Re: Allocation of dwarf2_frame_state_reg_info

[Daniel Jacobowitz <drow@false.org>]

On Tue, Mar 14, 2006 at 04:16:23PM +0000, Christopher Philip SMITH wrote:
> In dwarf2-frame.c:dwarf2_frame_cache(), the two 
> dwarf2_frame_state_reg_infos, fs->regs and fs->initial, seem to be 
> allocated by calls to dwarf2_frame_state_alloc_regs() in only some cases 
> of execute_cfa_program(). In the case of the function above, none of the 
> cases exercised while going through the CIE result in an allocation, 
> which has the effect back in dwarf2_frame_cache() of leaving fs->initial 
> unallocated, and so uninitialised.

Well, it's not entirely uninitialized, is it?  At least it was memset
to zeroes.

> Later, when processing the FDE, a 
> DW_CFA_restore is encountered which reads from fs->initial and causes 
> the error...

          dwarf2_frame_state_alloc_regs (&fs->regs, reg + 1);
          if (reg < fs->initial.num_regs)
            fs->regs.reg[reg] = fs->initial.reg[reg];
          else 
            fs->regs.reg[reg].how = DWARF2_FRAME_REG_UNSPECIFIED;

Are we looking at the same version of GDB here?

> My question is, what is the reasoning behind only allocating the 
> dwarf2_frame_state_reg_infos in execute_cfa_program()? It seems that if 
> both fs->regs and fs->initial were allocated with the dwarf2_frame_state 
> at the beginning of dwarf2_frame_cache(), then all the registers would 
> be set as "undefined" unless explicitly changed when processing the 
> DWARF (standard behaviour?) and the uninitialised read could never 
> happen? Is it only a case of avoiding unnecessary allocation, or is 
> there something more subtle going on here?

The problem is that we have no idea how many columns there are.

-- 
Daniel Jacobowitz
CodeSourcery