Re: [PATCH] dl: Use "adr" assembler command to get proper load address

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Oct 11, 2021 at 4:47 AM Lukasz Majewski <lukma@denx.de> wrote:
>
> Hi Szabolcs,
>
> Thank you very much for your input.
>
> > The 10/11/2021 10:56, Lukasz Majewski wrote:
> > > Do we have _any_ plan how to fix it? BSPs for many boards are built
> > > with Yocto/OE nowadays...
> > >
> > > The approach with using 'adr' (pseudo) assembler instruction to
> > > calculate the on-runtime addresses was working previously.
> > >
> > > Shall we revert changes which were introduced recently (to use
> > > __ehdr_start)?
> >
> > the reason for the change was to avoid relying on GOT[0].
>
> And why it is requested to not relying on GOT[0]?
>
> >
> > i guess we can revert elf_machine_load_address on arm, but
> > keep the new elf_machine_dynamic that does not rely on GOT[0].
>
> This was the approach proposed by this patch.
>
> >
> > it is also useful to have the same c code across targets
> > for the load address. if we want to do that and support
> > vaddr!=0 for ehdr then i think it can be (untested):
> >
> > __attribute__ ((const)) ElfW(Addr)
> > load_addr (void)
> > {
> >   extern const ElfW(Ehdr) __ehdr_start __attribute__ ((visibility
> > ("hidden"))); ElfW(Addr) addr = (ElfW(Addr)) &__ehdr_start;
> >   ElfW(Word) phnum = __ehdr_start.e_phnum;
> >   const ElfW(Phdr) *phdr = (const void *) (addr +
> > __ehdr_start.e_phoff); const ElfW(Phdr) *ph;
> >   assert (__ehdr_start.e_phentsize == sizeof *phdr);
> >   for (ph = phdr; ph < &phdr[phnum]; ++ph)
> >     if (ph->p_type == PT_LOAD && ph->p_offset == 0)
> >       {
> >         addr -= ph->p_vaddr;
>
> The above line is a bit strange for me. Isn't the p_offset set by
> prelink as well?
>
> And most of all - why do we need to perform relocation in such very
> early stage of the ld-linux-armhf.so.3 ?
>
> >         break;
> >       }
> >   return addr;
> > }
> >
> > i don't have a strong preference either way:
> > 1) fix yocto
>
> Yocto by default uses prelinkg from the very long time. I think that it
> would be very difficult to change that.

Just don't run prelink on ld.so since the load address is controlled
by kernel.

> (moreover, IMHO running prelink on any binary and glibc is a valid use
> case)
>
> > 2) partial revert
>
> Ok.
>
> > 3) new way to compute the load address.
>
> This would require some input from developers deeply involved in ELF
> loader development. (Florian IIRC).
>
> Best regards,
>
> Lukasz Majewski
>
> --
>
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-- 
H.J.