From: "H.J. Lu" <hjl.tools@gmail.com>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Lukasz Majewski <lukma@denx.de>,
Florian Weimer <fweimer@redhat.com>,
libc-alpha <libc-alpha@sourceware.org>,
Andreas Schwab <schwab@linux-m68k.org>,
Joseph Myers <joseph@codesourcery.com>
Subject: Re: [PATCH v2] dl: Use "adr" assembler command to get proper load address on ARM
Date: Fri, 15 Oct 2021 05:21:23 -0700 [thread overview]
Message-ID: <CAMe9rOpPM7RmA65MzTNr2DtoC_wMFt87mnyJ4pgvmK5TAorCCQ@mail.gmail.com> (raw)
In-Reply-To: <20211015120915.GD1982710@arm.com>
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.
next prev parent reply other threads:[~2021-10-15 12:22 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-07 13:16 [PATCH] dl: Use "adr" assembler command to get proper load address Lukasz Majewski
2021-09-07 16:49 ` Fangrui Song
2021-09-07 17:32 ` Lukasz Majewski
2021-09-07 17:44 ` Fangrui Song
2021-09-08 15:05 ` Lukasz Majewski
2021-09-08 17:41 ` Fāng-ruì Sòng
2021-09-08 19:19 ` Adhemerval Zanella
2021-09-08 20:34 ` Lukasz Majewski
2021-09-09 7:18 ` Lukasz Majewski
2021-09-09 9:49 ` Lukasz Majewski
2021-09-10 10:10 ` Lukasz Majewski
2021-09-17 8:29 ` Lukasz Majewski
2021-09-17 13:27 ` Joseph Myers
2021-09-17 16:17 ` Andreas Schwab
2021-09-26 19:58 ` Lukasz Majewski
2021-09-27 16:00 ` Joseph Myers
2021-10-05 7:45 ` Lukasz Majewski
2021-10-06 7:57 ` Fangrui Song
2021-10-06 9:03 ` Lukasz Majewski
2021-10-06 11:43 ` Lukasz Majewski
2021-10-06 12:55 ` Szabolcs Nagy
2021-10-07 9:19 ` Lukasz Majewski
2021-10-07 10:00 ` Lukasz Majewski
2021-10-07 14:15 ` Szabolcs Nagy
2021-10-07 14:58 ` Lukasz Majewski
2021-10-07 14:16 ` Adhemerval Zanella
2021-10-07 14:29 ` H.J. Lu
2021-10-07 15:57 ` Szabolcs Nagy
2021-10-07 16:22 ` H.J. Lu
2021-10-07 16:53 ` Adhemerval Zanella
2021-10-07 17:05 ` H.J. Lu
2021-10-07 17:24 ` Fāng-ruì Sòng
2021-10-08 9:15 ` Szabolcs Nagy
2021-10-11 8:56 ` Lukasz Majewski
2021-10-11 10:18 ` Szabolcs Nagy
2021-10-11 11:47 ` Lukasz Majewski
2021-10-11 12:01 ` H.J. Lu
2021-10-11 13:10 ` Lukasz Majewski
2021-10-11 13:22 ` H.J. Lu
2021-10-11 14:31 ` Lukasz Majewski
2021-10-11 13:34 ` Adhemerval Zanella
2021-10-11 12:48 ` Szabolcs Nagy
2021-10-15 7:54 ` [PATCH v2] dl: Use "adr" assembler command to get proper load address on ARM Lukasz Majewski
2021-10-15 12:09 ` Szabolcs Nagy
2021-10-15 12:21 ` H.J. Lu [this message]
2021-10-15 12:59 ` Lukasz Majewski
2021-10-15 23:53 ` Fāng-ruì Sòng
2021-10-18 11:08 ` Szabolcs Nagy
2021-10-18 11:35 ` Florian Weimer
2021-10-19 12:03 ` Lukasz Majewski
2021-10-25 10:18 ` Lukasz Majewski
2021-10-25 10:25 ` Florian Weimer
2021-10-25 10:53 ` Lukasz Majewski
2021-10-25 13:34 ` Szabolcs Nagy
2021-10-25 14:04 ` Lukasz Majewski
2021-10-25 15:09 ` Szabolcs Nagy
2021-10-25 17:26 ` Joseph Myers
2021-10-26 13:52 ` Lukasz Majewski
2021-10-26 20:55 ` Joseph Myers
2021-10-27 9:38 ` Szabolcs Nagy
2021-10-25 18:25 ` Lukasz Majewski
2021-10-15 13:59 ` Lukasz Majewski
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