linux-kernel - Re: linux-next: Tree for Sep 18 (objtool)

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Message-ID: <20190923124901.3ayejcis5ijrsvbx@treble>
Date:   Mon, 23 Sep 2019 07:49:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Mark Brown <broonie@...nel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: Tree for Sep 18 (objtool)

On Mon, Sep 23, 2019 at 11:20:24AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 18, 2019 at 09:04:21PM -0700, Randy Dunlap wrote:
> > On 9/18/19 3:10 PM, Mark Brown wrote:
> > > Hi all,
> > > 
> > > Changes since 20190917:
> > > 
> > 
> > on x86_64:
> > 
> > drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x2fb: call to gen8_canonical_addr() with UACCESS enabled
> 
> I'm thinking that comes from:
> 
> 				offset = gen8_canonical_addr(offset & ~UPDATE);
> 				if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {
> 
> however, per commit 6ae865615fc4 (and 2a418cf3f5f1) the compiler really
> should not be sticking gen8_canonical_addr() after __uaccess_begin().
> 
> /me puzzled...

I think you're looking at the wrong code.  It has user_access_begin/end
around it:

		if (!user_access_begin(user_exec_list, count * sizeof(*user_exec_list)))
			goto end;

		for (i = 0; i < args->buffer_count; i++) {
			if (!(exec2_list[i].offset & UPDATE))
				continue;

			exec2_list[i].offset =
				gen8_canonical_addr(exec2_list[i].offset & PIN_OFFSET_MASK);
			unsafe_put_user(exec2_list[i].offset,
					&user_exec_list[i].offset,
					end_user);
		}
end_user:
		user_access_end();


-- 
Josh