Re: [PATCH v2 2/2] ftrace: recordmcount: Handle sections with no non-weak symbols

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 30 Apr 2022 01:03:01 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> > The point of this section is to know which functions in __mcount_loc may
> > have been overridden, as they would be found in the __mcount_loc_weak
> > section. And then we can do something "special" to them.  
> 
> I'm not sure I follow that. How are you intending to figure out which 
> functions were overridden by looking at entries in the __mcount_loc_weak 
> section?

If there's duplicates (or something strange with the offset) then we could
delete the one that has the match in the weak section.

> 
> In the final vmlinux image, we only get offsets into .text for all 
> mcount locations, but no symbol information. The only hint is the fact 
> that a single kallsym symbol has multiple mcount locations within it. 
> Even then, the symbol with duplicate mcount entries won't be the 
> function that was overridden.
> 
> We could do a kallsyms_lookup() on each entry and consult the 
> __mcount_loc_weak section to identify duplicates, but that looks to be 
> very expensive.
> 
> Did you have a different approach in mind?

We only need to look at the ones in the weak section. How many that is may
determine if that's feasible or not.

> 
> >   
> >> 
> >> As an example, in the issue described in this patch set, if powerpc 
> >> starts over-riding kexec_arch_apply_relocations(), then the current weak 
> >> implementation in kexec_file.o gets carried over to the final vmlinux, 
> >> but the instructions will instead appear under the previous function in 
> >> kexec_file.o: crash_prepare_elf64_headers(). This function may or may 
> >> not be traced to begin with, so we won't be able to figure out if this 
> >> is valid or not.  
> > 
> > If it was overridden, then there would be two entries for function that
> > overrides the weak function in the __mcount_loc section, right? One for the
> > new function, and one that was overridden.  
> 
> In the final vmlinux, we will have two entries: one pointing at the 
> correct function, while the other will point to some other function 
> name. So, at least from kallsym perspective, duplicate mcount entries 
> won't be for the function that was overridden, but some arbitrary 
> function that came before the weak function in the object file.

Right, and we should be able to find them.

> 
> > Of course this could be more
> > complex if the new function had been marked notrace.
> > 
> > I was thinking of doing this just so that we know what functions are weak
> > and perhaps need extra processing.
> > 
> > Another issue with weak functions and ftrace just came up here:
> > 
> >   https://lore.kernel.org/all/20220428095803.66c17c32@gandalf.local.home/  
> 
> I noticed this just yesterday:
> 
>   # cat available_filter_functions | sort | uniq -d | wc -l
>   430
> 
> I'm fairly certain that some of those are due to weak functions -- I 
> just wasn't sure if all of those were.

Probably :-/

> 
> I suppose this will now also be a problem with ftrace_location(), given 
> that it was recently changed to look at an entire function for mcount 
> locations?
> 

Maybe. I have to focus on other things at the moment, but I'll try to find
some time to look at this deeper.

-- Steve