linux-kernel - Re: [PATCH] regset: use vmalloc() for regset_get

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Date: Fri, 2 Feb 2024 16:55:24 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Doug Anderson <dianders@...omium.org>
Cc: Christian Brauner <brauner@...nel.org>,
	Eric Biederman <ebiederm@...ssion.com>, Jan Kara <jack@...e.cz>,
	Kees Cook <keescook@...omium.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Oleg Nesterov <oleg@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] regset: use vmalloc() for regset_get_alloc()

On Fri, Feb 02, 2024 at 04:49:47PM +0000, Al Viro wrote:
> > +folks from `./scripts/get_maintainer.pl -f arch/arm64/kernel/ptrace.c`
> > 
> > Trying to follow the macros to see where "n" comes from is a maze of
> > twisty little passages, all alike. Hopefully someone from the ARM
> > world can help tell if the value of 17474 for n here is correct or if
> > something is wonky.
> 
> It might be interesting to have it print the return value of __regset_get()
> in those cases; if *that* is huge, we really have a problem.  If it ends up
> small enough to fit into few pages, OTOH...
> 
> SVE_VQ_MAX is defined as 255; is that really in units of 128 bits?  IOW,
> do we really expect to support 32Kbit registers?  That would drive the
> size into that range, all right, but it would really suck on context
> switches.
> 
> I could be misreading it, though - the macros in there are not easy to
> follow and I've never dealt with SVE before, so take the above with
> a cartload of salt.

Worse - it's SVE_VQ_MAX is 512; sorry about the confusion.  OK, that would
certainly explain the size (header + 32 registers, each up to 512 * 16 bytes),
but... ouch.