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Date: Sat, 4 Mar 2017 21:41:24 -0600 (CST)
From: Paul Sturm <psturm@...putervoice.com>
To: musl <musl@...ts.openwall.com>
Subject: Re: Static PIE with musl and clang

OK. I tried to compile this program: 

#include <stdio.h> 

int main() { 
printf("%p\n", main); 
return 0; 
} 

Command line: 
clang /usr/lib/rcrt1.o ../aslr.c -o aslr -fPIE -shared -Bsymbolic -Wl,--no-dynamic-linker 

./aslr seg faults when I run it. 

readelf -e aslr looks like this: 

ELF Header: 
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
Class: ELF64 
Data: 2's complement, little endian 
Version: 1 (current) 
OS/ABI: UNIX - System V 
ABI Version: 0 
Type: DYN (Shared object file) 
Machine: Advanced Micro Devices X86-64 
Version: 0x1 
Entry point address: 0x788 
Start of program headers: 64 (bytes into file) 
Start of section headers: 6584 (bytes into file) 
Flags: 0x0 
Size of this header: 64 (bytes) 
Size of program headers: 56 (bytes) 
Number of program headers: 6 
Size of section headers: 64 (bytes) 
Number of section headers: 26 
Section header string table index: 23 

Section Headers: 
[Nr] Name Type Address Offset 
Size EntSize Flags Link Info Align 
[ 0] NULL 0000000000000000 00000000 
0000000000000000 0000000000000000 0 0 0 
[ 1] .hash HASH 0000000000000190 00000190 
0000000000000058 0000000000000004 A 2 0 8 
[ 2] .dynsym DYNSYM 00000000000001e8 000001e8 
0000000000000198 0000000000000018 A 3 1 8 
[ 3] .dynstr STRTAB 0000000000000380 00000380 
00000000000000de 0000000000000000 A 0 0 1 
[ 4] .rela.dyn RELA 0000000000000460 00000460 
00000000000000f0 0000000000000018 A 2 0 8 
[ 5] .rela.plt RELA 0000000000000550 00000550 
0000000000000048 0000000000000018 AI 2 19 8 
[ 6] .init PROGBITS 0000000000000598 00000598 
000000000000000d 0000000000000000 AX 0 0 1 
[ 7] .plt PROGBITS 00000000000005b0 000005b0 
0000000000000040 0000000000000010 AX 0 0 16 
[ 8] .plt.got PROGBITS 00000000000005f0 000005f0 
0000000000000018 0000000000000000 AX 0 0 8 
[ 9] .text PROGBITS 0000000000000610 00000610 
0000000000000497 0000000000000000 AX 0 0 16 
[10] .fini PROGBITS 0000000000000aa7 00000aa7 
0000000000000008 0000000000000000 AX 0 0 1 
[11] .rodata PROGBITS 0000000000000aaf 00000aaf 
0000000000000004 0000000000000001 AMS 0 0 1 
[12] .eh_frame_hdr PROGBITS 0000000000000ab4 00000ab4 
000000000000001c 0000000000000000 A 0 0 4 
[13] .eh_frame PROGBITS 0000000000000ad0 00000ad0 
0000000000000064 0000000000000000 A 0 0 8 
[14] .ctors PROGBITS 0000000000200e40 00000e40 
0000000000000010 0000000000000000 WA 0 0 8 
[15] .dtors PROGBITS 0000000000200e50 00000e50 
0000000000000010 0000000000000000 WA 0 0 8 
[16] .jcr PROGBITS 0000000000200e60 00000e60 
0000000000000008 0000000000000000 WA 0 0 8 
[17] .dynamic DYNAMIC 0000000000200e68 00000e68 
0000000000000150 0000000000000010 WA 3 0 8 
[18] .got PROGBITS 0000000000200fb8 00000fb8 
0000000000000048 0000000000000008 WA 0 0 8 
[19] .got.plt PROGBITS 0000000000201000 00001000 
0000000000000030 0000000000000008 WA 0 0 8 
[20] .data PROGBITS 0000000000201030 00001030 
0000000000000008 0000000000000000 WA 0 0 8 
[21] .bss NOBITS 0000000000201040 00001038 
0000000000000050 0000000000000000 WA 0 0 32 
[22] .comment PROGBITS 0000000000000000 00001038 
000000000000003c 0000000000000001 MS 0 0 1 
[23] .shstrtab STRTAB 0000000000000000 000018fb 
00000000000000b9 0000000000000000 0 0 1 
[24] .symtab SYMTAB 0000000000000000 00001078 
0000000000000648 0000000000000018 25 51 8 
[25] .strtab STRTAB 0000000000000000 000016c0 
000000000000023b 0000000000000000 0 0 1 
Key to Flags: 
W (write), A (alloc), X (execute), M (merge), S (strings), I (info), 
L (link order), O (extra OS processing required), G (group), T (TLS), 
C (compressed), x (unknown), o (OS specific), E (exclude), 
l (large), p (processor specific) 

Program Headers: 
Type Offset VirtAddr PhysAddr 
FileSiz MemSiz Flags Align 
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000b34 0x0000000000000b34 R E 200000 
LOAD 0x0000000000000e40 0x0000000000200e40 0x0000000000200e40 
0x00000000000001f8 0x0000000000000250 RW 200000 
DYNAMIC 0x0000000000000e68 0x0000000000200e68 0x0000000000200e68 
0x0000000000000150 0x0000000000000150 RW 8 
GNU_EH_FRAME 0x0000000000000ab4 0x0000000000000ab4 0x0000000000000ab4 
0x000000000000001c 0x000000000000001c R 4 
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 
0x0000000000000000 0x0000000000000000 RW 10 
GNU_RELRO 0x0000000000000e40 0x0000000000200e40 0x0000000000200e40 
0x00000000000001c0 0x00000000000001c0 R 1 

Section to Segment mapping: 
Segment Sections... 
00 .hash .dynsym .dynstr .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame 
01 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss 
02 .dynamic 
03 .eh_frame_hdr 
04 
05 .ctors .dtors .jcr .dynamic .got 



From: "dalias" <dalias@...c.org> 
To: "musl" <musl@...ts.openwall.com> 
Sent: Sunday, March 5, 2017 2:44:03 AM 
Subject: Re: [musl] Static PIE with musl and clang 

On Sat, Mar 04, 2017 at 07:47:31PM -0600, Paul Sturm wrote: 
> Has anyone managed to build static PIE with musl and clang, 
> preferably linking with lld? 
> 
> I see that this has been done using GCC and rcrt1.o, but I cannot 
> find a combination of command line parameters that will successfully 
> link and execute with clang. 
> 
> I am using clang 4.0.0RC2 and musl 1.1.16. I have also tried to use 
> binutils ld instead of llvm lld to no avail. 

I doubt lld can do it yet; you need some equivalent of the 
--no-dynamic-linker option I added to BFD ld. It should be possible 
with clang using binutils/BFD ld. You might also be able to hack it by 
using -shared -Bsymbolic and explicitly linking rcrt1.o on the link 
command line (essentially doing the same thing you'd do to make an 
executable shared library) but there are corner-case things that break 
when you try to do it this way. The right approach would be adding 
--no-dynamic-linker to lld and teaching clang to pass it when linking 
with -static -pie. 

Rich 

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