Re: [PATCH] fs: Fix memcpy_sz for remaining count/rwsize

[Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>]

On 12.10.2015 08:11, Sascha Hauer wrote:
> On Thu, Oct 08, 2015 at 11:19:45PM +0200, Sebastian Hesselbarth wrote:
>> When using memcpy_sz with rwsize != 1 integer division of
>> count/rwsize may leave some bytes of the request uncopied if
>> count is not a multiple of rwsize.
>>
>> Fix this behavior by decrementing count by rwsize instead of
>> integer division and use plain memcpy for the remaining bytes.
>>
>> Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
>> ---
>> Cc: barebox@lists.infradead.org
>> ---
>>   fs/fs.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/fs.c b/fs/fs.c
>> index c041e41bb51b..ccbda22d2692 100644
>> --- a/fs/fs.c
>> +++ b/fs/fs.c
>> @@ -1580,9 +1580,7 @@ static void memcpy_sz(void *dst, const void *src, size_t count, int rwsize)
>>
>>   	rwsize = rwsize >> O_RWSIZE_SHIFT;
>>
>> -	count /= rwsize;
>> -
>> -	while (count-- > 0) {
>> +	while (count > 0) {
>>   		switch (rwsize) {
>>   		case 1:
>>   			*((u8 *)dst) = *((u8 *)src);
>> @@ -1599,7 +1597,12 @@ static void memcpy_sz(void *dst, const void *src, size_t count, int rwsize)
>>   		}
>>   		dst += rwsize;
>>   		src += rwsize;
>> +		count -= rwsize;
>>   	}
>
> This doesn't look correct. When count > 0 you are inside the loop, so
>
>> +
>> +	/* copy remaining bytes with plain memcpy */
>> +	if (count)
>> +		memcpy(dst, src, count);
>
> here count <= 0 which is no meaningful argument for the copy size.
>
> Should the loop start with while (count >= rwsize) instead?

Dammit, last minute cosmetic change including breaking the
whole patch. Sorry for that.

> I wonder if the behaviour shouldn't rather be:
> - let memcpy_sz return the number of bytes copied and not copy the
>    remaining partial word.
> - return error from memcpy_sz when input count < rwsize
>
> This would allow us to catch wrongly aligned sizes.

I am open for any different resolution. I stumbled upon the odd
behavior of memcpy_sz while writing to NAND using memcpy. Maybe
it would be also good to always pick byte size for memcpy when
no specific size has been passed. It took me a while until I
realized it is not the NAND controller but memcpy that breaks
the data written by leaving some bytes uncopied.

Sebastian



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