On 2014/12/10 08:37PM, Michael Ellerman wrote: > On Tue, 2014-12-09 at 23:03 +0530, Naveen N. Rao wrote: > > Currently, all non-dot symbols are being treated as function descriptors > > in ABIv1. This is incorrect and is resulting in perf probe not working: > > I don't understand that first sentence. With ABIv1 non-dot symbols *are* > function descriptors?
Not always. '_text' is an example of a symbol that is not a function descriptor. However, most functions have a dot variant constituting the actual entry point and a non-dot variant constituting the function descriptor. > > > # perf probe do_fork > > Added new event: > > Failed to write event: Invalid argument > > Error: Failed to add events. > > # dmesg | tail -1 > > [192268.073063] Could not insert probe at _text+768432: -22 > > > > _text is being resolved incorrectly and is resulting in the above error. > > Fix this by changing how we lookup symbol addresses on ppc64. We first > > check for the dot variant of a symbol and look at the non-dot variant > > only if that fails. In this manner, we avoid having to look at the > > function descriptor. > > I'm not clear that ppc_local_function_entry() makes sense. On ABIv2 you return > the local entry point, which is fine. But on ABIv1 you just return the > unmodified address, which will be the descriptor if you actually passed it a > function pointer. I think you're assuming that you're passed the text address, > but if that's the case the function is badly named at least. > > I also don't understand why we need to ever guess which ABI we're using. We > know which ABI we're built with, so there should be no guess work required. > > So at the very least this needs much more explanation. > > But to be honest I'm not clear why it even needs a kernel change, don't we > just > need perf to understand dot symbols? The problem in this case is in the kernel. perf probe is now basing all probe addresses on _text and writes, for example, "p:probe/do_fork _text+768432" to /sys/kernel/debug/tracing/kprobe_events. This ends up in kprobe_lookup_name() for resolving address of _text, which invokes ppc_function_entry(), which ends up thinking _text is a function descriptor. Even though we know we are compiled for ABIv1, there is no easy way to identify if a given symbol is the actual entry point or if it is a function descriptor. To address this, my approach is to always check for a dot symbol first and if that exists, we know we have the actual function entry. If not, we know this isn't a function descriptor (since there is no related dot symbol). I agree that the function is named badly though. The real problem is that kprobe_lookup_name is a macro and I can't have a #ifdef to call ppc_function_entry() only for ABIv2. Thoughts? Suggestions? Thanks, Naveen _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
