[EMAIL PROTECTED] wrote on Tue, 25 Mar 2008 12:10 -0500:
> I've been debugging a problem with simul returning EACCES errors on open of
> a PVFS file. It turns out the bug was due to permissions being (re-)set on
> an inode without a mutex locking the write. This was causing the
> permissions checks in other opens of the same file to fail because the mode
> was wrong.
>
> For each (subsequent) open, the kernel calls d_revalidate. The PVFS
> implementation of d_revalidate does a lookup to verify that the handle of
> the inode and looked-up handle match, then it does a getattr on that
> handle, and copies the attributes into the inode. This is where the race
> occurred, during copying of the attrs. In pvfs2-utils.c:231, we set:
>
> inode->i_mode = 0;
>
> /* figure out the correct permissions for the file based on pvfs attrs */
>
> if (attrs->perms & PVFS_O_EXECUTE)
> perm_mode |= S_IXOTH;
> if (attrs->perms & PVFS_O_WRITE)
> perm_mode |= S_IWOTH;
> ....
>
> inode->i_mode |= perm_mode;
>
> That entire block is unprotected, so between setting the i_mode to 0 and
> then perm_mode, the i_mode can be accessed by other processes (in the
> kernel vfs permissions checking code, for example).
>
> I think the problem is exacerbated by the large block of code that converts
> the pvfs attrs into the perm_mode field all while i_mode is 0. Just
> setting inode->i_mode = perm_mode directly made the bug disappear for my
> test case, even though there's still a RMW race there.
>
> I'm tempted to throw a big mutex_lock(inode->i_mutex) around the entire
> code block that does copy_attrs_to_inode. Any objections against doing
> that?
Definitely wants locking, but I think it should be in the caller.
One of the two callers in that file already has the new inode,
locked, so it doesn't want another level of locking. Ditto for the
caller in dir.c. If it is true that that inode locking is
sufficient to keep other processes away.
-- Pete
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