Re: [PATCH] BaseTools/tools_def: add "-fno-unwind-tables" to GCC_AARCH64_CC_FLAGS

[Laszlo Ersek <lersek@redhat.com>]

On 05/23/18 16:21, Gao, Liming wrote:
> Laszlo:
>   I agree to add this change first in tools_def.txt. Then, update IA32/X64 arch when there is a real request. For long term, I prefer to file a BZ for GccBase.lds / tools_def.template, to discard or strip .eh_frame under GCC.
> 
>   Reviewed-by: Liming Gao <liming.gao@intel.com>

Thanks! I've pushed this patch now as commit cbf00651eda6, and filed
<https://bugzilla.tianocore.org/show_bug.cgi?id=955>.

Cheers,
Laszlo


>> -----Original Message-----
>> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Laszlo Ersek
>> Sent: Tuesday, May 22, 2018 5:30 PM
>> To: Leif Lindholm <leif.lindholm@linaro.org>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; edk2-devel-01 <edk2-devel@lists.01.org>; Gao, Liming <liming.gao@intel.com>;
>> pjones@redhat.com; Cole Robinson <crobinso@redhat.com>; Paolo Bonzini <pbonzini@redhat.com>; Danilo C. L. de Paula
>> <ddepaula@redhat.com>
>> Subject: Re: [edk2] [PATCH] BaseTools/tools_def: add "-fno-unwind-tables" to GCC_AARCH64_CC_FLAGS
>>
>> On 05/21/18 12:30, Leif Lindholm wrote:
>>> On Fri, May 18, 2018 at 08:23:01PM +0200, Laszlo Ersek wrote:
>>>> The ElfConvert routines in GenFw don't handle the ".eh_frame" ELF section
>>>> emitted by gcc. For this reason, Leif disabled the generation of that
>>>> section for AARCH64 with "-fno-asynchronous-unwind-tables" in commit
>>>> 28e80befa4fe [1], and Ard did the same for IA32 and X64 in commit
>>>> 26ecc55c027d [2]. (The CLANG38 toolchain received the same flag at its
>>>> inception, in commit 6f756db5ea05 [3].)
>>>>
>>>> However, ".eh_frame" is back now; in upstream gcc commit 9cbee213b579 [4]
>>>> (part of tag "gcc-8_1_0-release"), both "-fasynchronous-unwind-tables" and
>>>> "-funwind-tables" were made the default for AARCH64. (The patch author
>>>> described the effects on the gcc mailing list [5].) We have to counter the
>>>> latter flag with "-fno-unwind-tables", otherwise GenFw chokes on
>>>> ".eh_frame" again (triggered for example on Fedora 28).
>>>>
>>>> "-f[no-]unwind-tables" goes back to at least gcc-4.4 [6], so it's safe to
>>>> add to GCC_AARCH64_CC_FLAGS.
>>>
>>> I'm not against this patch (which fixes a real problem), but would
>>> it be possible to take the opposite approach, as Peter Jones did for
>>> GRUB when the same issue hit there?:
>>>
>>> http://lists.gnu.org/archive/html/grub-devel/2018-02/msg00095.html
>>>
>>> I.e., rather than keep adding flags when new "special" sections are
>>> added by toolchains, explicitly filter those out as part of the link
>>> step.
>>
>> I guess this could be implemented in two places in BaseTools:
>>
>> (1) In GenFw / ElfConvert. I'm afraid that's beyond my binutils-fu at
>> the moment.
>>
>> (2) In "BaseTools/Scripts/GccBase.lds". This is similarly beyond my
>> binutils-fu, but at least I can make more comments on it:
>>
>> We already discard a good number of sections, but we keep (and realign)
>> "eh_frame". From Ard's commit message 26ecc55c027d, I believe the idea
>> here was to "retain the .eh_frame data for external debugging", and then
>> strip it with "objcopy -R .eh_frame" finally (see
>> [Dynamic-Library-File], <Command.GCC, Command.GCCLD> in
>> "BaseTools/Conf/build_rule.template").
>>
>> However, in practice we stub out objcopy (see under "GCC Common" in
>> "BaseTools/Conf/tools_def.template"), and then we must prevent the
>> generation of .eh_frame in the first place.
>>
>> Rather than adding "-fno-unwind-tables", I considered moving .eh_frame
>> to /DISCARD/, or else turning $(OBJCOPY) into the real thing. However,
>> that would affect more targets than just AARCH64/GCC, and I really
>> didn't (don't) remember why we have this redundancy with "objcopy -R
>> .eh_frame" vs. "-no-fasynchronous-unwind-tables". The latest practice
>> seemed to be "add more -fno-..." (see the commits I referenced above),
>> so I went with that.
>>
>>
>> I suggest we push this patch now, plus I could:
>>
>> - submit a followup for IA32/X64, if that's deemed best,
>> - file a BZ for GenFw / ElfConvert about the programmatic relocation
>>   filtering,
>> - file a BZ for GccBase.lds / tools_def.template, to discard or strip
>>   .eh_frame under GCC.
>>
>> Thanks
>> Laszlo
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