linux-kernel - Re: mainline build failure for arm64 allmodconfig with clang

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Message-ID: <CAHk-=wivP4zipYnwNWCLF5cd24GLs3m8=Sp7M-CmmPva_UC+3Q@mail.gmail.com>
Date:   Thu, 11 Aug 2022 08:39:36 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     "Sudip Mukherjee (Codethink)" <sudipm.mukherjee@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: mainline build failure for arm64 allmodconfig with clang

On Thu, Aug 11, 2022 at 8:05 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Right, these are exposed by commit 258fafcd0683 ("Makefile.extrawarn:
> re-enable -Wformat for clang").

Christ. Why is clang's format warning SO COMPLETELY BROKEN?

The warning is *WRONG*, for chrissake. Printing an 'int' with '%hhu'
is perfectly fine, and has well-defined semantics, and is what you
*want* to do in some cases.

I'm going to turn it off again, because honestly, this is a clang bug.
I don't care one whit if there are pending "fixes" for this clang bug,
until those fixes are in *clang*, not in the correct kernel code.

For chrissake, the value it is trying to print out as a char is '3'.

But even if it wasn't, and even if you wanted to print out "0xf365" as
a "char" value, then that is how C varargs *work*. It's an "int".

In fact, even a *character* is an "int". This program:

        #include <stdio.h>

        int main(int argc, char **argv)
        {
                printf("%hhu\n", 'a');
        }

generates a warning with "clang -Wformat", and dammit, if you are a
clang developer and you see no problem with that warning, then I don't
know what to say.

Nathan, please make clang people see some sense.

Because no, I'm not in the least interested in getting kernel "fixes"
for this issue. -Wformat for clang goes away until people have gotten
their heads extracted from their derrières.

This is ridiculous.

              Linus