linux-kernel - Re: [RFC PATCH v1] objtool: Use target file endianness instead of a compiled constant

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Date:   Thu, 31 Mar 2022 10:08:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Sathvika Vasireddy <sv@...ux.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH v1] objtool: Use target file endianness instead of a
 compiled constant

On Thu, Mar 31, 2022 at 09:52:07AM +0200, Christophe Leroy wrote:
> Some architectures like powerpc support both endianness, it's
> therefore not possible to fix the endianness via arch/endianness.h
> because there is no easy way to get the target endianness at
> build time.
> 
> Use the endianness recorded in the file objtool is working on.
> 

> +#include <objtool/elf.h>
>  
>  /*
> - * Does a byte swap if target endianness doesn't match the host, i.e. cross
> + * Does a byte swap if target file endianness doesn't match the host, i.e. cross
>   * compilation for little endian on big endian and vice versa.
>   * To be used for multi-byte values conversion, which are read from / about
>   * to be written to a target native endianness ELF file.
>   */
> -#define bswap_if_needed(val)						\
> +static inline bool need_bswap(GElf_Ehdr *ehdr)
> +{
> +	return (__BYTE_ORDER == __LITTLE_ENDIAN) ^
> +	       (ehdr->e_ident[EI_DATA] == ELFDATA2LSB);
> +}
> +
> +#define bswap_if_needed(ehdr, val)					\
>  ({									\
>  	__typeof__(val) __ret;						\
> +	bool __need_bswap = need_bswap(ehdr);				\
>  	switch (sizeof(val)) {						\
> -	case 8: __ret = __NEED_BSWAP ? bswap_64(val) : (val); break;	\
> -	case 4: __ret = __NEED_BSWAP ? bswap_32(val) : (val); break;	\
> -	case 2: __ret = __NEED_BSWAP ? bswap_16(val) : (val); break;	\
> +	case 8: __ret = __need_bswap ? bswap_64(val) : (val); break;	\
> +	case 4: __ret = __need_bswap ? bswap_32(val) : (val); break;	\
> +	case 2: __ret = __need_bswap ? bswap_16(val) : (val); break;	\
>  	default:							\
>  		BUILD_BUG(); break;					\
>  	}								\

Far less painfull that I imagined it would be,.. but I think I prefer
passing in elf, as opposed to elf->ehdr, would that work?