Messages in this thread |  | | Date | Tue, 4 Sep 2001 03:16:08 +0200 (CEST) | From | Jean-Marc Saffroy <> | Subject | Re: [RFD] readonly/read-write semantics |
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On Fri, 31 Aug 2001, Alexander Viro wrote:
> > In 2.4.9, I have encountered a strange condition while playing with file > > structs chained on a superblock list (sb->s_files) : some of them can have > > a NULL f_dentry pointer. The only case I found which can cause this is > > when fput is called and f_count drops to zero. Is that the only case ? > > Yes, it is, and yes, it's legitimate - code that scans that list should > (and in-tree one does) deal with such case.
AFAICT fput (and also dentry_open, BTW) nullifies f_dentry without any lock held, so code that scans the list (such as fs_may_remount_ro, I haven't looked for other instances) can never assume that a file struct found in the list has or even will keep what looks like a valid f_dentry.
> fs_may_remount_ro() is, indeed, racy and had been since very long.
Sure, let's consider code in fs_may_remount_ro :
file_list_lock(); /* loop over files in sb->s_files */ if (!file->f_dentry) continue; /* now a concurrent fput may set f_dentry to NULL */ inode = file->f_dentry->d_inode; /* oops */
Maybe the file struct should be removed from the list /before/ f_dentry is assigned NULL ?
> However, the main problem is not in opening something after the > check - the check itself is not exact enough.
I agree fs_may_remount_ro can report wrong results (ie. "you may remount ro" while you really can't) because of how it is used, but as stated above, I think it also has a small but real potential for directly crashing the system, and should be fixed.
-- Jean-Marc Saffroy - Research Engineer - Silicomp Research Institute mailto:saffroy@ri.silicomp.fr
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