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

[Lukasz Majewski <lukma@denx.de>]

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On Fri, 15 Oct 2021 05:21:23 -0700
"H.J. Lu" <hjl.tools@gmail.com> wrote:

> 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.
> 

Could you explain why?

Was the relocation of ld.so (I guess that ld.so = ld-linux-arm.so) a
bug from the very beginning and it was apparent just now?

From my point of view - the original change to use __ehdr_start broke
working setups, so it is a regression and shall be fixed in glibc.

Anyway, it would be beneficial to have input from other glibc
developers how to proceed with this issue.


Best regards,

Lukasz Majewski

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