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Date:   Tue, 11 Feb 2020 15:54:06 -0800
From:   Kees Cook <keescook@chromium.org>
To:     shuah <shuah@kernel.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Hector Marco-Gisbert <hecmargi@upv.es>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Jann Horn <jannh@google.com>,
        Russell King <linux@armlinux.org.uk>, x86@kernel.org,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v3 7/7] selftests/exec: Add READ_IMPLIES_EXEC tests
Message-ID: <202002111549.CF18B7B3B@keescook>
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On Tue, Feb 11, 2020 at 02:06:53PM -0700, shuah wrote:
> On 2/11/20 12:25 PM, Kees Cook wrote:
> > On Tue, Feb 11, 2020 at 11:11:21AM -0700, shuah wrote:
> > > On 2/10/20 12:30 PM, Kees Cook wrote:
> > > > In order to check the matrix of possible states for handling
> > > > READ_IMPLIES_EXEC across native, compat, and the state of PT_GNU_STACK,
> > > > add tests for these execution conditions.
> > > > 
> > > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > 
> > > No issues for this to go through tip.
> > > 
> > > A few problems to fix first. This fails to compile when 32-bit libraries
> > > aren't installed. It should fail the 32-bit part and run other checks.
> > 
> > Do you mean the Makefile should detect the missing compat build deps and
> > avoid building them? Testing compat is pretty important to this test, so
> > it seems like missing the build deps causing the build to fail is the
> > correct action here. This is likely true for the x86/ selftests too.
> > 
> > What would you like this to do?
> > 
> 
> selftests/x86 does this already and runs the dependency check in
> x86/Makefile.
> 
> 
> check_cc.sh:# check_cc.sh - Helper to test userspace compilation support
> Makefile:CAN_BUILD_I386 := $(shell ./check_cc.sh $(CC)
> trivial_32bit_program.c -m32)
> Makefile:CAN_BUILD_X86_64 := $(shell ./check_cc.sh $(CC)
> trivial_64bit_program.c)
> Makefile:CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh $(CC)
> trivial_program.c -no-pie)
> 
> Take a look and see if you can leverage this.

I did before, and it can certainly be done, but their stuff is somewhat
specific to x86_64/ia32. I'm looking at supporting _all_ compat for any
64-bit architecture. I can certainly write some similar build tooling,
but the question I have for you is one of coverage:

If a builder is 64-bit, it needs to be able to produce 32-bit compat
binaries for testing, otherwise the test is incomplete. (i.e. the tests
will only be able to test native behavior and not compat). This doesn't
seem like an "XFAIL" situation to me, and it doesn't seem right to
silently pass. It seems like the build should explicitly fail because
the needed prerequisites are missing. Do you instead want me to just
have it skip building the compat binaries if it can't build them?

-- 
Kees Cook