Cont.: Adding a new section to an ELF file aborts with Assertion `shdr != NULL'

[☯ D.Snezhkov]

Thank you, Mark. It worked.

Sequence I followed:

```
 // Open ELF readwrite
    if ((e = elf_begin(fd, ELF_C_RDWR, NULL)) == NULL)
        errx(EXIT_FAILURE, "elf_begin() failed: %s.", elf_errmsg(-1));

    // Read existing section
    scn = elf_nextscn(e, NULL);
    shdr = elf64_getshdr(scn);

    // Create new section (index after last)
    if ((scn = elf_newscn(e)) == NULL)
        errx(EXIT_FAILURE, "elf_newscn() failed: %s.", elf_errmsg(-1));
```


On Fri, Dec 24, 2021 at 4:15 PM Mark Wielaard <mark@klomp.org> wrote:

> Hi,
>
> On Fri, Dec 24, 2021 at 02:42:05PM -0600, P.B. via Elfutils-devel wrote:
> > Sorry I for some reason could not reply to the original thread here to
> > follow up:
> > https://www.mail-archive.com/elfutils-devel@sourceware.org/msg04076.html
> >
> > But I am facing the same issue, adding the section to a very simple ELF
> > file fails with  Assertion `shdr != NULL'. I am new to this so I am sure
> I
> > am doing something wrong, but not exactly certain what it is. I have also
> > tried to open ELF with ELF_C_WRITE as opposed to ELF_C_RDWR but then my
> > error is "elf_newdata() failed: executable header not created first" so I
> > am not sure what the proper sequence of events would be to add a section.
> >
> > Below is my code and a run with the message.
>
> Unfortunately the original report didn't come with full example
> sources and your sources aren't complete either. But I just played a
> bit with some simple code that adds a section and I think it is a bug
> in libelf. As the original report said (in a followup) actually
> reading any of the existing section headers works around the assert. I
> think we missed this bug because normally you would at least read the
> existing section headers before adding new sections.
>
> Could you try first reading some section header before adding a new one?
> e.g. just add something like:
>     scn = elf_nextscn(e, NULL);
>     shdr = elf64_getshdr(scn);
>
> Hope that helps to workaround the bug for now.
>
> Cheers,
>
> Mark
>