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

[Lukasz Majewski <lukma@denx.de>]

[-- Attachment #1: Type: text/plain, Size: 5934 bytes --]

Hi H.J, Adhemerval, Szabolcs,

> On Thu, Oct 7, 2021 at 7:18 AM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> >
> >
> > On 07/10/2021 07:00, Lukasz Majewski wrote:  
> > > On Thu, 7 Oct 2021 11:19:26 +0200
> > > Lukasz Majewski <lukma@denx.de> wrote:
> > >  
> > >> Hi Szabolcs,
> > >>  
> > >>> The 10/06/2021 13:43, Lukasz Majewski wrote:  
> > >>>> Please find in-depth analyze about the issue:
> > >>>>
> > >>>> It was tested with Beagle Bone Black (BBB) and QEMU (the same
> > >>>> binary rootfs images).
> > >>>> (If needed I will upload images and script to run QEMU to some
> > >>>> server for reproduction).
> > >>>> Branch:
> > >>>> https://github.com/lmajewski/y2038_glibc/commits/y2038_edge  
> > >>>
> > >>> i think it is easier to look at if you upload the broken
> > >>> ld.so binary. or at least readelf -aW ld.so output.  
> > >>
> > >> Please find working and broken ld-linux-armhf.so.3:
> > >> https://owncloud.denx.de/s/wtAfktG6pXLffSA
> > >>  
> > >>>  
> > >>>> On working setup to trigger the core dump:
> > >>>> /home/root/ld-linux-armhf.so.3 /sbin/init
> > >>>> gdb ./ld-linux-armhf.so.3 core
> > >>>>
> > >>>> (and the /home/root/ld-linux-armhf.so.3 is the "broken" one).
> > >>>>
> > >>>>
> > >>>> Not working (patch [1] not applied):
> > >>>> ====================================
> > >>>>
> > >>>> All the code is located in _dl_start in elf/rtld.c and
> > >>>> elf/get-dynamic-info.h files:
> > >>>>
> > >>>> (gdb) p/x $r5
> > >>>> $46 = 0xb6fc8000
> > >>>> r5 is set from the elf_machine_load_address()
> > >>>>
> > >>>> Then we enter the elf_get_dynamic_info()
> > >>>>
> > >>>> 0xb6fc95fc      99            ADJUST_DYN_INFO (DT_SYMTAB);
> > >>>>    0xb6fc95f8 <_dl_start+308>:  04 30 92 15     ldrne   r3,
> > >>>> [r2, #4] => 0xb6fc95fc <_dl_start+312>:  05 30 83 10     addne
> > >>>>   r3, r3, r5 0xb6fc9600 <_dl_start+316>:  04 30 82 15
> > >>>> strne   r3, [r2, #4] (gdb) p/x $r3
> > >>>> $63 = 0x410003f4
> > >>>> (gdb) p/x $r5
> > >>>> $64 = 0xb6fc8000  
> > >>>
> > >>> it seems r5 is already wrong here, it's not the runtime
> > >>> address of 0. (looks more like the runtime address of
> > >>> 0x41000000)  
> > >>
> > >> The r5 is set according to the change in patch which now I'm
> > >> trying to fix.
> > >>
> > >> To be more specific - r5 is set according to code in patch SHA1:
> > >> bca0f5cbc9257c13322b99e55235c4f21ba0bd82
> > >>  
> > >>>
> > >>> likely something goes wrong with the computation of r5.  
> > >>
> > >> Before the SHA1: bca0f5cbc9257c13322b99e55235c4f21ba0bd82 patch
> > >> r5 was computed with using 'adr' assembler instruction, not set
> > >> from some constant value.
> > >>
> > >> asm ("adr %0, _dl_start" : "=r" (pcrel_addr));
> > >>
> > >> And r5 value was 0x75fc8000
> > >>  
> > >>>  
> > >>>> Links:
> > >>>> [1] -
> > >>>> https://github.com/lmajewski/y2038_glibc/commit/e67e0f589b88a44be8f8b9b770b08950dd7e5bd5
> > >>>>
> > >>>> readelf -e ld-linux-armhf.so.3
> > >>>>
> > >>>> [10] .plt              PROGBITS        41000994 000994 000050
> > >>>> 04 AX  0   0  4 [11] .text             PROGBITS        41000a00
> > >>>> 000a00 01fed0 00  AX  0   0 64 [12] .rodata           PROGBITS
> > >>>> 410208d0 0208d0 004b24 00   A  0   0  4 [13] .ARM.extab
> > >>>> PROGBITS        410253f4 0253f4 000018 00   A  0   0  4 [14]
> > >>>> .ARM.exidx        ARM_EXIDX       4102540c 02540c 0000c8 00
> > >>>> AL 11 0  4 [15] .data.rel.ro      PROGBITS        41036200
> > >>>> 026200 000cf4 00  WA  0   0  8 [16] .dynamic          DYNAMIC
> > >>>> 41036ef4 026ef4 0000c8 08  WA  5   0  4 [17] .got
> > >>>> PROGBITS 41036fbc 026fbc 000040 04  WA  0   0  4  
> > >>>
> > >>> why are all addresses >0x41000000 ?
> > >>> in a shared library i expect all those addresses
> > >>> to be close to 0.  
> > >>
> > >> On this real HW system (the rootfs which is running) - libc.so.6
> > >> also has address > 0x41000000
> > >> libm.so.6 also has the value > 0x41200000
> > >> (Entry point address:               0x412c9190)
> > >>
> > >> The offset of > 0x41000000 looks a bit strange indeed, but it is
> > >> still less than the kernel space. Moreover, with position
> > >> independent code it shall not matter.
> > >>  
> > >>>
> > >>> is this made by some modified binutils?  
> > >>
> > >> I've double checked the ld-linux-armhf.so.3 and after build it
> > >> has: (Entry point address:               0xa00) which seems to
> > >> be correct.
> > >>
> > >> So it looks like during installation of the glibc (on the
> > >> Yocto/OE created rootfs) the elf header is modified and this
> > >> 0x41000000 offset is added.
> > >>  
> > >
> > > And indeed it is the case. Yocto/OE by default perform prelinking
> > > (use prelink program) to speedup start time of dynamic program.
> > >
> > > The prelink [1] itself assigns some virtual addresses to all
> > > required shared objects (in our case for /sbin/init), so no
> > > clashes are encountered.
> > >
> > > And using prelink is a _default_ behaviour in Yocto/OE poky
> > > distro.  
> >
> > Does it work without prelink? Also, does it fail with prelink in
> > real hardware?
> >
> > It indeed might be a prelink issue in fact.  
> 
> This will fail everywhere if prelink is used.
> 

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)?


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

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]