Why is the stack segment executable on Raspberry Pi? - linux

Question

I have a Raspberry Pi 3 with the Raspbian GNU/Linux 8 (Jessie) OS.
I wrote this simple program. I compiled it with gcc -o hello hello.c.
#include <stdio.h>
void main(){
printf("hello!\n");
}
From from the readelf output everything seems OK:
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
EXIDX 0x0004cc 0x000104cc 0x000104cc 0x00008 0x00008 R 0x4
PHDR 0x000034 0x00010034 0x00010034 0x00120 0x00120 R E 0x4
INTERP 0x000154 0x00010154 0x00010154 0x00019 0x00019 R 0x1
[Requesting program interpreter: /lib/ld-linux-armhf.so.3]
LOAD 0x000000 0x00010000 0x00010000 0x004d8 0x004d8 R E 0x10000
LOAD 0x000f0c 0x00020f0c 0x00020f0c 0x0011c 0x00120 RW 0x10000
DYNAMIC 0x000f18 0x00020f18 0x00020f18 0x000e8 0x000e8 RW 0x4
NOTE 0x000170 0x00010170 0x00010170 0x00044 0x00044 R 0x4
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x10
GNU_RELRO 0x000f0c 0x00020f0c 0x00020f0c 0x000f4 0x000f4 R 0x1
But when I run the program the stack is executable:
0x7efdf000 0x7f000000 0x00000000 rwx [stack]
I try to compile also with the option -z noexecstack, but nothing change.
I try also to download the version of libarmmem.so that have this code:
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
But nothing change.
Why is the stack segment executable on Raspberry Pi?
Edit I add the output of the LD_DEBUG=files ./hello command
23110:
23110: file=/usr/lib/arm-linux-gnueabihf/libarmmem.so [0]; needed by ./hello [0]
23110: file=/usr/lib/arm-linux-gnueabihf/libarmmem.so [0]; generating link map
23110: dynamic: 0x76f273fc base: 0x76f13000 size: 0x00014524
23110: entry: 0x76f13568 phdr: 0x76f13034 phnum: 6
23110:
23110:
23110: file=libc.so.6 [0]; needed by ./hello [0]
23110: file=libc.so.6 [0]; generating link map
23110: dynamic: 0x76f0ef20 base: 0x76dd4000 size: 0x0013e550
23110: entry: 0x76dea840 phdr: 0x76dd4034 phnum: 10
23110:
23110:
23110: calling init: /lib/arm-linux-gnueabihf/libc.so.6
23110:
23110:
23110: calling init: /usr/lib/arm-linux-gnueabihf/libarmmem.so
23110:
23110:
23110: initialize program: ./hello
23110:
23110:
23110: transferring control: ./hello
23110:
hello!
23110:
23110: calling fini: ./hello [0]
23110:
23110:
23110: calling fini: /usr/lib/arm-linux-gnueabihf/libarmmem.so [0]
23110:
Add more info:
I edit the file architecture.S, and after the make I received:
gcc -std=gnu99 -O2 -c -o trampoline.o trampoline.c
gcc -shared -o libarmmem.so architecture.o memcmp.o memcpymove.o memcpymove-a7.o memset.o trampoline.o
`architecture' referenced in section `.text' of trampoline.o: defined in discarded section `.note.GNU-stack' of architecture.o
`architecture' referenced in section `.text' of trampoline.o: defined in discarded section `.note.GNU-stack' of architecture.o
`architecture' referenced in section `.text' of trampoline.o: defined in discarded section `.note.GNU-stack' of architecture.o
`architecture' referenced in section `.text' of trampoline.o: defined in discarded section `.note.GNU-stack' of architecture.o
collect2: error: ld returned 1 exit status
Makefile:13: recipe for target 'libarmmem.so' failed
make: *** [libarmmem.so] Error 1

Answer 1

It is likely that /usr/lib/arm-linux-gnueabihf/libarmmem.so is causing this. I found this source file:
https://github.com/bavison/arm-mem/blob/master/architecture.S
It lacks the non-executable stack annotation, so glibc conservatively makes the stack executable when the DSO is preloaded. The other source files have this:
/* Prevent the stack from becoming executable */
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
So you just need to copy this into architecture.S (at the end of the file) and rebuild.
You can verify with eu-readelf -l /usr/lib/arm-linux-gnueabihf/libarmmem.so if this DSO is indeed the culprit. It should show either no GNU_STACK program header at all, or a GNU_STACK program header which is marked RWE in the penultimate column.