linux-kernel - RE: [PATCH] Makefile: LTO: have linker check -Wframe-larger-than

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Date:   Mon, 15 Mar 2021 10:57:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nick Desaulniers' <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>
CC:     Sami Tolvanen <samitolvanen@...gle.com>,
        Candle Sun <candlesea@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: RE: [PATCH] Makefile: LTO: have linker check -Wframe-larger-than

From: Nick Desaulniers
> Sent: 12 March 2021 17:55
> 
> On Thu, Mar 11, 2021 at 5:09 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > -Wframe-larger-than= requires stack frame information, which the
> > frontend cannot provide. This diagnostic is emitted late during
> > compilation once stack frame size is available.
> >
> > When building with LTO, the frontend simply lowers C to LLVM IR and does
> > not have stack frame information, so it cannot emit this diagnostic.
> > When the linker drives LTO, it restarts optimizations and lowers LLVM IR
> > to object code. At that point, it has stack frame information but
> > doesn't know to check for a specific max stack frame size.

With LTO the linker ought to be able to do a stack frame check
across multiples functions in the call stack.

Clearly recursive calls cause issues.
Indirect ones as well - but does CFI include enough info
about what can be called from where to help?

	David

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