[U-Boot] [PATCH] fs/fat: Fix unaligned __u16 reads for FAT12 access

[Oleksandr Tymoshenko]

Brüns, Stefan (Stefan.Bruens at rwth-aachen.de) wrote:
> Doing unaligned reads is not supported on all architectures, use
> byte sized reads of the little endian buffer.
> Rename off16 to off8, as it reflects the buffer offset in byte
> granularity (offset is in entry, i.e. 12 bit, granularity).
> Fix a regression introduced in 8d48c92b45aea91e2a2be90f2ed93677e85526f1
> 
> Reported-by: Oleksandr Tymoshenko <gonzo at bluezbox.com>
> Signed-off-by: Stefan Brüns <stefan.bruens at rwth-aachen.de>
> ---
>  fs/fat/fat.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fat/fat.c b/fs/fat/fat.c
> index fe899d0442..06088e2421 100644
> --- a/fs/fat/fat.c
> +++ b/fs/fat/fat.c
> @@ -179,7 +179,7 @@ int flush_dirty_fat_buffer(fsdata *mydata)
>  static __u32 get_fatent(fsdata *mydata, __u32 entry)
>  {
>  	__u32 bufnum;
> -	__u32 off16, offset;
> +	__u32 offset, off8;
>  	__u32 ret = 0x00;
>  
>  	if (CHECK_CLUST(entry, mydata->fatsize)) {
> @@ -242,8 +242,9 @@ static __u32 get_fatent(fsdata *mydata, __u32 entry)
>  		ret = FAT2CPU16(((__u16 *) mydata->fatbuf)[offset]);
>  		break;
>  	case 12:
> -		off16 = (offset * 3) / 2;
> -		ret = FAT2CPU16(*(__u16 *)(mydata->fatbuf + off16));
> +		off8 = (offset * 3) / 2;
> +		/* fatbut + off8 may be unaligned, read in byte granularity */
> +		ret = mydata->fatbuf[off8] + mydata->fatbuf[off8 + 1] << 8;

This patch when applied gives me "Invalid FAT entry" message. Reason
is: operator '<<' has lower precedence than '+' so this expression is
equivalent to (mydata->fatbuf[off8] + mydata->fatbuf[off8 + 1]) << 8
With explicit parentheses around shift it works as expected:

ret = mydata->fatbuf[off8] + (mydata->fatbuf[off8 + 1] << 8);

-- 
gonzo