[PATCH] fs: jffs2: fix error when reading blocks with offset

[Ahmad Fatoum]

On 02.12.21 11:48, Holger Assmann wrote:
> This bug resulted in a panic when trying to read a file partially with
> an offset, e.g. when starting Linux. In such a case a header is loaded
> by a separate call of jffs2_read(), which then copies only the first n
> bytes out of the respective block.
> When the remaining data of the block is then subsequently going to to be
> read the system deviates in its behaviour from that under Linux by not
> calling jffs2_read_inode_range() in a 4k-alignment, but with an offset.
> 
> jffs2_read_inode_range() originates from the Linux jffs2 driver. When
> being called with an offset it still reads 4096 bytes of data and
> eventually returns fragments of two consecutive blocks. jffs2_read()
> then reads this result whilst again applying the offset, therefore
> returning faulty data.
> 
> We fix that problem by calling jffs2_get_block() without an offset and
> therefore reading the whole block. The offset is then applied when we
> actually perform memcpy with the returned buffer. This fix might also
> increase the performance since the respective block is likely to be
> cached from the previous call.
> 
> Signed-off-by: Holger Assmann <h.assmann at pengutronix.de>
> ---
>  fs/jffs2/fs.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/jffs2/fs.c b/fs/jffs2/fs.c
> index 1a2b443792..3d4cb98c97 100644
> --- a/fs/jffs2/fs.c
> +++ b/fs/jffs2/fs.c
> @@ -74,6 +74,9 @@ static int jffs2_get_block(struct jffs2_file *jf, unsigned int pos)
>  	struct jffs2_inode_info *f = JFFS2_INODE_INFO(jf->inode);
>  	int ret;
>  
> +	/* pos always has to be 4096 bytes aligned here */
> +	WARN_ON(pos % 4096 != 0);
> +
>  	if (pos != jf->offset) {
>  		ret = jffs2_read_inode_range(c, f, jf->buf, pos,
>  					     JFFS2_BLOCK_SIZE);
> @@ -98,13 +101,14 @@ static int jffs2_read(struct device_d *_dev, FILE *f, void *buf,
>  	/* Read till end of current block */
>  	ofs = f->pos % JFFS2_BLOCK_SIZE;
>  	if (ofs) {
> -		ret = jffs2_get_block(jf, pos);
> +		ret = jffs2_get_block(jf, f->pos - ofs); /* Align down block */
>  		if (ret)
>  			return ret;
>  
>  		now = min(size, JFFS2_BLOCK_SIZE - ofs);
>  
> -		memcpy(buf, jf->buf + ofs, now);
> +		/* Complete block has been read, re-apply ofset now */
> +		memcpy(buf, jf->buf + ofs, now); 

Newly added stray space at end of line.

Apart from that:

Reviewed-by: Ahmad Fatoum <a.fatoum at pengutronix.de>

>  		size -= now;
>  		pos += now;
>  		buf += now;
> 


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