Re: syscall backtraces on arm-linux-gnu

[Daniel Jacobowitz <drow@false.org>]

On Fri, Aug 07, 2009 at 05:49:49PM +0200, Baurzhan Ismagulov wrote:
> It turned out that select is implemented in assembly in glibc, so no CFI
> is provided and gdb falls back to prologue analysis. The following hack
> fixes the use case for me:

The right answer is to somehow get CFI for this.  My preferred
solution is to teach gas how to generate DWARF-2 CFI from the ARM
EH directives (the functions in glibc are already annotated).
Worst case, someone can write the necessary DWARF by hand using
.byte directives.

> @@ -988,8 +990,6 @@ arm_scan_prologue (struct frame_info *th
>  			     regs[fp_start_reg++]);
>  	    }
>  	}
> -      else if ((insn & 0xf0000000) != 0xe0000000)
> -	break;			/* Condition not true, exit early */
>        else if ((insn & 0xfe200000) == 0xe8200000)	/* ldm? */
>  	break;			/* Don't scan past a block load */
>        else

Skipping a jump during scanning is definitely not safe.  That means
GDB's got no idea whether following instructions - pushes and stack
adjusts included - were executed.

You won't see much in the way of testsuite changes with this, because
the testsuite runs (A) with DWARF CFI from the compiler, and (B)
mostly without optimization.

-- 
Daniel Jacobowitz
CodeSourcery