Re: [RFC PATCH] ftrace: Skip __fentry__ location of overridden weak functions

[Zheng Yejian]

On 2024/6/11 17:21, Peter Zijlstra wrote:
> On Tue, Jun 11, 2024 at 09:56:51AM +0800, Zheng Yejian wrote:
> > On 2024/6/7 23:02, Peter Zijlstra wrote:
>
> > > Oh gawd, sodding weak functions again.
> > >
> > > I would suggest changing scipts/kallsyms.c to emit readily identifiable
> > > symbol names for all the weak junk, eg:
> > >
> > >     __weak_junk_NNNNN
> >
> > Sorry for the late reply, I just had a long noon holiday :>
> >
> > scripts/kallsyms.c is compiled and used to handle symbols in vmlinux.o
> > or vmlinux.a, see kallsyms_step() in scripts/link-vmlinux.sh, those
> > overridden weak symbols has been removed from symbol table of vmlinux.o
> > or vmlinux.a. But we can found those symbols from original xx/xx.o file,
> > for example, the weak free_initmem() in in init/main.c is overridden,
> > its symbol is not in vmlinx but is still in init/main.o .
> >
> > How about traversing all origin xx/xx.o and finding all weak junk symbols ?
>
> You don't need to. ELF symbl tables have an entry size for FUNC type
> objects, this means that you can readily find holes in the text and fill
> them with a symbol.
>
> Specifically, you can check the mcount locations against the symbol
> table and for every one that falls in a hole, generate a new junk
> symbol.
>
> Also see 4adb23686795 where objtool adds these holes to the
> ignore/unreachable code check.
>
>
> The lack of size for kallsyms is in a large part what is causing the
> problems.

Thanks for your suggestions, I'll try it soon.

--

Thanks,
ZYJ