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Date: Mon, 6 Apr 2020 15:04:03 -0700
From: Fangrui Song <i@...kray.me>
To: harald@...hat.com
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] V2 Enable linking to a static position
independent executable
On 2020-04-06, harald@...hat.com wrote:
>From: Harald Hoyer <harald@...hat.com>
>
>This also enables address space layout randomization (ASLR).
>
>$ cat hello.c
>
>int main()
>{
> printf("main = 0x%lxd\n", main);
> return 0;
>}
>
>$ gcc -static-pie -o hello hello.c -specs musl-gcc.specs
>
>$ ldd hello
> statically linked
>
>$ file hello
>hello: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=24fe0d02b0558dc37b8ae2268878b118804ae72d, with debug_info, not stripped
>
>$ ./hello
>main = 0x7f858c4e72b9d
>
>$ ./hello
>main = 0x7f0854d312b9d
>
>$ ./hello
>main = 0x7f7179a1d2b9d
>
>$ ./hello
>main = 0x7f37f981b2b9d
>
>$ readelf -l hello
>
>Elf file type is DYN (Shared object file)
>Entry point 0x1058
>There are 9 program headers, starting at offset 64
>
>Program Headers:
> Type Offset VirtAddr PhysAddr
> FileSiz MemSiz Flags Align
> LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
> 0x00000000000003c0 0x00000000000003c0 R 0x1000
> LOAD 0x0000000000001000 0x0000000000001000 0x0000000000001000
> 0x0000000000002cce 0x0000000000002cce R E 0x1000
> LOAD 0x0000000000004000 0x0000000000004000 0x0000000000004000
> 0x0000000000001550 0x0000000000001550 R 0x1000
> LOAD 0x0000000000005e50 0x0000000000006e50 0x0000000000006e50
> 0x00000000000002e0 0x00000000000009a0 RW 0x1000
> DYNAMIC 0x0000000000005e70 0x0000000000006e70 0x0000000000006e70
> 0x0000000000000170 0x0000000000000170 RW 0x8
> NOTE 0x0000000000000238 0x0000000000000238 0x0000000000000238
> 0x0000000000000024 0x0000000000000024 R 0x4
> GNU_EH_FRAME 0x0000000000004b60 0x0000000000004b60 0x0000000000004b60
> 0x00000000000001ec 0x00000000000001ec R 0x4
> GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
> 0x0000000000000000 0x0000000000000000 RW 0x10
> GNU_RELRO 0x0000000000005e50 0x0000000000006e50 0x0000000000006e50
> 0x00000000000001b0 0x00000000000001b0 R 0x1
>
> Section to Segment mapping:
> Segment Sections...
> 00 .note.gnu.build-id .gnu.hash .dynsym .dynstr .rela.dyn
> 01 .init .plt .text .fini
> 02 .rodata .eh_frame_hdr .eh_frame
> 03 .init_array .fini_array .data.rel.ro .dynamic .got .got.plt .data .bss
> 04 .dynamic
> 05 .note.gnu.build-id
> 06 .eh_frame_hdr
> 07
> 08 .init_array .fini_array .data.rel.ro .dynamic .got
>---
> tools/musl-gcc.specs.sh | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
> mode change 100644 => 100755 tools/musl-gcc.specs.sh
>
>diff --git a/tools/musl-gcc.specs.sh b/tools/musl-gcc.specs.sh
>old mode 100644
>new mode 100755
>index 30492574..72f5f487
>--- a/tools/musl-gcc.specs.sh
>+++ b/tools/musl-gcc.specs.sh
>@@ -8,7 +8,7 @@ cat <<EOF
> -nostdinc -isystem $incdir -isystem include%s %(old_cpp_options)
>
> *cc1:
>-%(cc1_cpu) -nostdinc -isystem $incdir -isystem include%s
>+%(cc1_cpu) %{static-pie:-fPIE} -nostdinc -isystem $incdir -isystem include%s
The intention of the compiler driver option -static-pie is that it only
affects linking options (-static -pie --no-dynamic-linker -z text), not compile options (-fPIE).
(I don't know why -z text (which is not too relevant) ended up in GCC's default specs.
Anyway, I followed suit for clang: https://reviews.llvm.org/D62606)
> *link_libgcc:
> -L$libdir -L .%s
>@@ -17,13 +17,13 @@ cat <<EOF
> libgcc.a%s %:if-exists(libgcc_eh.a%s)
>
> *startfile:
>-%{!shared: $libdir/Scrt1.o} $libdir/crti.o crtbeginS.o%s
>+%{shared:;static:$libdir/crt1.o%s; static-pie:$libdir/rcrt1.o%s; pie:$libdir/Scrt1.o%s; :$libdir/crt1.o%s} $libdir/crti.o%s %{static:crtbeginT.o%s; shared|static-pie|pie:crtbeginS.o%s; :crtbegin.o%s}
>
> *endfile:
>-crtendS.o%s $libdir/crtn.o
>+%{static:crtend.o%s; shared|static-pie|pie:crtendS.o%s; :crtend.o%s} $libdir/crtn.o%s
>
> *link:
>--dynamic-linker $ldso -nostdlib %{shared:-shared} %{static:-static} %{rdynamic:-export-dynamic}
>+%{!r:--build-id} --no-add-needed %{!static|static-pie:--eh-frame-hdr} --hash-style=gnu %{shared:-shared} %{!shared:%{!static:%{!static-pie:%{rdynamic:-export-dynamic} -dynamic-linker $ldso}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}}
>
> *esp_link:
>
>--
>2.25.2
>
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