Excessive calls to iterate_phdr during exception handling

[Ryan Johnson]

On 29/05/2013 12:01 AM, Ian Lance Taylor wrote:
> On Tue, May 28, 2013 at 8:50 PM, Ryan Johnson
> <ryan.johnson@cs.utoronto.ca> wrote:
>> For example, it should be reasonably safe to let __cxa_allocate_exception
>> call dl_iterate_phdr in order to build a list of object headers valid at the
>> time unwind begins. It already calls malloc, so allocating space for a cache
>> (holding pointers to at most a few dozen object headers) wouldn't be so
>> terrible; __cxa_free_exception could release the space after the dust
>> settles. In order to optimize non-throw uses of unwinding, it might make
>> sense to build the cache somewhere besides __cxa_allocate_exception, but the
>> basic concept doesn't change.
> Actually, when everything is working correctly with modern tools on a
> GNU/Linux system, the unwind code never calls malloc.  That is a good
> thing, as otherwise it would be unsafe to throw from a signal handler.
Hmm... How recent is "modern" ? Because the only obvious 
__cxa_allocate_exception in the gcc-4.8 sources 
(libstdc++-v3/libsupc++/eh_alloc.cc) sure seems to call malloc, and the 
traces I got from throwing in code compiled by both gcc-4.6 and gcc-4.8 
(gold linker) show calls to malloc as well.

Assuming my system is somehow out of date, though, what is the preferred 
mechanism I'm missing out on? The code has enough #define tricks that 
I've been having trouble learning how things work in a stock gnu/linux 
system just by inspecting the code.

Thanks,
Ryan