Re: ftrace: Proposal for an Alternative RecordMcount framework

[Alan Kao]

On Wed, Feb 28, 2018 at 05:12:52AM +0800, Steven Rostedt wrote:
> On Tue, 27 Feb 2018 18:04:26 +0800
> Alan Kao <alankao@xxxxxxxxxxxxx> wrote:
>  
> > 1. During the final linking stages, do "objdump vmlinux.o | grep ..." [2]
> 
> Note, doing it at that stage takes the longest time. It makes small
> changes much longer to compile. That said...
>

What if we can have some option to *disable* all the recordmcount.pl lauches
after every .o?  There will be only an oneshot grep for a near-complete
vmlinux binary.

> > 2. Form the output as an ELF objecj
> > 3. Link the object to __mcount_loc_start symbol
> > 4. Done
> > 
> > With the similar reason as the patch [3], we should mark _mcount to be
> > a weak symbol to prevent it from being relaxed later.
> > 
> > We would like to know your opinion and comments on this.
> > Thanks!
> 
> What about just having your arch use recordmcount.c instead? It doesn't
> do any grepping. It is an elf reader and modifier and modifies the .o
> file directly.

Thanks for the hint.  But after a quick scan, it seems that recordmcount.c
processes .o files in a per-file basis, which means that we will still
suffer from the linker relaxation problem.

> 
> Note, I will be rewriting that code in the near future too, to
> consolidate it with objtool.
> 
> -- Steve

Please allow me to state the problem more clearly here.  I hope this helps.

1. locations of mcount are recorded in a per-file basis.
2. to optimize the binary, the linker turns on some aggressive
   options, including relaxation.
3. the optimizations changes the original offset.
4. already recorded mcount call-sites no longer point to their
   real positions.
5. still a linked vmlinux is made.
6. dynamic ftrace breaks the real logic in the kernel space,
   panics happen.

Thanks,
Alan