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

[Lukasz Majewski <lukma@denx.de>]

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On Mon, 11 Oct 2021 05:01:23 -0700
"H.J. Lu" <hjl.tools@gmail.com> wrote:

> 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.

prelink is used also to speed things up. In some use cases - boot time
optimization (like kiosk, or automotive) the bootup time is crucial.

IMHO, the change with __ehdr_start introduced regression and hence
shall be fixed so the previous behaviour is preserved.

> 
> > (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
> >
> > --
> >
> > DENX Software Engineering GmbH,      Managing Director: Wolfgang
> > Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell,
> > Germany Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email:
> > lukma@denx.de  
> 
> 
> 




Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@denx.de

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