Re: [PATCH] mips: avoid explicit UB in assignment of mips_io_port_base
From: Nick Desaulniers
Date: Fri Aug 23 2019 - 13:16:18 EST
On Tue, Aug 20, 2019 at 10:15 AM Nick Desaulniers
> Hi Paul,
> Bumping this thread; we'd really like to be able to boot test another
> ISA in our CI. This lone patch is affecting our ability to boot. Can
> you please pick it up?
Following up with this link that explains the undefined behavior issue more:
Please reconsider accepting this patch.
> On Wed, Aug 7, 2019 at 2:12 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> > Sorry for the delayed response, literally sent the patch then went on vacation.
> > On Mon, Jul 29, 2019 at 3:16 PM Maciej W. Rozycki <macro@xxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, 29 Jul 2019, Nick Desaulniers wrote:
> > >
> > > > The code in question is modifying a variable declared const through
> > > > pointer manipulation. Such code is explicitly undefined behavior, and
> > > > is the lone issue preventing malta_defconfig from booting when built
> > > > with Clang:
> > > >
> > > > If an attempt is made to modify an object defined with a const-qualified
> > > > type through use of an lvalue with non-const-qualified type, the
> > > > behavior is undefined.
> > > >
> > > > LLVM is removing such assignments. A simple fix is to not declare
> > > > variables const that you plan on modifying. Limiting the scope would be
> > > > a better method of preventing unwanted writes to such a variable.
> > This is now documented in the LLVM release notes for Clang-9:
> > https://github.com/llvm/llvm-project/commit/e39e79358fcdd5d8ad809defaa821f0bbfa809a5
> > > >
> > > > Further, the code in question mentions "compiler bugs" without any links
> > > > to bug reports, so it is difficult to know if the issue is resolved in
> > > > GCC. The patch was authored in 2006, which would have been GCC 4.0.3 or
> > > > 4.1.1. The minimal supported version of GCC in the Linux kernel is
> > > > currently 4.6.
> > >
> > > It's somewhat older than that. My investigation points to:
> > >
> > > commit c94e57dcd61d661749d53ee876ab265883b0a103
> > > Author: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> > > Date: Sun Nov 25 09:25:53 2001 +0000
> > >
> > > Cleanup of include/asm-mips/io.h. Now looks neat and harmless.
> > Oh indeed, great find!
> > So it looks to me like the order of events is:
> > 1. https://github.com/jaaron/linux-mips-ip30/commit/c94e57dcd61d661749d53ee876ab265883b0a103
> > in 2001 first introduces the UB. mips_io_port_base is defined
> > non-const in arch/mips/kernel/setup.c, but then declared extern const
> > (and modified via UB) in include/asm-mips/io.h. A setter is created,
> > but not a getter (I'll revisit this below). This appears to work (due
> > to luck) for a few years until:
> > 2. https://github.com/mpe/linux-fullhistory/commit/966f4406d903a4214fdc74bec54710c6232a95b8
> > in 2006 adds a compiler barrier (reload all variables) and this
> > appears to work. The commit message mentions that reads after
> > modification of the const variable were buggy (likely GCC started
> > taking advantage of the explicit UB around this time as well). This
> > isn't a fix for UB (more thoughts below), but appears to work.
> > 3. https://github.com/llvm/llvm-project/commit/b45631090220b732e614b5530bbd1d230eb9d38e
> > in 2019 removes writes to const variables in LLVM as that's explicit
> > UB. We observe the boot failure in mips and narrow it down to this
> > instance.
> > I can see how throwing a compiler barrier in there made subsequent
> > reads after UB writes appear to work, but that was more due to luck
> > and implementation details of GCC than the heart of the issue (ie. not
> > writing code that is explicitly undefined behavior)(and could change
> > in future versions of GCC). Stated another way, the fix for explicit
> > UB is not hacks, but avoiding the UB by rewriting the problematic
> > code.
> > > However the purpose of the arrangement does not appear to me to be
> > > particularly specific to a compiler version.
> > >
> > > > For what its worth, there was UB before the commit in question, it just
> > > > added a barrier and got lucky IRT codegen. I don't think there's any
> > > > actual compiler bugs related, just runtime bugs due to UB.
> > >
> > > Does your solution preserves the original purpose of the hack though as
> > > documented in the comment you propose to be removed?
> > The function modified simply writes to a global variable. It's not
> > clear to my why the value about to be modified would EVER be loaded
> > before modification.
> > > Clearly it was defined enough to work for almost 18 years, so it would be
> > > good to keep the optimisation functionally by using different means that
> > > do not rely on UB.
> > "Defined enough" ???
> > https://youtu.be/Aq_1l316ow8?t=17
> > > This variable is assigned at most once throughout the
> > > life of the kernel and then early on, so considering it r/w with all the
> > > consequences for all accesses does not appear to me to be a good use of
> > > it.
> > Note: it's not possible to express the semantics of a "write once
> > variable" in C short of static initialization (AFAIK, without explicit
> > violation of UB, but Cunningham's Law may apply).
> > (set_io_port_base is called in ~20 places)
> > Thinking more about this while I was away, I think what this code has
> > needed since 2001 is proper encapsulation. If you want a variable
> > that is written from one place only, but readable throughout, then the
> > pattern I'd use is:
> > 1. declare a getter in a .h file.
> > 2. define/qualify `mips_io_port_base` as `static` and non-const in a
> > .c file where it's modified.
> > 3. define the getter and setter in the above .c file.
> > That would rely on linkage to limit the visibility of the symbol for
> > modification. But, we'd then need to export the getter, vs the symbol
> > itself. There's also on the order of ~20 call sites that would need
> > to be changed to invoke the getter rather than read the raw variable.
> > Also, it's unlikely the getter gets inlined across translation units
> > (short of LTO, which the mainline kernel doesn't support today).
> > I think my patch here (https://lkml.org/lkml/2019/7/29/1636) is
> > minimally and much less invasive.
> > > Maybe a piece of inline asm to hide the initialisation or suchlike then?
> > I think that would still be UB as the definition would not be changed;
> > you'd still be modifying a variable declared const.
> > --
> > Thanks,
> > ~Nick Desaulniers
> ~Nick Desaulniers