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Re: libelf RDWR and elf_newscn do not work

On 11/05/2013 04:31 PM, Mark Wielaard wrote:
> On Fri, 2013-10-11 at 13:36 +0200, Mark Wielaard wrote:
>> On Thu, 2013-10-10 at 17:34 +0200, Jiri Slaby wrote:
>>> On 10/10/2013 03:16 PM, Mark Wielaard wrote:
>>>> @@ -764,7 +765,8 @@ __elfw2(LIBELFBITS,updatefile) (Elf *elf, int change_bo, size_t shnum)
>>>>  	    (*shdr_fctp) (&shdr_data[scn->index],
>>>>  			  scn->shdr.ELFW(e,LIBELFBITS),
>>>>  			  sizeof (ElfW2(LIBELFBITS,Shdr)), 1);
>>>> -	  else if (elf->state.ELFW(elf,LIBELFBITS).shdr == NULL)
>>>> +	  else if (elf->state.ELFW(elf,LIBELFBITS).shdr == NULL
>>>> +		   || (elf->flags & ELF_F_DIRTY))
>>> I seem to miss where is elf->flags |= ELF_F_DIRTY in the newscn path...
>>> Should it be added too?
>> No, I don't think it should be set there. But you do raise a good point.
>> I had assumed that since newscn increases e_shnum it would mark the
>> whole Elf dirty. But now that I look it doesn't seem to. if e_shnum is
>> changed then the ehdr->flags do get ELF_F_DIRTY set (see elf_update ->
>> elf32_updatenull). But I cannot immediately see why the whole Elf file
>> is marked dirty (although it is in your example).
>> So either I am missing something that makes it correct anyway, or the
>> check should be against ehdr->flags. I am digging...
> So elf_update will first determine the new number of sections from the
> list updated by newscn. Then it will call updatenull to determine what
> actually changed, including comparing the new shnum with the
> ehdr->e_shnum with update_if_changed, which will set the ehdr_flags to
> ELF_F_DIRTY. Then ehdr is written and the ehdr_flags dirty bit is
> cleared. So we cannot directly use that.
> But update_null will also calculate the new ehdr->e_shoff, which will
> change is any section is added/removed/changed size, etc. And in that
> case it will set the ELF_F_DIRTY bit of the elf->flags. So I do think
> the patch is correct.

Ok, it makes sense, do you plan to push the patch?

suse labs

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