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From: Peter Zijlstra <>
To: Nick Desaulniers <>
Cc: Masahiro Yamada <>,
	Josh Poimboeuf <>,
	Michal Marek <>,
	Linux Kernel Mailing List <>,
	Linux Kbuild mailing list <>,
	Sam Ravnborg <>, X86 ML <>,
	Arnd Bergmann <>,
	Changbin Du <>,,
	clang-built-linux <>
Date: Fri, 8 Apr 2022 22:32:28 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Apr 08, 2022 at 01:08:47PM -0700, Nick Desaulniers wrote:
> Lore thread start for newly cc'ed ML readers:
> On Fri, Apr 8, 2022 at 12:14 PM Peter Zijlstra <> 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?

Any CALL instruction is a no-no. Only 'simple' code is allowed between

> > 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]

So in the case of that latest __always_inline patch, there was only a
single caller. New syntax would buy us absolutely nothing there.

If we're talking extentions, I'd much rather have function spaces. That
is, being able to tag functions *AND* function pointers with an address
space qualifier.

I want to be able to create a function pointer that can only be assigned
functions from the noinstr space for example. Ideally calling such a
functino pointer would only be possible from within that space.

Anyway, let me go read that blog you linked.

  parent reply	other threads:[~2022-04-08 20:32 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
     [not found] ` <>
     [not found]   ` <>
2022-04-08 20:08     ` [PATCH] kbuild: Remove CONFIG_DEBUG_SECTION_MISMATCH Nick Desaulniers
2022-04-08 20:16       ` Nick Desaulniers
2022-04-08 20:32       ` Peter Zijlstra [this message]
2022-04-08 20:48         ` Peter Zijlstra

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