Re: RFC: Add a mechanism to stop backtraces using dwarf2 frame information

[Daniel Jacobowitz <drow@false.org>]

On Wed, Mar 02, 2005 at 05:15:53PM -0500, Daniel Jacobowitz wrote:
> I picked an idiom which GDB currently doesn't handle to mean "no backtrace
> information is available": DW_CFA_undefined in the return address column.
> Seems a plausible interpretation to me.  This idiom implies that not only
> is no DWARF unwinding data available, but also that more conventional means
> of unwinding are unlikely to succeed.  Obviously, if GDB has an earlier
> sniffer which recognizes the particular location, we can continue
> backtracing.  This just stops us from falling back to the prologue
> analyzers.

For Your Entertainment, here's two examples of how to use this feature.
One uses the gas .cfi directives; it is simpler to write but has a
couple of drawbacks:
  - It goes in .eh_frame, which is loadable and thus takes up RAM
    unless explicitly discarded.
  - It requires you to know the number of the return address column
    for your architecture.

The other uses .debug_frame, which only has one alternative - it's a
bit bulkier in the source code.  Though that can be worked around with
macros.
 
An unpatched GDB will backtrace normally from foo to main, and get
stuck in a loop backtracing from baz.  A patched GDB will stop cleanly
after either foo or baz.

	.globl main
	.type main, %function
main:
	call	foo
	call	baz
	ret

	.globl foo
	.type foo, %function
foo:
	.cfi_startproc
	.cfi_escape 0x7, 0x8
	ret
	.cfi_endproc

baz:
	ret
.Lend_baz:


	.section	.debug_frame,"",@progbits
.Lframe0:
	.long	.LECIE0-.LSCIE0	# Length of Common Information Entry
.LSCIE0:
	.long	0xffffffff	# CIE Identifier Tag
	.byte	0x1	# CIE Version
	.ascii "\0"	# CIE Augmentation
	.uleb128 0x1	# CIE Code Alignment Factor
	.sleb128 -4	# CIE Data Alignment Factor
	.byte	0x0	# CIE RA Column
	.byte	0xc	# DW_CFA_def_cfa
	.uleb128 0x0
	.uleb128 0x0
	.byte	0x7	# DW_CFA_undefined
	.byte	0x0	# ... column 0
	.align 4
.LECIE0:
.LSFDE0:
	.long	.LEFDE0-.LASFDE0	# FDE Length
.LASFDE0:
	.long	.Lframe0	# FDE CIE offset
	.long	baz	# FDE initial location
	.long	.Lend_baz-baz	# FDE address range
	.align 4
.LEFDE0:

-- 
Daniel Jacobowitz
CodeSourcery, LLC