linux-kernel - Re: [RFC PATCH 4/8] mm: Separate fault info out of 'struct vm

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Message-ID: <CAHk-=wjWQFSZci+By4wXAEx6EDH4nss7tf5QimQpF53rnHa7Wg@mail.gmail.com>
Date:   Fri, 15 Jan 2021 13:32:57 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Hugh Dickins <hughd@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC PATCH 4/8] mm: Separate fault info out of 'struct vm_fault'

On Fri, Jan 15, 2021 at 1:23 AM Will Deacon <will@...nel.org> wrote:
>
> Hmm. The feedback on the clang bug suggests that GCC is the one in the
> wrong here (although the argument is based on C11 and I haven't trawled
> through the standards to see how this has evolved):

Oh well.

That writing is absolutely the _worst_ kind of weaselwording standards
language reading, trying to make excuses for bad behavior by basically
depending on "this language is unclear", and trying to say that the
buggy behavior is required by C11.

What a disappointment.

Absolutely nothing in the quoted C11 language says to me what that bug
entry claims it says.

The argument seems to hinge on

   "The members of an anonymous structure or union are considered to
be members of the containing structure or union"

and then it makes the completely uncalled-for leap that that means
that because it was "int" in the const struct, it must be "int" in the
containing structure too.

Which is complete BS, and doesn't follow logically _or_ grammatically.
It would be a "member of the containing structure" even with the
"const" qualifier, so the argument they make is just inane.

In fact, the _other_ sentence they quote clearly points to this being
just a clang bug:

 "A modifiable lvalue is [...] if it is a structure or union, does not
have any member (including, recursively, any member or element of all
contained aggregates or unions) with a const- qualified type"

and clearly this recursively is an element with a const-qualified
recursive struct.

Whatever. It's one of those "read the documentation squint-eyed to
avoid doing the right thing" arguments.

It's not worth arguing with people like that, and let's just ignore
the clang bug here.

            Linus