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Subject[RFC][PATCH 30/30] r/o bind mounts: debugging for missed calls
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Date

There have been a few oopses caused by 'struct file's with NULL f_vfsmnts.
There was also a set of potentially missed mnt_want_write()s from
dentry_open() calls.

This patch provides a very simple debugging framework to catch these kinds of
bugs. It will WARN_ON() them, but should stop us from having any oopses or
mnt_writer count imbalances.

I'm quite convinced that this is a good thing because it found bugs in the
stuff I was working on as soon as I wrote it.

[hch: made it conditional on a debug option.
But it's still a little bit too ugly]

[hch: merged forced remount r/o fix from Dave and akpm's fix for the fix]

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
linux-2.6.git-dave/fs/file_table.c | 11 ++++++-
linux-2.6.git-dave/fs/open.c | 12 +++++++-
linux-2.6.git-dave/fs/super.c | 3 ++
linux-2.6.git-dave/include/linux/fs.h | 49 ++++++++++++++++++++++++++++++++++
linux-2.6.git-dave/lib/Kconfig.debug | 10 ++++++
5 files changed, 82 insertions(+), 3 deletions(-)
diff -puN fs/file_table.c~keep-track-of-mnt_writer-state-of-struct-file fs/file_table.c
--- linux-2.6.git/fs/file_table.c~keep-track-of-mnt_writer-state-of-struct-file 2008-02-08 13:05:02.000000000 -0800
+++ linux-2.6.git-dave/fs/file_table.c 2008-02-08 13:05:02.000000000 -0800
@@ -42,6 +42,7 @@ static inline void file_free_rcu(struct
static inline void file_free(struct file *f)
{
percpu_counter_dec(&nr_files);
+ file_check_state(f);
call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
}

@@ -207,6 +208,7 @@ int init_file(struct file *file, struct
* that we can do debugging checks at __fput()e
*/
if ((mode & FMODE_WRITE) && !special_file(dentry->d_inode->i_mode)) {
+ file_take_write(file);
error = mnt_want_write(mnt);
WARN_ON(error);
}
@@ -237,8 +239,13 @@ void drop_file_write_access(struct file
struct inode *inode = dentry->d_inode;

put_write_access(inode);
- if (!special_file(inode->i_mode))
- mnt_drop_write(mnt);
+
+ if (special_file(inode->i_mode))
+ return;
+ if (file_check_writeable(file) != 0)
+ return;
+ mnt_drop_write(mnt);
+ file_release_write(file);
}
EXPORT_SYMBOL_GPL(drop_file_write_access);

diff -puN fs/open.c~keep-track-of-mnt_writer-state-of-struct-file fs/open.c
--- linux-2.6.git/fs/open.c~keep-track-of-mnt_writer-state-of-struct-file 2008-02-08 13:05:02.000000000 -0800
+++ linux-2.6.git-dave/fs/open.c 2008-02-08 13:05:02.000000000 -0800
@@ -810,6 +810,8 @@ static struct file *__dentry_open(struct
error = __get_file_write_access(inode, mnt);
if (error)
goto cleanup_file;
+ if (!special_file(inode->i_mode))
+ file_take_write(f);
}

f->f_mapping = inode->i_mapping;
@@ -851,8 +853,16 @@ cleanup_all:
fops_put(f->f_op);
if (f->f_mode & FMODE_WRITE) {
put_write_access(inode);
- if (!special_file(inode->i_mode))
+ if (!special_file(inode->i_mode)) {
+ /*
+ * We don't consider this a real
+ * mnt_want/drop_write() pair
+ * because it all happenend right
+ * here, so just reset the state.
+ */
+ file_reset_write(f);
mnt_drop_write(mnt);
+ }
}
file_kill(f);
f->f_path.dentry = NULL;
diff -puN fs/super.c~keep-track-of-mnt_writer-state-of-struct-file fs/super.c
--- linux-2.6.git/fs/super.c~keep-track-of-mnt_writer-state-of-struct-file 2008-02-08 13:05:02.000000000 -0800
+++ linux-2.6.git-dave/fs/super.c 2008-02-08 13:05:02.000000000 -0800
@@ -578,6 +578,9 @@ retry:
if (!(f->f_mode & FMODE_WRITE))
continue;
f->f_mode &= ~FMODE_WRITE;
+ if (file_check_writeable(f) != 0)
+ continue;
+ file_release_write(f);
mnt = f->f_path.mnt;
file_list_unlock();
/*
diff -puN include/linux/fs.h~keep-track-of-mnt_writer-state-of-struct-file include/linux/fs.h
--- linux-2.6.git/include/linux/fs.h~keep-track-of-mnt_writer-state-of-struct-file 2008-02-08 13:05:02.000000000 -0800
+++ linux-2.6.git-dave/include/linux/fs.h 2008-02-08 13:05:02.000000000 -0800
@@ -776,6 +776,9 @@ static inline int ra_has_index(struct fi
index < ra->start + ra->size);
}

