Re: mmotm 2020-01-23-21-12 uploaded (efi)

Re: mmotm 2020-01-23-21-12 uploaded (efi)

[Ard Biesheuvel]

On Sat, 25 Jan 2020 at 01:09, Randy Dunlap <rdunlap@infradead.org> wrote:
>
> On 1/23/20 9:13 PM, akpm@linux-foundation.org wrote:
> > The mm-of-the-moment snapshot 2020-01-23-21-12 has been uploaded to
> >
> >    http://www.ozlabs.org/~akpm/mmotm/
> >
> > mmotm-readme.txt says
> >
> > README for mm-of-the-moment:
> >
> > http://www.ozlabs.org/~akpm/mmotm/
> >
> > This is a snapshot of my -mm patch queue.  Uploaded at random hopefully
> > more than once a week.
> >
>
> on x86_64:
> CONFIG_X86_UV is not set/enabled.
>
> from linux-next.patch (in mmotm):
>
> ld: arch/x86/platform/efi/efi_64.o: in function `efi_set_virtual_address_map':
> efi_64.c:(.init.text+0x11aa): undefined reference to `efi_uv1_memmap_phys_prolog'
> ld: efi_64.c:(.init.text+0x123c): undefined reference to `efi_uv1_memmap_phys_epilog'
>
>
> Full randconfig file is attached.
>

Should be fixed by

https://lore.kernel.org/linux-efi/20200121093912.5246-1-ardb@kernel.org/

Re: mmotm 2020-01-23-21-12 uploaded (efi)

[Qian Cai]

> On Jan 25, 2020, at 2:06 AM, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> 
> Should be fixed by
> 
> https://lore.kernel.org/linux-efi/20200121093912.5246-1-ardb@kernel.org/

Cc kasan-devel@

If everyone has to disable KASAN for the whole subdirectories like this, I am worried about we are losing testing coverage fairly quickly. Is there a bug in compiler?

Re: mmotm 2020-01-23-21-12 uploaded (efi)

[Dmitry Vyukov]

On Tue, Jan 28, 2020 at 7:15 AM Qian Cai <cai@lca.pw> wrote:
> > Should be fixed by
> >
> > https://lore.kernel.org/linux-efi/20200121093912.5246-1-ardb@kernel.org/
>
> Cc kasan-devel@
>
> If everyone has to disable KASAN for the whole subdirectories like this, I am worried about we are losing testing coverage fairly quickly. Is there a bug in compiler?

My understanding is that this is invalid C code in the first place,
no? It just happened to compile with some compilers, some options and
probably only with high optimization level.
There is a known, simple fix that is used throughout the kernel -
provide empty static inline stub, or put whole calls under ifdef.

Re: mmotm 2020-01-23-21-12 uploaded (efi)

[Ard Biesheuvel]

On Tue, 28 Jan 2020 at 07:26, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, Jan 28, 2020 at 7:15 AM Qian Cai <cai@lca.pw> wrote:
> > > Should be fixed by
> > >
> > > https://lore.kernel.org/linux-efi/20200121093912.5246-1-ardb@kernel.org/
> >
> > Cc kasan-devel@
> >
> > If everyone has to disable KASAN for the whole subdirectories like this, I am worried about we are losing testing coverage fairly quickly. Is there a bug in compiler?
>
> My understanding is that this is invalid C code in the first place,
> no? It just happened to compile with some compilers, some options and
> probably only with high optimization level.

No, this is not true. The whole point of favoring IS_ENABLED(...) over
#ifdef ... has always been that the code remains visible to the
compiler, regardless of whether the option is selected or not, but
that it gets optimized away entirely. The linker errors prove that
there is dead code remaining in the object files, which means we can
no longer rely on IS_ENABLED() to work as intended.

> There is a known, simple fix that is used throughout the kernel -
> provide empty static inline stub, or put whole calls under ifdef.

No, sorry, that doesn't work for me. I think it is great that we have
diagnostic features that are as powerful as KASAN, but if they require
code changes beyond enable/disable, I am not going to rely on them.

Re: mmotm 2020-01-23-21-12 uploaded (efi)

[Dmitry Vyukov]

On Tue, Jan 28, 2020 at 8:33 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> > > > Should be fixed by
> > > >
> > > > https://lore.kernel.org/linux-efi/20200121093912.5246-1-ardb@kernel.org/
> > >
> > > Cc kasan-devel@
> > >
> > > If everyone has to disable KASAN for the whole subdirectories like this, I am worried about we are losing testing coverage fairly quickly. Is there a bug in compiler?
> >
> > My understanding is that this is invalid C code in the first place,
> > no? It just happened to compile with some compilers, some options and
> > probably only with high optimization level.
>
> No, this is not true. The whole point of favoring IS_ENABLED(...) over
> #ifdef ... has always been that the code remains visible to the
> compiler, regardless of whether the option is selected or not, but
> that it gets optimized away entirely. The linker errors prove that
> there is dead code remaining in the object files, which means we can
> no longer rely on IS_ENABLED() to work as intended.

I agree that exposing more code to compiler is good, I prefer to do it
as well. But I don't see how this proves anything wrt this particular
code being invalid C. Called functions still need to be defined. There
is no notion of dead code in C. Yes, this highly depends on compiler,
options, optimization level, etc. Some combinations may work, some
won't. E.g. my compiler compiles it just fine (clang 10) without
disabling instrumentation... what does it prove? I don't know.

To clarify: I completely don't object to patching this case in gcc
with -O2, it just may be hard to find anybody willing to do this work
if we are talking about fixing compilation of invalid code.



> > There is a known, simple fix that is used throughout the kernel -
> > provide empty static inline stub, or put whole calls under ifdef.
>
> No, sorry, that doesn't work for me. I think it is great that we have
> diagnostic features that are as powerful as KASAN, but if they require
> code changes beyond enable/disable, I am not going to rely on them.

Re: mmotm 2020-01-23-21-12 uploaded (efi)

[Ard Biesheuvel]

On Tue, 28 Jan 2020 at 10:08, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Tue, Jan 28, 2020 at 8:33 AM Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
> > > > > Should be fixed by
> > > > >
> > > > > https://lore.kernel.org/linux-efi/20200121093912.5246-1-ardb@kernel.org/
> > > >
> > > > Cc kasan-devel@
> > > >
> > > > If everyone has to disable KASAN for the whole subdirectories like this, I am worried about we are losing testing coverage fairly quickly. Is there a bug in compiler?
> > >
> > > My understanding is that this is invalid C code in the first place,
> > > no? It just happened to compile with some compilers, some options and
> > > probably only with high optimization level.
> >
> > No, this is not true. The whole point of favoring IS_ENABLED(...) over
> > #ifdef ... has always been that the code remains visible to the
> > compiler, regardless of whether the option is selected or not, but
> > that it gets optimized away entirely. The linker errors prove that
> > there is dead code remaining in the object files, which means we can
> > no longer rely on IS_ENABLED() to work as intended.
>
> I agree that exposing more code to compiler is good, I prefer to do it
> as well. But I don't see how this proves anything wrt this particular
> code being invalid C. Called functions still need to be defined. There
> is no notion of dead code in C. Yes, this highly depends on compiler,
> options, optimization level, etc. Some combinations may work, some
> won't. E.g. my compiler compiles it just fine (clang 10) without
> disabling instrumentation... what does it prove? I don't know.
>
> To clarify: I completely don't object to patching this case in gcc
> with -O2, it just may be hard to find anybody willing to do this work
> if we are talking about fixing compilation of invalid code.
>

I don't mind simply disabling KASAN altogether for this code if nobody
can be bothered to fix the compiler.