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

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

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.

-- 
H.J.