+#define FILE_MNT_WRITE_TAKEN 1
+#define FILE_MNT_WRITE_RELEASED 2
+
struct file {
/*
* fu_list becomes invalid after file_free is called and queued via
@@ -810,6 +813,9 @@ struct file {
spinlock_t f_ep_lock;
#endif /* #ifdef CONFIG_EPOLL */
struct address_space *f_mapping;
+#ifdef CONFIG_DEBUG_WRITECOUNT
+ unsigned long f_mnt_write_state;
+#endif
};
extern spinlock_t files_lock;
#define file_list_lock() spin_lock(&files_lock);
@@ -818,6 +824,49 @@ extern spinlock_t files_lock;
#define get_file(x) atomic_inc(&(x)->f_count)
#define file_count(x) atomic_read(&(x)->f_count)

+#ifdef CONFIG_DEBUG_WRITECOUNT
+static inline void file_take_write(struct file *f)
+{
+ WARN_ON(f->f_mnt_write_state != 0);
+ f->f_mnt_write_state = FILE_MNT_WRITE_TAKEN;
+}
+static inline void file_release_write(struct file *f)
+{
+ f->f_mnt_write_state |= FILE_MNT_WRITE_RELEASED;
+}
+static inline void file_reset_write(struct file *f)
+{
+ f->f_mnt_write_state = 0;
+}
+static inline void file_check_state(struct file *f)
+{
+ /*
+ * At this point, either both or neither of these bits
+ * should be set.
+ */
+ WARN_ON(f->f_mnt_write_state == FILE_MNT_WRITE_TAKEN);
+ WARN_ON(f->f_mnt_write_state == FILE_MNT_WRITE_RELEASED);
+}
+static inline int file_check_writeable(struct file *f)
+{
+ if (f->f_mnt_write_state == FILE_MNT_WRITE_TAKEN)
+ return 0;
+ printk(KERN_WARNING "writeable file with no "
+ "mnt_want_write()\n");
+ WARN_ON(1);
+ return -EINVAL;
+}
+#else /* !CONFIG_DEBUG_WRITECOUNT */
+static inline void file_take_write(struct file *filp) {}
+static inline void file_release_write(struct file *filp) {}
+static inline void file_reset_write(struct file *filp) {}
+static inline void file_check_state(struct file *filp) {}
+static inline int file_check_writeable(struct file *filp)
+{
+ return 0;
+}
+#endif /* CONFIG_DEBUG_WRITECOUNT */
+
#define MAX_NON_LFS ((1UL<<31) - 1)

/* Page cache limit. The filesystems should put that into their s_maxbytes
diff -puN lib/Kconfig.debug~keep-track-of-mnt_writer-state-of-struct-file lib/Kconfig.debug
--- linux-2.6.git/lib/Kconfig.debug~keep-track-of-mnt_writer-state-of-struct-file 2008-02-08 13:05:02.000000000 -0800
+++ linux-2.6.git-dave/lib/Kconfig.debug 2008-02-08 13:05:02.000000000 -0800
@@ -433,6 +433,16 @@ config DEBUG_VM

If unsure, say N.

+config DEBUG_WRITECOUNT
+ bool "Debug filesystem writers count"
+ depends on DEBUG_KERNEL
+ help
+ Enable this to catch wrong use of the writers count in struct
+ vfsmount. This will increase the size of each file struct by
+ 32 bits.
+
+ If unsure, say N.
+
config DEBUG_LIST
bool "Debug linked list manipulation"
depends on DEBUG_KERNEL
_

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