linux-kernel - Re: [PATCH 1/7] powerpc/spufs: simplify spufs core dumping

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Date:   Tue, 21 Apr 2020 20:25:36 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jeremy Kerr <jk@...abs.org>, Arnd Bergmann <arnd@...db.de>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] powerpc/spufs: simplify spufs core dumping

On Tue, Apr 21, 2020 at 08:19:09PM +0100, Al Viro wrote:
> On Tue, Apr 21, 2020 at 09:01:48PM +0200, Christoph Hellwig wrote:
> > On Tue, Apr 21, 2020 at 07:49:41PM +0100, Al Viro wrote:
> > > >  	spin_lock(&ctx->csa.register_lock);
> > > > -	ret = __spufs_proxydma_info_read(ctx, buf, len, pos);
> > > > +	__spufs_proxydma_info_read(ctx, &info);
> > > > +	ret = simple_read_from_buffer(buf, len, pos, &info, sizeof(info));
> > > 
> > > IDGI...  What's that access_ok() for?  If you are using simple_read_from_buffer(),
> > > the damn thing goes through copy_to_user().  Why bother with separate access_ok()
> > > here?
> > 
> > I have no idea at all, this just refactors the code.
> 
> Wait a bloody minute, it's doing *what* under a spinlock?

... and yes, I realize that it's been broken the same way.  It still needs fixing.
AFAICS, that got broken in commit bf1ab978be23 "[POWERPC] coredump: Add SPU elf
notes to coredump." when spufs_proxydma_info_read() had copy_to_user() (until
then done after dropping the spinlock) moved into an area where blocking is very
much *not* allowed.