Re: unwinding for vsyscall code

[Richard Henderson <rth@twiddle.net>]

On Sat, Apr 12, 2003 at 08:57:16PM -0700, Ulrich Drepper wrote:
> +	/* What follows are the instructions for the table generation.
> +	   We have to record all changes of the stack pointer.  */
> +		0x04,			/* DW_CFA_advance_loc4 */
> +		0x01, 0x00, 0x00, 0x00,	/* Size of push %ecx */
> +		0x0e,			/* DW_CFA_def_cfa_offset */
> +		0x08,			/* RA at offset 8 now */
> +		0x04,			/* DW_CFA_advance_loc4 */
> +		0x01, 0x00, 0x00, 0x00,	/* Size of push %edx */
> +		0x0e,			/* DW_CFA_def_cfa_offset */
> +		0x0c,			/* RA at offset 12 now */
> +		0x04,			/* DW_CFA_advance_loc4 */
> +		0x01, 0x00, 0x00, 0x00,	/* Size of push %ebp */
> +		0x0e,			/* DW_CFA_def_cfa_offset */
> +		0x10,			/* RA at offset 16 now */

Not only changes to the stack pointer, but also changes to call-saved
registers, such as %ebp.  You're intending to be able to unwind through
asynchronous signals here, which means that you can't leave even a single
insn window with a register modified but not recorded.

So you also need a

	0x85 0x04			DW_CFA_offset %ebp -16

there at the end of the prologue.

> +	/* Finally the epilogue.  */
> +		0x04,			/* DW_CFA_advance_loc4 */
> +		0x0e, 0x00, 0x00, 0x00,	/* Offset til pop %edx */
> +		0x0e,			/* DW_CFA_def_cfa_offset */
> +		0x12,			/* RA at offset 12 now */

And of course you need a corresponding bit here, since once we
pop off the slot in which we stored %ebp, we can't restore it
from there, because we will have clobbered that slot in the
signal handler.

(Btw, typo here in comment; it's "%ebp" not "%edx".)

So here we also need a 

	0xc5				DW_CFA_restore %ebp

here before the next DW_CFA_advance_loc.

Oh, and you don't need to use DW_CFA_advance_loc4.  You should
be using

	DW_CFA_advance_loc+N		N <= 0x3f
	DW_CFA_advance_loc1 N		N <= 0xff

We have to use DW_CFA_advance_loc4 in GCC because we don't know
the true sizes of instructions.  This gets fixed for us in GAS
through some truely disgusting magic based on section names.



r~