linux-kernel - Re: [PATCH v2 05/17] vdso: Avoid call to memset() by getrandom

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Message-ID: <20240828124519.GE29862@gate.crashing.org>
Date: Wed, 28 Aug 2024 07:45:19 -0500
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: "Jason A . Donenfeld" <Jason@...c4.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>, Naveen N Rao <naveen@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, "Theodore Ts'o" <tytso@....edu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        shuah <shuah@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>, linux-mm@...ck.org,
        linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 05/17] vdso: Avoid call to memset() by getrandom

On Wed, Aug 28, 2024 at 12:24:12PM +0000, Arnd Bergmann wrote:
> On Wed, Aug 28, 2024, at 11:18, Jason A. Donenfeld wrote:
> > On Tue, Aug 27, 2024 at 05:53:30PM -0500, Segher Boessenkool wrote:
> >> On Tue, Aug 27, 2024 at 11:08:19AM -0700, Eric Biggers wrote:
> >> > 
> >> > Is there a compiler flag that could be used to disable the generation of calls
> >> > to memset?
> >> 
> >> -fno-tree-loop-distribute-patterns .  But, as always, read up on it, see
> >> what it actually does (and how it avoids your problem, and mostly: learn
> >> what the actual problem *was*!)
> >
> > This might help with various loops, but it doesn't help with the matter
> > that this patch fixes, which is struct initialization. I just tried it
> > with the arm64 patch to no avail.
> 
> Maybe -ffreestanding can help here? That should cause the vdso to be built
> with the assumption that there is no libc, so it would neither add nor
> remove standard library calls. Not sure if that causes other problems,
> e.g. if the calling conventions are different.

"GCC requires the freestanding
environment provide 'memcpy', 'memmove', 'memset' and 'memcmp'."

This is precisely to implement things like struct initialisation.  Maybe
we should have a "-ffreeerstanding" or "-ffreefloating" or think of
something funnier still environment as well, this problem has been there
since the -ffreestanding flag has existed, but the problem is as old as
the night.

-fno-builtin might help a bit more, but just attack the problem at
its root, like I suggested?

(This isn't a new problem, originally it showed up as "GCC replaces
(part of) my memcpy() implementation by a (recursive) call to memcpy()"
and, well, that doesn't quite work!)


Segher