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

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Message-ID: <CAHmME9rjH4Ek3e8jmBvRp3wy+mrzJAeYxk5=o+OHjoT5sTOQPg@mail.gmail.com>
Date: Wed, 28 Aug 2024 14:26:08 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Segher Boessenkool <segher@...nel.crashing.org>, 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 2:24 PM Arnd Bergmann <arnd@...db.de> 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.

>From https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90701:

| You need -ffreestanding but that is documented to emit memset and
memcpy still.