elftoolchain-developers

[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!

Re: [Elftoolchain-developers] strange behaviour of elf_update()

[Kai W. <kai...@gm...>]

Hi Luca,

On Tue, Mar 02, 2010 at 02:12:31PM +0100, Luca Pizzamiglio wrote:
> 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.

Thanks for reporting this problem.

It should be already fixed by elftoolchain svn repository revsion 305.
However it's not yet been merged back to FreeBSD.

You can fetch the diff (try svn diff -c305 in trunk/libelf) and apply
it to the libelf in FreeBSD 7.3.

Cheers,
Kai

Re: [Elftoolchain-developers] strange behaviour of elf_update()

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

Hi Kay,

I've tried the patch and it works fine!

Thank you very much!

Best regards,

Luca



On Tue, Mar 2, 2010 at 2:34 PM, Kai Wang <kai...@gm...> wrote:
> Hi Luca,
>
> On Tue, Mar 02, 2010 at 02:12:31PM +0100, Luca Pizzamiglio wrote:
>> 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.
>
> Thanks for reporting this problem.
>
> It should be already fixed by elftoolchain svn repository revsion 305.
> However it's not yet been merged back to FreeBSD.
>
> You can fetch the diff (try svn diff -c305 in trunk/libelf) and apply
> it to the libelf in FreeBSD 7.3.
>
> Cheers,
> Kai
>