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Date: Wed, 11 Dec 2013 14:54:40 +1100
From: Paul Mackerras <paulus@samba.org>
To: Tom <tommusta@gmail.com>
Subject: Re: [V2 PATCH 3/3] powerpc: Fix Unaligned LE Floating Point Loads
 and Stores
Message-ID: <20131211035440.GA9399@drongo>
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On Thu, Oct 31, 2013 at 01:38:58PM -0500, Tom wrote:
> From: Tom Musta <tommusta@gmail.com>
> 
> This patch addresses unaligned single precision floating point loads
> and stores in the single-step code.  The old implementation
> improperly treated an 8 byte structure as an array of two 4 byte
> words, which is a classic little endian bug.
> 
> Signed-off-by: Tom Musta <tommusta@gmail.com>
> ---
>  arch/powerpc/lib/sstep.c |   52 +++++++++++++++++++++++++++++++++++----------
>  1 files changed, 40 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index c8743e1..1cfd150 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -355,22 +355,36 @@ static int __kprobes do_fp_load(int rn, int (*func)(int, unsigned long),
>  				struct pt_regs *regs)
>  {
>  	int err;
> -	unsigned long val[sizeof(double) / sizeof(long)];
> +	union {
> +		double dbl;
> +		unsigned long ul[2];
> +		struct {
> +#ifdef __BIG_ENDIAN__
> +			unsigned _pad_;
> +			unsigned word;
> +#endif
> +#ifdef __LITTLE_ENDIAN__
> +			unsigned word;
> +			unsigned _pad_;
> +#endif
> +		} single;
> +	} data;
>  	unsigned long ptr;
>  
>  	if (!address_ok(regs, ea, nb))
>  		return -EFAULT;
>  	if ((ea & 3) == 0)
>  		return (*func)(rn, ea);
> -	ptr = (unsigned long) &val[0];
> +	ptr = (unsigned long) &data.ul;
>  	if (sizeof(unsigned long) == 8 || nb == 4) {
> -		err = read_mem_unaligned(&val[0], ea, nb, regs);
> -		ptr += sizeof(unsigned long) - nb;
> +		err = read_mem_unaligned(&data.ul[0], ea, nb, regs);
> +		if (nb == 4)
> +			ptr = (unsigned long)&(data.single.word);
>  	} else {
>  		/* reading a double on 32-bit */
> -		err = read_mem_unaligned(&val[0], ea, 4, regs);
> +		err = read_mem_unaligned(&data.ul[0], ea, 4, regs);
>  		if (!err)
> -			err = read_mem_unaligned(&val[1], ea + 4, 4, regs);
> +			err = read_mem_unaligned(&data.ul[1], ea + 4, 4, regs);

This breaks 32-bit big-endian (as well as making the code longer and
more complex).

In fact, to make this work for 64-bit little-endian, all you really
needed to do was ifdef out the statement:

		ptr += sizeof(unsigned long) - nb;

Paul.