Messages in this thread | | | Date | Mon, 12 Nov 2007 23:08:10 +0900 | From | Tejun Heo <> | Subject | Re: [PATCH] sysfs: fix off-by-one error in fill_read_buffer |
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Miao Xie wrote: > Hi,everyone. > I found that there is a off-by-one problem in the following code. > > Version: 2.6.24-rc2 > File: fs/sysfs/file.c:118-122 > Function: fill_read_buffer > -------------------------------------------------------------------- > count = ops->show(kobj, attr_sd->s_attr.attr, buffer->page); > > sysfs_put_active_two(attr_sd); > > BUG_ON(count > (ssize_t)PAGE_SIZE); > -------------------------------------------------------------------- > Because according to the specification of the sysfs and the implement of > the > show methods, the show methods return the number of bytes which would be > generated for the given input, excluding the trailing null.So if the return > value of the show methods equals PAGE_SIZE - 1, the buffer is full in fact. > And if the return value equals PAGE_SIZE, the resulting string was already > truncated,or buffer overflow occurred. > > This patch fixes an off-by-one error in fill_read_buffer. > > Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
It isn't strictly a bug. If the ->show() op fills full PAGE_SIZE and returns PAGE_SIZE, the user will get full PAGE_SIZE bytes correctly, so it will work. However, considering normal use cases, return value of PAGE_SIZE very likely indicates an error condition, so considering it a BUG condition is a good idea.
Miao, can you please note that the code works fine with PAGE_SIZE return but it's likely to indicate truncated result or overflow in normal use cases as a comment on top of the BUG_ON()?
Thanks.
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