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Date: Thu, 30 May 2019 14:20:44 +0800
From: Fangrui Song <i@...kray.me>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] fix musl-gcc.specs.sh to correctly handle
-static-pie
On 2019-05-29, Rich Felker wrote:
>On Tue, May 28, 2019 at 10:19:18PM +0200, Ferdi265 wrote:
>> Hello,
>>
>> Today I wanted to use musl-gcc to build -static-pie binaries. I noticed
>> that this does not work (musl-libc is still linked as a shared library
>> and is also still requested as an interpreter).
>>
>> Looking into the musl-gcc.specs file the bug was obvious: rcrt1.o was
>> not used as a startfile, and neither -no-dynamic-linker nor -static were
>> passed to the linker.
>>
>> This patch fixes this by actually using rcrt1.o and passing the linker
>> options when -static-pie is given.
>>
>> I don't have much experience with the specifics of gcc .spec files and
>> which options need to be passed, but this seems to work with all
>> variations of -shared, -static, and -static-pie that I've tried.
>>
>> Here (https://github.com/Ferdi265/musl) is the repository with the patch
>> on GitHub, and I've also attached the patch below.
>>
>> Greetings,
>> Ferdinand "Ferdi265" Bachmann
>>
>> --- PATCH BELOW ---
>>
>> From 070bce8f7e508a951d3b65da227b2fca3a65f37b Mon Sep 17 00:00:00 2001
>> From: Ferdinand Bachmann <theferdi265@...il.com>
>> Date: Tue, 28 May 2019 21:53:25 +0200
>> Subject: [PATCH] fix musl-gcc.specs.sh to correctly handle -static-pie
>>
>> ---
>> tools/musl-gcc.specs.sh | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/musl-gcc.specs.sh b/tools/musl-gcc.specs.sh
>> index 30492574..7206cb25 100644
>> --- a/tools/musl-gcc.specs.sh
>> +++ b/tools/musl-gcc.specs.sh
>> @@ -17,13 +17,13 @@ cat <<EOF
>> libgcc.a%s %:if-exists(libgcc_eh.a%s)
>>
>> *startfile:
>> -%{!shared: $libdir/Scrt1.o} $libdir/crti.o crtbeginS.o%s
>> +%{static-pie: $libdir/rcrt1.o} %{!static-pie: %{!shared:
>> $libdir/Scrt1.o}} $libdir/crti.o crtbeginS.o%s
>>
>> *endfile:
>> crtendS.o%s $libdir/crtn.o
>>
>> *link:
>> --dynamic-linker $ldso -nostdlib %{shared:-shared} %{static:-static}
>> %{rdynamic:-export-dynamic}
>> +%{static-pie:-no-dynamic-linker -static} %{!static-pie:-dynamic-linker
>> $ldso} -nostdlib %{shared:-shared} %{static:-static}
>> %{rdynamic:-export-dynamic}
>>
>> *esp_link:
>>
>
>This looks okay-ish, but could be improved. GCC spec syntax doesn't
>require separate static-pie and !static-pie conditions; it supports
>"else" with notation:
>
> %{static-pie:-no-dynamic-linker -static;-dynamic-linker $ldso}
>
>There's also the question of whether we should make -static -pie work
>like it's intended to by us, or like gcc does it upstream (ignoring
>-pie). I think it's hard to duplicate the behavior we want since it
>depends on knowing if the compiler was built as default-pie, so the
>way you've done it is probably the best we can easily do, and at least
>non-broken.
>
>Anyone else have comments on this?
gcc -dumpspecs says:
%{static-pie:-static -pie --no-dynamic-linker -z text}
Do we want to specify -z text?
In ld.bfd and gold, -z notext is the default. If text relocations are needed:
-z notext: allow and emit DF_TEXTREL. DF_TEXTREL is not emitted if there is no text relocation.
-z text: error
In lld, -z text is the default (this change is a no-op).
-z text: error on text relocations
-z notext: allow text relocations, and emit DF_TEXTREL no matter whether text relocations exist.
While comparing the differences, I also came across this rule:
%{!static|static-pie:--eh-frame-hdr}
> It means "pass --eh-frame-hdr to ld if -static isn't used or -static-pie
> is used since -static-pie needs PT_GNU_EH_FRAME segment.
This is related to PT_GNU_EH_FRAME (it is produced if .eh_frame_hdr exists) and various unwinders.
I'm not particularly clear about the internals. Some related links:
https://reviews.llvm.org/D19029 FreeBSD folk said LLVM's libunwind needed this on ARM
I think this piece of code is relevant
https://github.com/llvm-mirror/libunwind/blob/master/src/AddressSpace.hpp#L532
https://reviews.llvm.org/D43203 clang -static adds --eh-frame-hdr unconditionally
https://www.openwall.com/lists/musl/2014/06/16/2 Re: gcc'c crtstuff.c: a musl-related experience
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