Re: Fwd: Segfaulting with Ubuntu

> That worked!  What exactly causes this issue? Certainly not a buffer
> overflow in dietlibc?

I added a workaround to the Makefile.

The issue is this: Ubuntu adds -fstack-protector to the gcc default
flags, without telling anyone.  On x86_64, this emits code like this:

   b:   64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
  12:   00 00 
  14:   48 89 45 f8             mov    %rax,-0x8(%rbp)

at the beginning of each function, and

  24:   48 8b 55 f8             mov    -0x8(%rbp),%rdx
  28:   64 48 33 14 25 28 00    xor    %fs:0x28,%rdx
  2f:   00 00 
  31:   74 05                   je     38 <foo+0x38>
  33:   e8 00 00 00 00          callq  38 <foo+0x38>
                        34: R_X86_64_PC32       __stack_chk_fail+0xfffffffffffffffc

at the end of each function.  Now, %fs is not initialized, and accessing
%fs:0x28 segfaults.  The libc startup code is expected to set this up,
so that that memory can be accessed and there is a random value at that
address.  Now, since Ubuntu enables the stack protector globally, the
function in dietlibc that sets up %fs and %fs:0x28 is ALSO compiled with
this extra code, and the first part accesses %fs:0x28 before the
function has a chance to set it up, causing a segfault.

If you ask me, the Ubuntu people deserve a kick in the nuts for this.

I added a workaround to the cvs Makefile.

Please note that dietlibc does not support -fstack-protector on all
platforms, because I do not have access to all platforms.  So you might
still get segfaults on non-x86 platforms.  If you do, please contact me
so we can add stack-protector support for your platform.

Felix