On Friday 10/07 at 17:18 -0400, Calvin Owens wrote: > On Friday 10/07 at 21:48 +0100, Al Viro wrote: > > On Fri, Oct 07, 2016 at 01:35:52PM -0700, Calvin Owens wrote: > > > Omitting the owner field in file_operations declared in modules is an > > > easy mistake to make, and can result in crashes when the module is > > > unloaded while userspace is poking the file. > > > > > > This patch modifies fops_get() to WARN when it encounters a NULL owner, > > > since in this case it cannot take a reference on the containing module. > > > > NAK. This is complete crap - we do *NOT* need ->owner on a lot of > > file_operations. > > This isn't a theoretical issue: I have a proprietary module that makes this > mistake and crashes when poking a chrdev it exposes in userspace races with > unloading the module. > > Of course, the bug is in this silly module. I'm not arguing that it isn't. I > was hesitant to even mention this because I know waving at something in an OOT > module is a poor argument for changing anything in the proper kernel. > > But what I'm trying to do here is prevent people from making that mistake in > the future by yelling at them when they do. The implicit ignoring of a NULL > owner in try_module_get() in fops_get() is not necessarily obvious. Let's drop this, I should never have sent the patch in the first place. > > * we do not need that on file_operations of a regular file or > > directory on a normal filesystem, since that filesystem is not going > > away until the file has been closed - ->f_path.mnt is holding a reference > > to vfsmount, which is holding a reference to superblock, which is holding > > a reference to file_system_type, which is holding a reference to _its_ > > ->owner. > > * we do not need that on anything on procfs - module removal is > > legal while a procfs file is opened; its cleanup will be blocked for the > > duration of ->read(), ->write(), etc. calls. > > I see why this is true, and it's something I considered. But when there is > zero cost to being explicit and setting ->owner, why not do it? > > > If anything, we would be better off with modifications that would get > > rid of ->owner on file_operations. It's not trivial to do, but it might > > be not impossible. I'll look into this, I'm interested. Thanks, Calvin > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
