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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 8 Apr 2022 13:08:47 -0700
Message-ID: <CAKwvOd=2xeZOg+0HosLPgCegKZxe7F-Cprw0MjOiWf2q=AbNDQ@mail.gmail.com>
Subject: Re: [PATCH] kbuild: Remove CONFIG_DEBUG_SECTION_MISMATCH
To:     Peter Zijlstra <peterz@infradead.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Michal Marek <michal.lkml@markovi.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Sam Ravnborg <sam@ravnborg.org>, X86 ML <x86@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Changbin Du <changbin.du@gmail.com>,
        linux-toolchains@vger.kernel.org,
        clang-built-linux <llvm@lists.linux.dev>
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Lore thread start for newly cc'ed ML readers:
https://lore.kernel.org/lkml/7fad83ecde03540e65677959034315f8fbb3755e.1649434832.git.jpoimboe@redhat.com/

On Fri, Apr 8, 2022 at 12:14 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Sat, Apr 09, 2022 at 03:29:21AM +0900, Masahiro Yamada wrote:
> > Is [2] caused by dead code that was not optimized out
> > due to the unusual inlining decisions by the compiler ?
>
> The complaint is due to SMAP validation; objtool will scream if there's
> a CALL in between STAC/CLAC. The thinking is that since they open a
> security window, we want tight code between them. We also very much
> don't want tracing and other funnies to happen there. As such, any CALL
> is dis-allowed.

Just indirect calls, which might be manipulated, or static calls, too?

>
> This weird option is having us upgrade quite a few 'inline' to
> '__always_inline'.

As is, the assumption that __init functions only call other __init
functions or __always_inline is a brittle house of cards that leads to
a "what color is your function" [0] scenario, and leads to code that
happens to not emit warnings for compiler X (or compiler X version Y).
There's also curious exceptions in modpost that look like memory leaks
to me.

We already have such toolchain portability issues for different
toolchains and different configs; warnings from section mismatches,
and objtool STAC/CLAC checks.  I feel that Josh's patch would sweep
more of those under the rug, so I'm not in favor of it, but could be
convinced otherwise.

TBH, I kind of think that we could use a C extension to permit
__attribute__((always_inline)) to additionally be a statement
attribute, rather than just a function attribute because of cases like
this; we need the flexibility to make one call site __always_inline
without necessarily forcing ALL callsites to be __always_inline'd.

void y (void);
void x (void) { __attribute__((always_inline)) y(); };

(This is already expressable in LLVM IR; not (yet) in C. I'm not sure
yet _why_ this was added to LLVM; whether a different language front
end can express this, if C can and I'm mistaken, or whether it's only
used for optimizations).

I think that would give developers maximal flexibility to defer as
much to the compiler's inlining decisions when they don't care, and
express precisely what they need when they do [care].

[0] https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/
-- 
Thanks,
~Nick Desaulniers