd.h>

#include "libdwelf.h"


int main(int argc, char **argv) {
  const char *fname = argv[1];

  elf_version(EV_CURRENT);
  int fd = open(argv[1], O_RDWR);

  Elf *elf;
  elf = elf_begin(fd, ELF_C_RDWR, NULL);

  dwelf_relocation_debug_sections(elf, fname);
  elf_update(elf, ELF_C_WRITE);

  return 0;
}

// $ gcc reloc.c -l elf -l dwelf -l dw -l ebl -l ebl_backends -l cpu -L ../libcpu -L ../libebl -L ../backends -L ../libdwelf -I ../libdwelf -I ../libdw -g -o reloc

```

Then, I am thinking of making strip.c use this dwelf_relocation_debug_sections() so that I can reduce some code redundancy.

```
diff --git a/src/Makefile.am b/src/Makefile.am
index 1d592d4d..746c8817 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -43,6 +43,8 @@ if BUILD_STATIC
 libasm = ../libasm/libasm.a
 libdw = ../libdw/libdw.a -lz $(zip_LIBS) $(libelf) -ldl -lpthread
 libelf = ../libelf/libelf.a -lz $(zstd_LIBS)
+libebl = ../libebl/libebl.a ../backends/libebl_backends.a ../libcpu/libcpu.a
+libdwelf = ../libdwelf/libdwelf.a
 if LIBDEBUGINFOD
 libdebuginfod = ../debuginfod/libdebuginfod.a -lpthread $(libcurl_LIBS)
 else
@@ -60,6 +62,7 @@ endif
 endif
 libebl = ../libebl/libebl.a ../backends/libebl_backends.a ../libcpu/libcpu.a
 libeu = ../lib/libeu.a
+libdwelf = ../libdwelf/libdwelf.a
 
 if DEMANGLE
 demanglelib = -lstdc++
@@ -81,7 +84,7 @@ readelf_LDADD = $(libdw) $(libebl) $(libelf) $(libeu) $(argp_LDADD)
 nm_LDADD = $(libdw) $(libebl) $(libelf) $(libeu) $(argp_LDADD) $(obstack_LIBS) \
           $(demanglelib)
 size_LDADD = $(libelf) $(libeu) $(argp_LDADD)
-strip_LDADD = $(libebl) $(libelf) $(libdw) $(libeu) $(argp_LDADD)
+strip_LDADD = $(libebl) $(libelf) $(libdw) $(libeu) $(libdwelf) $(argp_LDADD)
 elflint_LDADD  = $(libebl) $(libdw) $(libelf) $(libeu) $(argp_LDADD)
 findtextrel_LDADD = $(libdw) $(libelf) $(libeu) $(argp_LDADD)
 addr2line_LDADD = $(libdw) $(libelf) $(libeu) $(argp_LDADD) $(demanglelib)
```

After the above Makefile.am change, still, elfutils building kept failing due to some
 undefined reference error.

```
/usr/bin/ld: strip.o: in function `handle_debug_relocs':
/home/dichen/elfutils/src/strip.c:576: undefined reference to `dwelf_relocation_debug_sections'
/usr/bin/ld: strip.o: in function `handle_elf':
/home/dichen/elfutils/src/strip.c:2123: undefined reference to `dwelf_relocation_debug_sections'
collect2: error: ld returned 1 exit status
```

I believe I am missing something in the building process(Makefile.am, Makefile.in, etc). But I don't have many clue now, could you please give a look?

CC: @amerey

(In reply to Di Chen from comment #4)
> diff --git a/src/Makefile.am b/src/Makefile.am
> index 1d592d4d..746c8817 100644
> --- a/src/Makefile.am
> +++ b/src/Makefile.am
> @@ -43,6 +43,8 @@ if BUILD_STATIC
>  libasm = ../libasm/libasm.a
>  libdw = ../libdw/libdw.a -lz $(zip_LIBS) $(libelf) -ldl -lpthread
>  libelf = ../libelf/libelf.a -lz $(zstd_LIBS)
> +libebl = ../libebl/libebl.a ../backends/libebl_backends.a ../libcpu/libcpu.a
> +libdwelf = ../libdwelf/libdwelf.a
>  if LIBDEBUGINFOD
>  libdebuginfod = ../debuginfod/libdebuginfod.a -lpthread $(libcurl_LIBS)
>  else
> @@ -60,6 +62,7 @@ endif
>  endif
>  libebl = ../libebl/libebl.a ../backends/libebl_backends.a ../libcpu/libcpu.a
>  libeu = ../lib/libeu.a
> +libdwelf = ../libdwelf/libdwelf.a
>  
>  if DEMANGLE
>  demanglelib = -lstdc++
> @@ -81,7 +84,7 @@ readelf_LDADD = $(libdw) $(libebl) $(libelf) $(libeu)
> $(argp_LDADD)
>  nm_LDADD = $(libdw) $(libebl) $(libelf) $(libeu) $(argp_LDADD)
> $(obstack_LIBS) \
>            $(demanglelib)
>  size_LDADD = $(libelf) $(libeu) $(argp_LDADD)
> -strip_LDADD = $(libebl) $(libelf) $(libdw) $(libeu) $(argp_LDADD)
> +strip_LDADD = $(libebl) $(libelf) $(libdw) $(libeu) $(libdwelf)
> $(argp_LDADD)
>  elflint_LDADD  = $(libebl) $(libdw) $(libelf) $(libeu) $(argp_LDADD)
>  findtextrel_LDADD = $(libdw) $(libelf) $(libeu) $(argp_LDADD)
>  addr2line_LDADD = $(libdw) $(libelf) $(libeu) $(argp_LDADD) $(demanglelib)

These Makefile changes aren't needed since strip is already built with libdwelf and using libdwelf functions. Reverting these Makefile changes should fix the linker error.

@amerey

I believe your elfutils was lacking the dwelf_relocation_debug_sections replacement in strip.c while compiling.

Could you give a double check for whether the dwelf_relocation_debug_sections is introduced in strip binary?

$ eu-readelf -s {strip} | grep dwelf_relocation

This issue is in code review.

https://sourceware.org/pipermail/elfutils-devel/2024q3/007388.html