Re: [PATCH] BaseTools/GenFw ARM: don't permit R_ARM_GOT_PREL relocations

["Gao, Liming" <liming.gao@intel.com>]

That's good. Reviewed-by: Liming Gao <liming.gao@intel.com>

>-----Original Message-----
>From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
>Sent: Tuesday, December 11, 2018 9:46 PM
>To: Gao, Liming <liming.gao@intel.com>
>Cc: Leif Lindholm <leif.lindholm@linaro.org>; edk2-devel@lists.01.org; Laszlo
>Ersek <lersek@redhat.com>; Feng, Bob C <bob.c.feng@intel.com>
>Subject: Re: [PATCH] BaseTools/GenFw ARM: don't permit
>R_ARM_GOT_PREL relocations
>
>On Tue, 11 Dec 2018 at 14:40, Gao, Liming <liming.gao@intel.com> wrote:
>>
>> Ard:
>>   With this change, GenFw will report what error message if ELF image has
>R_ARM_GOT_PREL relocations.
>>
>
>Numerous occurrences of
>
>GenFw: ERROR 3000: Invalid
>  WriteSections ():
>/home/ard/build/edk2-workspace/Build/ArmVirtQemuKernel-
>ARM/RELEASE_CLANG38/ARM/ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreReloc
>atable/DEBUG/ArmVirtPrePiUniCoreRelocatable.dll
>unsupported ELF EM_ARM relocation 0x60.
>
>
>> > -----Original Message-----
>> > From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
>> > Sent: Tuesday, December 11, 2018 7:21 PM
>> > To: Leif Lindholm <leif.lindholm@linaro.org>
>> > Cc: edk2-devel@lists.01.org; Laszlo Ersek <lersek@redhat.com>; Feng,
>Bob C <bob.c.feng@intel.com>; Gao, Liming
>> > <liming.gao@intel.com>
>> > Subject: Re: [PATCH] BaseTools/GenFw ARM: don't permit
>R_ARM_GOT_PREL relocations
>> >
>> > On Tue, 11 Dec 2018 at 12:19, Ard Biesheuvel <ard.biesheuvel@linaro.org>
>wrote:
>> > >
>> > > On Tue, 11 Dec 2018 at 10:53, Leif Lindholm <leif.lindholm@linaro.org>
>wrote:
>> > > >
>> > > > On Tue, Dec 11, 2018 at 10:37:15AM +0100, Ard Biesheuvel wrote:
>> > > > > We currently permit R_ARM_GOT_PREL relocations in the ELF32
>conversion
>> > > > > routines, under the assumption that relative relocations are fine as
>> > > > > long as the section layout is the same between ELF and PE/COFF.
>> > > > >
>> > > > > However, as is the case with any proxy generating relocation, it is
>> > > > > up to the linker to emit an entry in the GOT table and populate it
>> > > > > with the correct absolute address, which should also be fixed up at
>> > > > > PE/COFF load time. Unfortunately, the relocations covering the GOT
>> > > > > section are not emitted into the static relocation sections processed
>> > > > > by GenFw, but only in the dynamic relocation section as a
>R_ARM_RELATIVE
>> > > > > relocation, and so GenFw fails to emit the correct PE/COFF relocation
>> > > > > data for GOT entries.
>> > > > >
>> > > > > Since GOT indirection is pointless anyway for PE/COFF modules
>running
>> > > > > in UEFI context, let's just drop the references to R_ARM_GOT_PREL
>from
>> > > > > GenFw, resulting in a build time failure rather than a runtime failure
>> > > > > if such relocations do occur.
>> > > > >
>> > > > > Cc: Bob Feng <bob.c.feng@intel.com>
>> > > > > Cc: Liming Gao <liming.gao@intel.com>
>> > > > > Cc: Leif Lindholm <leif.lindholm@linaro.org>
>> > > > > Contributed-under: TianoCore Contribution Agreement 1.1
>> > > > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> > > >
>> > > > Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
>> > > >
>> > > > Ouch. This sounds like the best move for now. But how do we deal
>with
>> > > > builds that actually break?
>> > > >
>> > >
>> > > So the only builds that are breaking due to this are ones where we run
>> > > the linker in PIE mode (which only happens in
>> > > ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf), and using the
>> > > GNU gold linker. The reason we need the -pie option is to force the
>> > > linker to emit dynamic relocations into the binary so it can relocate
>> > > itself. This is necessary because the firmware image may execute from
>> > > a a priori unknown memory offset.
>> > >
>> > > I am playing around with hidden visibility and other tweaks to coerce
>> > > the linker into emitting direct relative references instead of GOT
>> > > based ones, and it is very tedious. The GOLD linker really doesn't
>> > > appear to be set up for bare metal binaries.
>> >
>> > Oh, and on AARCH64 it is even more annoying, given that the relative
>> > GOT references are emitted as ADRP/ADD pairs, which means we have
>the
>> > 4 KB alignment issue as well.