linux-kernel - Re: [PATCH v3 1/7] compiler-clang: add build check for clang 10.0.1

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Date:   Wed, 4 Nov 2020 02:38:26 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Will Deacon <will@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/7] compiler-clang: add build check for clang 10.0.1

On Tue, Nov 03, 2020 at 10:48:27AM -0800, Nick Desaulniers wrote:
> On Mon, Nov 2, 2020 at 8:55 PM Jarkko Sakkinen <jarkko@...nel.org> wrote:
> >
> > On Wed, Sep 02, 2020 at 03:59:05PM -0700, Nick Desaulniers wrote:
> > > +#define CLANG_VERSION (__clang_major__ * 10000       \
> > > +                  + __clang_minor__ * 100    \
> > > +                  + __clang_patchlevel__)
> > > +
> > > +#if CLANG_VERSION < 100001
> > > +# error Sorry, your version of Clang is too old - please use 10.0.1 or newer.
> > > +#endif
> >
> >
> > I'm trying to compile a BPF enabled test kernel for a live system and I
> > get this error even though I have much newer clang:
> >
> > ➜  ~ (master) ✔ clang --version
> > Ubuntu clang version 11.0.0-2
> > Target: x86_64-pc-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> >
> > Tried to Google for troubleshooter tips but this patch is basically the
> > only hit I get :-)
> 
> To check the values of the above preprocessor defines, please run:
> $ clang -dM -E - < /dev/null | grep -e __clang_m -e __clang_p
> 
> If you have multiple versions of clang installed, you might not be
> running the version you think you are.  Particularly, if you're using
> bcc, idk if it includes a copy of clang?  If that's the case, we may
> have to work out how we can support older versions of clang for the
> express purposes of bpf.

➜  ~ (master) ✔ clang -dM -E - < /dev/null | grep -e __clang_m -e __clang_p
#define __clang_major__ 11
#define __clang_minor__ 0
#define __clang_patchlevel__ 0

I'm compiling the kernel itself with GCC.

Here's an example BPF script that fails on me:

struct sgx_enclave_add_pages {
	unsigned long src;
	unsigned long offset;
	unsigned long length;
	unsigned long secinfo;
	unsigned long flags;
	unsigned long count;
};

kprobe:sgx_ioctl
{
	if (arg1 == 0xc030a401) {
		printf("sgx_ioctl: %d, %lu\n", pid, ((struct sgx_enclave_add_pages *)(arg2))->offset);
	}

}
Note that it relies on code not yet in the mainline.

If I don't declare structs, things work just fine. E.g. the following
works:

kprobe:sgx_encl_get_backing
{
	printf("%s\n", func)
}

BTW, I don't really understand how scripts/clang-version.sh is even
supposed to work, if you compile the kernel itself with GCC. In that
case there would be no output, right? And thus version gets set to
zero...

> -- 
> Thanks,
> ~Nick Desaulniers

/Jarkko