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

[Lukasz Majewski <lukma@denx.de>]

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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.

Links:
[1] - https://linux.die.net/man/8/prelink

> 
> 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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