Messages in this thread Patch in this message | | | Subject | [RFC][PATCH 2/2] fix mnt_want_write_file() on special files | From | Dave Hansen <> | Date | Mon, 03 Aug 2009 14:59:42 -0700 |
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mnt_want_write_file() uses the basic assumption that we can use a refernce to a 'struct file' with FMODE_WRITE set in lieu of all of the expensive checks to avoid remount,ro races.
The problem is that FMODE_WRITE is not enough. Special files never had a mnt_want_write() done for them, so we have to exclude them.
This also adds a commented-out BUG_ON() that will reliably detect if anyone tries this again. However, it comes at the cost of destroying any and all performance gains that mnt_clone_write() would have offered (and then some).
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linux-2.6.git-dave/fs/namespace.c | 24 +++++++++++++++++++----- 1 file changed, 19 insertions(+), 5 deletions(-)
diff -puN fs/namespace.c~mnt_want_write_file-0 fs/namespace.c --- linux-2.6.git/fs/namespace.c~mnt_want_write_file-0 2009-08-03 14:51:51.000000000 -0700 +++ linux-2.6.git-dave/fs/namespace.c 2009-08-03 14:52:39.000000000 -0700 @@ -294,9 +294,17 @@ EXPORT_SYMBOL_GPL(mnt_want_write); * * After finished, mnt_drop_write must be called as usual to * drop the reference. + * + * Be very careful using this. You must *guarantee* that + * this vfsmount has at least one existing, persistent writer + * that can not possibly go away, before calling this. */ int mnt_clone_write(struct vfsmount *mnt) { + /* This would kill the performance + * optimization in this function + BUG_ON(count_mnt_writers(mnt) <= 0); + */ /* superblock may be r/o */ if (__mnt_is_readonly(mnt)) return -EROFS; @@ -312,14 +320,20 @@ EXPORT_SYMBOL_GPL(mnt_clone_write); * @file: the file who's mount on which to take a write * * This is like mnt_want_write, but it takes a file and can - * do some optimisations if the file is open for write already + * do some optimisations if the file is open for write already. + * We do not do mnt_want_write() on read-only or special files, + * so we can not use mnt_clone_write() for them. */ int mnt_want_write_file(struct file *file) { - if (!(file->f_mode & FMODE_WRITE)) - return mnt_want_write(file->f_path.mnt); - else - return mnt_clone_write(file->f_path.mnt); + struct path *path = &file->f_path; + struct inode *inode = path->dentry->d_inode; + + if ((file->f_mode & FMODE_WRITE) && + !special_file(inode->i_mode)) + return mnt_clone_write(path->mnt); + + return mnt_want_write(path->mnt); } EXPORT_SYMBOL_GPL(mnt_want_write_file); diff -puN ./lib/Kconfig.debug~mnt_want_write_file-0 ./lib/Kconfig.debug _
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