linux-kernel - Re: [PATCH] powerpc: Make set_endian() return EINVAL when not supporting little endian

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Date:   Thu, 26 Aug 2021 13:41:39 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: Make set_endian() return EINVAL when not
 supporting little endian

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> There is no point in modifying MSR_LE bit on CPUs not supporting
> little endian.

Isn't that an ABI break?

set_endian(PR_ENDIAN_BIG) should work on a big endian CPU, even if it
does nothing useful.

cheers

> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 185beb290580..b2b9919795a2 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1995,6 +1995,10 @@ int set_endian(struct task_struct *tsk, unsigned int val)
>  {
>  	struct pt_regs *regs = tsk->thread.regs;
>  
> +	if (!cpu_has_feature(CPU_FTR_PPC_LE) &&
> +	    !cpu_has_feature(CPU_FTR_REAL_LE))
> +		return -EINVAL;
> +
>  	if ((val == PR_ENDIAN_LITTLE && !cpu_has_feature(CPU_FTR_REAL_LE)) ||
>  	    (val == PR_ENDIAN_PPC_LITTLE && !cpu_has_feature(CPU_FTR_PPC_LE)))
>  		return -EINVAL;
> -- 
> 2.25.0