Re: [PATCH 00/35] Shadow stacks for userspace

[Florian Weimer]

* David Laight:

> Was there any 'spare' space in struct jmpbuf ?

jmp_buf in glibc looks like this:

(gdb) ptype/o jmp_buf
type = struct __jmp_buf_tag {
/*      0      |      64 */    __jmp_buf __jmpbuf;
/*     64      |       4 */    int __mask_was_saved;
/* XXX  4-byte hole      */
/*     72      |     128 */    __sigset_t __saved_mask;

                               /* total size (bytes):  200 */
                             } [1]
(gdb) ptype/o __jmp_buf
type = long [8]

The glibc ABI reserves space for 1024 signals, something that Linux is
never going to implement.  We can use that space to store a few extra
registers in __save_mask.  There is a complication because the
pthread_cancel unwinding allocates only space for the __jmpbuf member.
Fortunately, we do not need to unwind the shadow stack for thread
cancellation, so we don't need that extra space in that case.

Thanks,
Florian