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

From: Nadav Amit
Date: Thu May 25 2023 - 15:41:05 EST

> 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?