For linux, it’s not using the ip directly to determine is_signal_frame, but it’s using information from processing dwarf data, which it can only find if it has the right ip in the first place.
It seems like the init_cursor caller should be passing an argument to declare whether this is a signal handler frame or not. I’d expect her to know that. It’s a lot more reliable that having libunwind try to figure it out. Doug > On Mar 29, 2017, at 3:33 PM, Doug Moore <do...@rice.edu> wrote: > >> But I think we need to know the IP to detect signal frames? > > > We need to know an IP, yes. It’s not clear that we need the decremented-by-1 > IP, though. Especially since we’re deciding whether to decrement it based on > whether it’s in a signal frame or not. > > I looked at the x86_64 implementations of unw_is_signal_frame for freebsd and > linux. > > For freebsd, unw_is_signal_frame uses the (unmodified) ip in the cursor > argument to determine whether this is a signal frame. So, for freebsd, we > could call unw_is_signal_frame early, passing the cursor we had already, and > use that to determine whether or not to use ip or ip-1 for searching > dwarf/eh_frame structures. > > For linux, unw_is_signal_frame does not look at the ip; it returns a value > under the assumption that another function, either tdep_fetch_frame or > tdep_cache_frame, has already set that value. And tdep_fetch_frame does not > look at the ip field of the cursor passed to it either. Really, I’m not sure > yet where the ip gets examined in the linux version of unw_is_signal_frame. > > Doug > > >> On Mar 29, 2017, at 10:12 AM, Dave Watson <davejwat...@fb.com> wrote: >> >> On 03/27/17 05:36 PM, Doug Moore wrote: >>> It looks like the call to tdep_fetch_frame in fetch_proc_info would >>> determine whether or not this was a signal frame, and thus whether or not >>> we needed to use the previous instruction for the lookup. >>> >>> Of course, we call tdep_fetch_frame only after we’ve already tried that >>> lookup. But perhaps some of the work done to identify signal frames could >>> be moved up a bit, so that a better decision could be made about whether or >>> not to use the previous instruction. >>> >>> Does that make any sense? >> >> But I think we need to know the IP to detect signal frames? >> >> My thought was that if IP-1 lookup failed, we could try IP lookup, and >> if it was a signal frame, use that instead (probably with verify=1) >> >> !DSPAM:10223,58dbcee539347198113515! >> >> > _______________________________________________ Libunwind-devel mailing list Libunwind-devel@nongnu.org https://lists.nongnu.org/mailman/listinfo/libunwind-devel
