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

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Message-Id: <fc19bf63-d519-46e2-be70-80202c85ff92@app.fastmail.com>
Date: Wed, 28 Aug 2024 12:24:12 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Jason A . Donenfeld" <Jason@...c4.com>,
 "Segher Boessenkool" <segher@...nel.crashing.org>
Cc: "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 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.

       Arnd