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Date:   Thu, 7 Apr 2022 17:16:40 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Richard Biener <rguenther@suse.de>
Cc:     linux-toolchains@vger.kernel.org, Michael Matz <matz@suse.de>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: older gccs and case labels producing integer constants
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On Tue, Apr 05, 2022 at 01:41:09PM +0200, Richard Biener wrote:
> As was noted in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66880
> this is invalid C99+ but compilers are not required to diagnose that
> (you get it diagnosed with -pedantic).  -fsanitize=shift exposes
> it though since the non-integral-constant gets instrumented.

Right, just to close this: I was still unsure which of the cmdline
options would cause it and bisected the kernel (big fat box can build
allmodconfigs in no time :)).

The single change which fixes the whole build is

---
diff --git a/Makefile b/Makefile
index 8c7de9a72ea2..3582089cfeb6 100644
--- a/Makefile
+++ b/Makefile
@@ -523,7 +523,7 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
 		   -Werror=implicit-function-declaration -Werror=implicit-int \
 		   -Werror=return-type -Wno-format-security \
-		   -std=gnu11
+		   -std=gnu89
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=

with that

gcc (SUSE Linux) 7.4.1 20190905 [gcc-7-branch revision 275407]

but as we saw, only -std=gnu11 alone doesn't cause it:

$ gcc -std=gnu11 -o switch.o switch.c
$

And so we had the -fsanitize=shift already enabled since 2020 in the
kernel build and the gnu11 change then triggered the undefined behavior
due to the -fsanitize instrumentation as it was already explained:

$ gcc -std=gnu11 -fsanitize=shift -o switch.o switch.c
switch.c: In function ‘foo’:
switch.c:10:7: error: case label does not reduce to an integer constant
       case (((0xfc08) << 16) | (0x0101)):;
       ^~~~

Ok, now I can sleep at night again.

:-)))

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette