Where is segment %fs for static elf images setup? - linux

Question

I'm trying to figure out how the %fs register is initialized
when creating a elf image by hand.
The simple snippet I'd like to run is:
.text
nop
movq %fs:0x28, %rax;
1: jmp 1b
Which should read at offset 0x28 in the %fs segment. Normally this is where the stack canary is stored. Because I create the elf image by hand the %fs segment is not setup at all by my code this fails expectedly(?) .
Here is how I create the elf image:
0000000000000000 <.text>:
0: 90 nop
1: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
8: 00 00
a: eb fe jmp 0xa
I create the .text segment via
echo 9064488b042528000000ebfe | xxd -r -p > r2.bin
Then I convert to elf:
ld -b binary -r -o raw.elf r2.bin
objcopy --rename-section .data=.text --set-section-flags .data=alloc,code,load raw.elf
At that point raw.elf contains my instructions. I then link with
ld -T raw.ld -o out.elf -M --verbose where raw.ld is:
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64", "elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_entry)
PHDRS {
phdr4000000 PT_LOAD;
}
SECTIONS
{
_entry = 0x4000000;
.text 0x4000000 : { raw.elf (.text) } :phdr4000000
}
I can now start out.elf with gdb:
gdb --args out.elf
and set a breakpoint at 0x4000000:
(gdb)break *0x4000000
(gdb)run
The first nop can be stepped via stepi, however the stack canary read mov %fs:0x28,%rax segfaults.
I suppose that is expected given that maybe the OS is not setting up %fs.
For a simple m.c: int main() { return 0; } program compiled with gcc --static m.c -o m I can read from %fs. Adding:
long can()
{
long v = 0;
__asm__("movq %%fs:0x28, %0;"
: "=r"(val)::);
return v;
}
lets me read from %fs - even though I doubt that %fs:28 is setup because ld.so is not run (it is a static image).
Question:
Can anyone point out where %fs is setup in the c runtime for static images?

Answer 1

You need to call arch_prctl with an ARCH_SET_FS argument before you can use the %fs segment prefix. You will have to allocate the backing store somewhere (brk, mmap, or an otherwise unused part of the stack).
glibc does this in __libc_setup_tls in csu/libc-tls.c for statically linked binaries, hidden behind the TLS_INIT_TP macro.