Messages in this thread | | | Date | Tue, 26 Nov 2013 14:28:59 -0800 | From | "H. Peter Anvin" <> | Subject | Re: copy_from_user_*() and buffer zeroing |
| |
On 11/26/2013 01:54 PM, Andrew Morton wrote: > > Nine years ago: > > commit 7079f897164cb14f616c785d3d01629fd6a97719 > Author: mingo <mingo> > Date: Fri Aug 27 17:33:18 2004 +0000 > > [PATCH] Add a few might_sleep() checks > > Add a whole bunch more might_sleep() checks. We also enable might_sleep() > checking in copy_*_user(). This was non-trivial because of the "copy_*_user() > in atomic regions" trick would generate false positives. Fix that up by > adding a new __copy_*_user_inatomic(), which avoids the might_sleep() check. > > Only i386 is supported in this patch. > > > I can't think of any reason why __copy_from_user_inatomic() should be > non-zeroing. But maybe I'm missing something - this would pretty > easily permit uninitialised data to appear in pagecache and someone > surely would have noticed.. >
Yes, and the might_sleep() check is indeed bypassed.
However, the non-zeroing bit is currently motivated by the following comment:
/** * __copy_from_user: - Copy a block of data from user space, with less checking. * @to: Destination address, in kernel space. * @from: Source address, in user space. * @n: Number of bytes to copy. * * Context: User context only. This function may sleep. * * Copy data from user space to kernel space. Caller must check * the specified block with access_ok() before calling this function. * * Returns number of bytes that could not be copied. * On success, this will be zero. * * If some data could not be copied, this function will pad the copied * data to the requested size using zero bytes. * * An alternate version - __copy_from_user_inatomic() - may be called from * atomic context and will fail rather than sleep. In this case the * uncopied bytes will *NOT* be padded with zeros. See fs/filemap.h * for explanation of why this is needed. */
This comment is only present in the 32-bit code. fs/filemap.h of course no longer exists, however, the original commit seems to be 01408c4939479ec46c15aa7ef6e2406be50eeeca which puts a comment in the (now defunct mm/filemap.h).
I have to say I don't follow the explanation in that patch. It seems like if you're concurrently reading a buffer being written you should expect to get any kind of mismash...
Neil, is this still an issue?
-hpa
| |