Re: dwarf-frame.c question

[Andrew Cagney <ac131313@redhat.com>]

>    Date: Tue, 27 May 2003 17:18:35 +0200
>    From: Michal Ludvig <mludvig@suse.cz>
> 
>    Hi Mark,
> 
>    why do you decrement unwound PC in dwarf_frame_cache() before using it?
> 
> The unwound PC is the return address, i.e. the instruction that will
> be executed when the function returns.

Yes, the resume address, or the next instruction that will be executed 
when the frame resumes.

frame_address_in_block() also tries to handle this.

>  This is the instruction after
> the call instruction.  The problem is that if the call instruction is
> the last instruction of a function, the return address might point to
> the next function:

> foo:
>    ...
>    call abort
> 
> bar:
>    push %ebp
>    mov %esp, %ebp
>    ...
> 
> That's why the GCC unwinder does the same thing.  Note that the
> decrementing the PC is wrong for "interrupt frames", which is why the
> if-statement is there in the code fragment you cite:
> 
>    dwarf-frame.c:
>    478       /* Unwind the PC.  */
>    479       fs->pc = frame_pc_unwind (next_frame);
>    480       if (get_frame_type (next_frame) == NORMAL_FRAME
>    481           && frame_relative_level (next_frame) >= 0)
>    482         fs->pc--;
> 
>    This makes a problem for a signal trampoline.
>    If it is sitting on addresses say 0x40000140-0x40000150, the return 
>    address from signal handler is 0x40000140, but dwarf_frame_cache() says 
>    it is 0x4000013f and couldn't find it's CFI...
> 
> Do you have signal trampolnes with CFI?  If the CFI is hand-generated,
> you should probably "cheat" by adding a nop before the trampoline and
> include it in the address range of the FDE (see the
> arch/i386/kernel/vsyscall-sigreturn.S in the Linux kernel sources).
> 
>    This has a tragic consequence few lines below were you look for FDE but 
>    don't check if you find one. If you don't, line 488 segfaults.
>    484       /* Find the correct FDE.  */
>    485       fde = dwarf_frame_find_fde (&fs->pc);
> 
> Hmm, a gdb_assert() is in order here.
> 
>    487       /* Extract any interesting information from the CIE.  */
>    488       fs->data_align = fde->cie->data_alignment_factor;
>    489       fs->code_align = fde->cie->code_alignment_factor;
>    490       fs->retaddr_column = fde->cie->return_address_register;
> 
>    The problem is, that dwarf_frame_p() looks for the real return address 
>    and says that there is a debug info and sets unwinder to use dwarf2 
>    methods for this farme.
> 
> Hmm, it should do something similar as dwarf_frame_cache().
> 
>    But the in unwinder itself in dwarf_frame_cache() looks for the 
>    decreased one and of course doesn't find it.
> 
> That defenitely is a bug.  It isn't immediately obvious to me how to
> solve this :-(.

First an FYI.  CFI has that return-address column.  I'm left wondering 
if frame_pc_unwind() should try the frame for the unwound pc before 
trying for registers.  However, there has so far been zero evidence 
supporting this need so I think, until there is, let it be.  It also 
wouldn't help with this case - it to would still point back to beyond 
the function :-(

Second, another FYI.  This isn't just a CFI problem.  There have been 
earlier posts about how GDB, already gets confused by this - printing 
out the wrong function address for instance.  This problem is generic.

Anyway, is it safe to always decrement the resume address before looking 
for the CFI info?  Given a more complex sequence like:

	1: setup call
	2: call xxx with lots of side effects
	3: delay slot saved r0++
	4: discard call

then the CFI info for 4 could be very different to that for 2/3.

 >  Andrew, it seems that we should tweak the frame code
 > to make sure that frame_unwind_by_pc is always passed a PC *within* the
 > function.

True, but how?  It would effectively be frame_unwind_address_in_block() 
but how reliably/where could it be used?

Andrew