Re: get_frame_func() VS get_frame_id().code

Andrew Cagney
2004-03-09T02:54:19Z

> On Fri, Mar 05, 2004 at 11:05:54AM -0500, Andrew Cagney wrote:
>
>>> Hello,
>>>
>>> The current get_frame_func() is implemented as roughly:
>>>
>>> fi->prev_func.addr = get_pc_function_start (addr_in_block);
>>>
>>> Unfortunatly this isn't valid for a signal trampoline (or at least the
>>> evil ones that consist of random bytes in a random memory location).
>>> For such trampolines, get_pc_function_start [rightly] fails and "func"
>>> ends up as zero -- not good -- a properly constructed frame ID requires
>>> non-zero code and stack addresses.
>>>
>>> Fortunatly, with a bit of extra instruction pattern matching, it is
>>> possible to identify the first instruction of a signal trampoline and
>>> hence correctly compute the trampolines "func" address. Similarly, more
>>> normal frames can determine the function start using the symbol table's
>>> get_pc_function_start.
>>>
>>> Consequently, I think there should be mechanism for obtaining both the
>>> symbol table and frame's idea of a function's start address. This would
>>> mean introducing:
>>>
>>> - get_frame_func_by_symtab
>>> Returns the function start according to the symbol table. Much of the
>>> existing code (especially unwinders) would need to be updated to use this.
>
>
> How about just leaving this as frame_func_unwind?

Because the func, based on the frame ID, is more likely to be correct.
The current get_frame_func and frame_func_unwind can return 0 or the
wrong function :-(

BTW, there is get_frame_function() which returns the symbol for the
function.

> The only current use
> of frame_func_unwind that I see outside of unwinders is the
> implementation of get_frame_func.

... get_frame_func is hardly called ...

> So we could define frame_func_unwind
> to attempt to find the beginning of the function at the unwound PC
> using the symtab.

... so it may even be possible to simply pull get_frame_func from the
frame interface.

> It is already the unwinder's duty to propogate the function address
> into the ID.

Andrew