Re: [PATCH v2] x86/lib: Do not use local symbols with SYM_CODE_START_LOCAL()

[Jiri Slaby]

On 25. 05. 23, 21:39, Nadav Amit wrote:
> > On May 25, 2023, at 12:05 PM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >
> > On 5/25/23 11:42, Nadav Amit wrote:
> > > From: Nadav Amit <namit@xxxxxxxxxx>
> > >
> > > When SYM_CODE_START_LOCAL() is used, the symbols are local but need to
> > > be preserved in the object. However, using the ".L" label prefix does
> > > not retain the symbol in the object.
> > >
> > > It is beneficial to be able to map instruction pointers back to symbols,
> > > for instance for profiling. Otherwise, there are code addresses that do
> > > not map back to any symbol.  Consequently, the ".L" label prefix should
> > > not be used when SYM_CODE_START_LOCAL() is used.
> > >
> > > Few symbols, such as .Lbad_put_user_clac and currently have both the
> > > SYM_CODE_START_LOCAL() invocation and the ".L" prefix.  This commit
> > > removes the ".L" prefix from these symbols.
> > >
> > > No functional change, other then emitting these symbols into the object,
> > > is intended.
> >
> > Nadav, could you perhaps do a bit of research on how this situation came
> > to be?  Was it an accident or on purpose that these symbols came to be
> > .L?  Then, could you CC the folks who made this change and ask them
> > directly if they intended to induce the effects that you find undesirable?
>
> Fair enough. I actually thought it is an oversight, but it now seems
> intentional (although I am not sure I understand/agree with the reason).
>
> So apparently, for one of the symbols from my v1 (which was already
> removed), I see that Borislav Petkov suggested to prepend the “.L” in
> order to for them not to be visible [1].
>
> The reason that is given for not making the functions visible is that
> these are "functions with very local names”.
>
> I do not think in this tradeoff not exposing local names worth
> preventing profilers (and their users) from understanding where a
> sample/trace is was taken. If for instance you look at a branch
> trace (e.g., using Intel PT) you want to see the symbol to which a
> branch goes to.
>
> Borislav, Jiri - do you agree?

Ah, if it makes tools' output harder to follow, I would indeed switch 
back to emitting these symbols, i.e. remove the .L prefix from them.

That said:

Acked-by: Jiri Slaby <jirislaby@xxxxxxxxxx>

thanks,
--
js
suse labs