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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Josh Poimboeuf <jpoimboe@kernel.org>, linux-kernel@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
        linux-s390@vger.kernel.org
Subject: Re: [PATCH] bug: Use normal relative pointers in 'struct bug_entry'
In-Reply-To: <afddb4548e93f6458ec1d9ec185a834c348eda33.1651798983.git.jpoimboe@kernel.org>
References: <afddb4548e93f6458ec1d9ec185a834c348eda33.1651798983.git.jpoimboe@kernel.org>
Date:   Mon, 09 May 2022 22:31:14 +1000
Message-ID: <871qx2ubu5.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Josh Poimboeuf <jpoimboe@kernel.org> writes:
> With CONFIG_GENERIC_BUG_RELATIVE_POINTERS, the addr/file relative
> pointers are calculated weirdly: based on the beginning of the bug_entry
> struct address, rather than their respective pointer addresses.
>
> Make the relative pointers less surprising to both humans and tools by
> calculating them the normal way.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
...
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index ecbae1832de3..76252576d889 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -13,7 +13,8 @@
>  #ifdef CONFIG_DEBUG_BUGVERBOSE
>  .macro __EMIT_BUG_ENTRY addr,file,line,flags
>  	 .section __bug_table,"aw"
> -5001:	 .4byte \addr - 5001b, 5002f - 5001b
> +5001:	 .4byte \addr - .
> +	 .4byte 5002f - .
>  	 .short \line, \flags
>  	 .org 5001b+BUG_ENTRY_SIZE
>  	 .previous
> @@ -24,7 +25,7 @@
>  #else
>  .macro __EMIT_BUG_ENTRY addr,file,line,flags
>  	 .section __bug_table,"aw"
> -5001:	 .4byte \addr - 5001b
> +5001:	 .4byte \addr - .
>  	 .short \flags
>  	 .org 5001b+BUG_ENTRY_SIZE
>  	 .previous

Embarrassingly, we have another copy of the logic, used in the C
versions, they need updating too:

diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index ecbae1832de3..3fde35fd67f8 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -49,14 +49,14 @@
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 #define _EMIT_BUG_ENTRY				\
 	".section __bug_table,\"aw\"\n"		\
-	"2:\t.4byte 1b - 2b, %0 - 2b\n"		\
+	"2:\t.4byte 1b - ., %0 - .\n"		\
 	"\t.short %1, %2\n"			\
 	".org 2b+%3\n"				\
 	".previous\n"
 #else
 #define _EMIT_BUG_ENTRY				\
 	".section __bug_table,\"aw\"\n"		\
-	"2:\t.4byte 1b - 2b\n"			\
+	"2:\t.4byte 1b - .\n"			\
 	"\t.short %2\n"				\
 	".org 2b+%3\n"				\
 	".previous\n"


With that added it seems to be working correctly for me.

Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)


cheers