linux-kernel - Re: [PATCH v2] x86/lib: Do not use local symbols with SYM_CODE_START

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Message-Id: <24E47178-C177-425F-A8EF-CFFAE22597D4@gmail.com>
Date:   Thu, 25 May 2023 12:39:47 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Jiri Slaby <jirislaby@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86/lib: Do not use local symbols with
 SYM_CODE_START_LOCAL()


> On May 25, 2023, at 12:05 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 5/25/23 11:42, Nadav Amit wrote:
>> From: Nadav Amit <namit@...are.com>
>> 
>> 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?


[1] https://lore.kernel.org/all/20190906075550.23435-2-jslaby@suse.cz/