linux-kernel - Re: [PATCH] bug: Use normal relative pointers in 'struct bug

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Message-ID: <yt9d35hnj9zk.fsf@linux.ibm.com>
Date:   Fri, 06 May 2022 11:17:35 +0200
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH] bug: Use normal relative pointers in 'struct bug_entry'

Josh Poimboeuf <jpoimboe@...nel.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@...nel.org>

Acked-by: Sven Schnelle <svens@...ux.ibm.com> # s390
> ---
>  arch/arm64/include/asm/asm-bug.h |  4 ++--
>  arch/powerpc/include/asm/bug.h   |  5 +++--
>  arch/riscv/include/asm/bug.h     |  4 ++--
>  arch/s390/include/asm/bug.h      |  5 +++--
>  arch/x86/include/asm/bug.h       |  2 +-
>  lib/bug.c                        | 15 +++++++--------
>  6 files changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
> index 03f52f84a4f3..c762038ba400 100644
> --- a/arch/arm64/include/asm/asm-bug.h
> +++ b/arch/arm64/include/asm/asm-bug.h
> @@ -14,7 +14,7 @@
>  	14472:	.string file;					\
>  		.popsection;					\
>  								\
> -		.long 14472b - 14470b;				\
> +		.long 14472b - .;				\
>  		.short line;
>  #else
>  #define _BUGVERBOSE_LOCATION(file, line)
> @@ -25,7 +25,7 @@
>  #define __BUG_ENTRY(flags) 				\
>  		.pushsection __bug_table,"aw";		\
>  		.align 2;				\
> -	14470:	.long 14471f - 14470b;			\
> +	14470:	.long 14471f - .;			\
>  _BUGVERBOSE_LOCATION(__FILE__, __LINE__)		\
>  		.short flags; 				\
>  		.popsection;				\
> 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
> diff --git a/arch/riscv/include/asm/bug.h b/arch/riscv/include/asm/bug.h
> index d3804a2f9aad..1aaea81fb141 100644
> --- a/arch/riscv/include/asm/bug.h
> +++ b/arch/riscv/include/asm/bug.h
> @@ -30,8 +30,8 @@
>  typedef u32 bug_insn_t;
>  
>  #ifdef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
> -#define __BUG_ENTRY_ADDR	RISCV_INT " 1b - 2b"
> -#define __BUG_ENTRY_FILE	RISCV_INT " %0 - 2b"
> +#define __BUG_ENTRY_ADDR	RISCV_INT " 1b - ."
> +#define __BUG_ENTRY_FILE	RISCV_INT " %0 - ."
>  #else
>  #define __BUG_ENTRY_ADDR	RISCV_PTR " 1b"
>  #define __BUG_ENTRY_FILE	RISCV_PTR " %0"
> diff --git a/arch/s390/include/asm/bug.h b/arch/s390/include/asm/bug.h
> index 0b25f28351ed..aebe1e22c7be 100644
> --- a/arch/s390/include/asm/bug.h
> +++ b/arch/s390/include/asm/bug.h
> @@ -15,7 +15,8 @@
>  		"1:	.asciz	\""__FILE__"\"\n"		\
>  		".previous\n"					\
>  		".section __bug_table,\"awM\",@progbits,%2\n"	\
> -		"2:	.long	0b-2b,1b-2b\n"			\
> +		"2:	.long	0b-.\n"				\
> +		"	.long	1b-.\n"				\
>  		"	.short	%0,%1\n"			\
>  		"	.org	2b+%2\n"			\
>  		".previous\n"					\
> @@ -30,7 +31,7 @@
>  	asm_inline volatile(					\
>  		"0:	mc	0,0\n"				\
>  		".section __bug_table,\"awM\",@progbits,%1\n"	\
> -		"1:	.long	0b-1b\n"			\
> +		"1:	.long	0b-.\n"				\
>  		"	.short	%0\n"				\
>  		"	.org	1b+%1\n"			\
>  		".previous\n"					\
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index aaf0cb0db4ae..a3ec87d198ac 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -18,7 +18,7 @@
>  #ifdef CONFIG_X86_32
>  # define __BUG_REL(val)	".long " __stringify(val)
>  #else
> -# define __BUG_REL(val)	".long " __stringify(val) " - 2b"
> +# define __BUG_REL(val)	".long " __stringify(val) " - ."
>  #endif
>  
>  #ifdef CONFIG_DEBUG_BUGVERBOSE
> diff --git a/lib/bug.c b/lib/bug.c
> index 45a0584f6541..c223a2575b72 100644
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -6,8 +6,7 @@
>  
>    CONFIG_BUG - emit BUG traps.  Nothing happens without this.
>    CONFIG_GENERIC_BUG - enable this code.
> -  CONFIG_GENERIC_BUG_RELATIVE_POINTERS - use 32-bit pointers relative to
> -	the containing struct bug_entry for bug_addr and file.
> +  CONFIG_GENERIC_BUG_RELATIVE_POINTERS - use 32-bit relative pointers for bug_addr and file
>    CONFIG_DEBUG_BUGVERBOSE - emit full file+line information for each BUG
>  
>    CONFIG_BUG and CONFIG_DEBUG_BUGVERBOSE are potentially user-settable
> @@ -53,10 +52,10 @@ extern struct bug_entry __start___bug_table[], __stop___bug_table[];
>  
>  static inline unsigned long bug_addr(const struct bug_entry *bug)
>  {
> -#ifndef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
> -	return bug->bug_addr;
> +#ifdef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
> +	return (unsigned long)&bug->bug_addr_disp + bug->bug_addr_disp;
>  #else
> -	return (unsigned long)bug + bug->bug_addr_disp;
> +	return bug->bug_addr;
>  #endif
>  }
>  
> @@ -131,10 +130,10 @@ void bug_get_file_line(struct bug_entry *bug, const char **file,
>  		       unsigned int *line)
>  {
>  #ifdef CONFIG_DEBUG_BUGVERBOSE
> -#ifndef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
> -	*file = bug->file;
> +#ifdef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
> +	*file = (const char *)&bug->file_disp + bug->file_disp;
>  #else
> -	*file = (const char *)bug + bug->file_disp;
> +	*file = bug->file;
>  #endif
>  	*line = bug->line;
>  #else