[Elftoolchain-developers] strange behaviour of elf_update()

[Luca P. <luc...@gm...>]

Hi to all.
I'm Luca and I'm using libelf on FreeBSD 7.3 to develop a tool that
manipulate ELF binaries. I hope that mailing list is right to post
this problem.

My tool changes some information in the dynamic symbol table (using
gelf_update_sym()), save the elf and quit.

elf_update(e, ELF_C_WRITE) fails with a DATA class error (Invalid data
buffer descriptor).

Investigations revealed that _libelf_xlate() fails because src->d_buf is NULL.
Further investigations revealed that the caller function
_libelf_write_elf() when wants to write the program header table, use
the e_phdr32 field of the _Elf struct. Seems that this field is not
correctly initialized.
As workaround I call gelf_getphdr() and the problem goes away.

I attach a very simple test that shows (and solve with -DWORKAROUND)
the problem.

Is this the right behavior of the library (and I need to read the
tutorial more deeply) or I've found an unlikely bug?
I've checked the source of libelf and _libelf_getphdr() (c)allocate
the phdr in _Elf struct. May be it has to be called by elf_begin()?
Have I to open a PR?

Thanks in advance

Luca


PS: In the tutorial (libelf-by-example-20100112), I've found some
errors in the Listing 3.1 (Program 2):
the functions elf_getshdrnum(), elf_getshdrstrndx() and elf_phdrnum()
doesn't exist. The right names are: elf_getshnum(), elf_getshstrndx()
and elf_phdrnum().
PS2: I like this topic and should be great if you need help and I
could be a little involved in this project!