[PATCH v2] dl: Use "adr" assembler command to get proper load address on ARM

[H.J. Lu]

On Fri, Oct 15, 2021 at 5:09 AM Szabolcs Nagy via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> The 10/15/2021 09:54, Lukasz Majewski wrote:
> > This change is a partial revert of commit
> > bca0f5cbc9257c13322b99e55235c4f21ba0bd82
> > "arm: Simplify elf_machine_{load_address,dynamic}" which imposed usage
> > of __ehdr_start linker variable to get the address of loaded program.
> >
> > The elf_machine_load_address() function is declared in the
> > sysdeps/arm/dl-machine.h header. It is called from (very early)
> > _dl_start() entry point for the program. It shall return the load
> > address of the dynamic linker program.
> >
> > With this revert the 'adr' assembler instruction is used instead of a
> > place holder:
> >
> > arm-poky-linux-gnueabi-objdump -t ld-linux-armhf.so.3 | grep ehdr
> > 00000000 l       .note.gnu.build-id     00000000      __ehdr_start
> >
> > which is pre-set by binutils.
> >
> > The problem starts when one runs 'prelink' on the rootfs created with
> > for example OE/Yocto.
> > Then the _ehdr_start stays as 0x0, but the ELF header's sections have
> > different addresses - for example 0x41000000 instead of the originally
> > set 0x0.
> >
> > This is crucial when /sbin/init is executed. Value set in __ehdr_start
> > symbol is not updated. This causes the program to crash very early
> > when ld-linux-armhf.so.3's _dl_start is executed, as calculated offset
> > for loader relocation is going to hit the kernel space (0xf7xxyyyy).
> >
> > It looks like the correct way to obtain the _dl_start offset on ARM is
> > to use assembler instruction 'adr' at execution time (so the prelink
> > assigned offset is taken into consideration) instead of __ehdr_start.
> >
> > With this patch we only modify the elf_machine_load_address() function,
> > as it is called very early, before the ld-linux-armhf.so.3 is performing
> > relocation (also its own one).
>
> i'd use an explanation like:
>
> __ehdr_start is a linker created symbol that points to the elf header.
> The elf header is at the beginning of the elf file and normally its
> virtual address is 0 in a shared library.  This means the runtime
> address of __ehdr_start is the load address of the module.  However if
> prelinking is applied to ld.so then all virtual addresses are moved by
> an offset so the runtime address of the elf header becomes the load
> address + prelink offset.  The kernel does not treat prelinked ld.so
> specially so the load address is not 0, it still has to be computed,
> but simply using __ehdr_start no longer gives a correct value for that.
>
> This issue affects all targets with prelinking support, but so far we
> only got reports from OE/Yocto builds for arm that has prelinked ld.so.
>
> but i think a better fix is possible than revert:

I think either prelink should be fixed not to prelink ld.so or Yocto
should be fixed not to prelink ld.so.

-- 
H.J.