linux-kernel - Re: [PATCH] powerpc/boot: Use address-of operator on section symbols

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Message-ID: <20200718153129.GA1815449@ubuntu-n2-xlarge-x86>
Date:   Sat, 18 Jul 2020 08:31:29 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Geoff Levand <geoff@...radead.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Joel Stanley <joel@....id.au>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] powerpc/boot: Use address-of operator on section symbols

On Sat, Jul 18, 2020 at 09:50:50AM +0200, Geert Uytterhoeven wrote:
> Hi Nathan,
> 
> On Wed, Jun 24, 2020 at 6:02 AM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> > arch/powerpc/boot/main.c:107:18: warning: array comparison always
> > evaluates to a constant [-Wtautological-compare]
> >         if (_initrd_end > _initrd_start) {
> >                         ^
> > arch/powerpc/boot/main.c:155:20: warning: array comparison always
> > evaluates to a constant [-Wtautological-compare]
> >         if (_esm_blob_end <= _esm_blob_start)
> >                           ^
> > 2 warnings generated.
> >
> > These are not true arrays, they are linker defined symbols, which are
> > just addresses.  Using the address of operator silences the warning
> > and does not change the resulting assembly with either clang/ld.lld
> > or gcc/ld (tested with diff + objdump -Dr).
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/212
> > Reported-by: Joel Stanley <joel@....id.au>
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  arch/powerpc/boot/main.c | 4 ++--
> >  arch/powerpc/boot/ps3.c  | 2 +-
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/boot/main.c b/arch/powerpc/boot/main.c
> > index a9d209135975..cae31a6e8f02 100644
> > --- a/arch/powerpc/boot/main.c
> > +++ b/arch/powerpc/boot/main.c
> > @@ -104,7 +104,7 @@ static struct addr_range prep_initrd(struct addr_range vmlinux, void *chosen,
> >  {
> >         /* If we have an image attached to us, it overrides anything
> >          * supplied by the loader. */
> > -       if (_initrd_end > _initrd_start) {
> > +       if (&_initrd_end > &_initrd_start) {
> >
> 
> Are you sure that fix is correct?
> 
>     extern char _initrd_start[];
>     extern char _initrd_end[];
>     extern char _esm_blob_start[];
>     extern char _esm_blob_end[];
> 
> Of course the result of their comparison is a constant, as the addresses
> are constant.  If clangs warns about it, perhaps that warning should be moved
> to W=1?
> 
> But adding "&" is not correct, according to C.
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 

Hi Geert,

Yes, I have done fairly extensive testing in the past to verify that
this fix is correct.

For example:

$ cat test.c
#include <stdio.h>

extern char _test[];

int main(void)
{
        printf("_test:  %p\n", _test);
        printf("&_test: %p\n", &_test);
        return 0;
}

$ cat test.lds
_test = .;

$ clang -Wl,-T test.lds test.c

$ ./a.out
_test:  0x204
&_test: 0x204

$ gcc -fuse-ld=lld -Wl,-T test.lds test.c

$ ./a.out
_test:  0x60a0f76301fb
&_test: 0x60a0f76301fb

I also did runtime verification in QEMU to confirm this is true when I
was testing these commits, which are already present in Linus' tree:

63174f61dfae ("kernel/extable.c: use address-of operator on section symbols")
bf2cbe044da2 ("tracing: Use address-of operator on section symbols")
8306b057a85e ("lib/dynamic_debug.c: use address-of operator on section symbols")
b0d14fc43d39 ("mm/kmemleak.c: use address-of operator on section symbols")

I did a lot of work to get this warning enabled as it can find bugs:

6def1a1d2d58 ("fanotify: Fix the checks in fanotify_fsid_equal")
79ba4f931067 ("IB/hfi1: Fix logical condition in msix_request_irq")

-Wno-tautological-compare disables a bunch of good subwarnings, as I
point out in the commit that enabled it:

afe956c577b2 ("kbuild: Enable -Wtautological-compare")

Cheers,
Nathan