[PATCH v3 0/7] fix debugfs file removal races
Original v2 thread is here:
http://lkml.kernel.org/g/87fux3memd@gmail.com
In the discussion of v2, it turned out that touching each and every of
the ~1000 debugfs users in order to make them save against file
removals is unfeasible.
Thus, v3 takes a different approach: every struct file_operations
handed to debugfs is wrapped by a protecting proxy in [2/7]. Only
those struct file_operations which are easy to fix directly,
i.e. those defined by debugfs itself, opt-out from this proxying in
[3-7/7].
The Coccinelle people are CC'd because of [3/7].
Many thanks to J. Lawall who helped me very much at #cocci@freenode
in getting this done!
The SRCU part really needs some fresh review: in v2 the
rcu_assign_pointer()'ed and srcu_derefence()'d ->d_fsdata has been
effectively used as an indication of whether a file is dead or not.
With the full proxy approach in v3, ->d_fsdata can't be cleared out at
file removal because open files might still hold a reference to it and
those must be released again from the proxy's ->release().
Thus, the properly memory- and compiler-barriered accesses to
->d_fsdata have now been replaced by completely unbarriered d_delete()
and d_unlinked() calls in debugfs_use_file_start() and
debugfs_remove() (all in [1/7] now).
I believe that no extra barriers are needed:
The SRCU read side critical sections around any file usage looks like this:
srcu_read_lock();
if(d_unlinked(dentry)) {
srcu_read_unlock();
return -EIO
}
cope_around_with(d_inode(dentry)->i_private);
srcu_read_unlock()
- srcu_read_lock() and srcu_read_unlock() already contain a barrier()
each. Thus, the compiler is forced to make the dentry's state being
read at least once within the read side critical section.
- I don't care for speculative reads to the file's private data
in cope_around_with().
- Writes in cope_around_with() should be properly handled by the control
dependency in that they don't occur on the bus if d_unlinked() holds.
Furthermore, any writes in cope_around_with() are emitted by the
compiler before the srcu_read_unlock().
For the file removing side of things, the SRCU usage looks like this:
d_delete(dentry);
synchronize_srcu();
free(d_inode(dentry)->i_private);
d_delete() is defined in another compilation unit. Thus, its call can't be
reorded with the one to synchronize_srcu() and the dentry state is written
(on that CPU) before synchronize_srcu() is entered.
Changes v2 -> v3:
[1/7] ("debugfs: prevent access to possibly dead file_operations at file
open")
- move the definition of the debugfs_use_file_start() and _end() from former
[2/2] to [1/7]. Also, they've been renamed from debugfs_file_use_data*().
- Make the ->open() proxy use the debugfs_use_file_*() helpers.
- In debugfs_use_file_start(), use d_unlinked() rather than
(->d_fsdata == NULL) as a flag whether the dentry is dead.
- Make the ->open() proxy include the forwarded call to the original fops'
->open
within the SRCU read side critical section.
- debugfs_proxy_file_operations has been renamed to
"debugfs_open_proxy_file_operations" to distinguish it from the full proxy
introduced in [2/7].
[2/7] ("debugfs: prevent access to removed files' private data")
- This one has changed completely: instead of providing file
removal-safe fops helpers to opt-into at the debugfs users, the
original struct file_operations get completely and
unconditionally proxied now.
[3-7/7]
New. Opt-out from the full proxying introduced in [2/7] for some
special case struct file_operations provided by debugfs itself.
Changes v1 -> v2:
[1/2] ("debugfs: prevent access to possibly dead file_operations at file
open")
- Resolve trivial diff conflict in debugfs_remove_recursive():
in the meanwhile, an unrelated 'mutex_unlock(...)' had been rewritten to
'inode_unlock(...)' which broke the diff's context.
- Introduce the fs/debugfs/internal.h header and move the declarations of
debugfs_noop_file_operations, debugfs_proxy_file_operations and
debugfs_rcu from include/linux/debugfs.h thereinto. Include this header
from file.c and inode.c.
- Add a word about the new internal header to the commit message.
- Move the inclusion of linux/srcu.h from include/linux/debugfs.h
into file.c and inode.c respectively.
[2/2] ("debugfs: prevent access to removed files' private data")
- Move the definitions of debugfs_file_use_data_start() and
debugfs_file_use_data_finish() from include/linux/debugfs.h to
file.c. Export them and keep their declarations in debugfs.h,
- In order to be able to attach proper __acquires() and __releases() tags
to the decalarations of debugfs_file_use_data_*() in debugfs.h,
move the debugfs_srcu declaration from internal.h into debugfs.h.
- Since the definitions as well as the docstrings of
debugfs_file_use_data_*() have been moved into file.c,
[PATCH v3 0/7] fix debugfs file removal races
Original v2 thread is here:
http://lkml.kernel.org/g/87fux3memd@gmail.com
In the discussion of v2, it turned out that touching each and every of
the ~1000 debugfs users in order to make them save against file
removals is unfeasible.
Thus, v3 takes a different approach: every struct file_operations
handed to debugfs is wrapped by a protecting proxy in [2/7]. Only
those struct file_operations which are easy to fix directly,
i.e. those defined by debugfs itself, opt-out from this proxying in
[3-7/7].
The Coccinelle people are CC'd because of [3/7].
Many thanks to J. Lawall who helped me very much at #cocci@freenode
in getting this done!
The SRCU part really needs some fresh review: in v2 the
rcu_assign_pointer()'ed and srcu_derefence()'d ->d_fsdata has been
effectively used as an indication of whether a file is dead or not.
With the full proxy approach in v3, ->d_fsdata can't be cleared out at
file removal because open files might still hold a reference to it and
those must be released again from the proxy's ->release().
Thus, the properly memory- and compiler-barriered accesses to
->d_fsdata have now been replaced by completely unbarriered d_delete()
and d_unlinked() calls in debugfs_use_file_start() and
debugfs_remove() (all in [1/7] now).
I believe that no extra barriers are needed:
The SRCU read side critical sections around any file usage looks like this:
srcu_read_lock();
if(d_unlinked(dentry)) {
srcu_read_unlock();
return -EIO
}
cope_around_with(d_inode(dentry)->i_private);
srcu_read_unlock()
- srcu_read_lock() and srcu_read_unlock() already contain a barrier()
each. Thus, the compiler is forced to make the dentry's state being
read at least once within the read side critical section.
- I don't care for speculative reads to the file's private data
in cope_around_with().
- Writes in cope_around_with() should be properly handled by the control
dependency in that they don't occur on the bus if d_unlinked() holds.
Furthermore, any writes in cope_around_with() are emitted by the
compiler before the srcu_read_unlock().
For the file removing side of things, the SRCU usage looks like this:
d_delete(dentry);
synchronize_srcu();
free(d_inode(dentry)->i_private);
d_delete() is defined in another compilation unit. Thus, its call can't be
reorded with the one to synchronize_srcu() and the dentry state is written
(on that CPU) before synchronize_srcu() is entered.
Changes v2 -> v3:
[1/7] ("debugfs: prevent access to possibly dead file_operations at file
open")
- move the definition of the debugfs_use_file_start() and _end() from former
[2/2] to [1/7]. Also, they've been renamed from debugfs_file_use_data*().
- Make the ->open() proxy use the debugfs_use_file_*() helpers.
- In debugfs_use_file_start(), use d_unlinked() rather than
(->d_fsdata == NULL) as a flag whether the dentry is dead.
- Make the ->open() proxy include the forwarded call to the original fops'
->open
within the SRCU read side critical section.
- debugfs_proxy_file_operations has been renamed to
"debugfs_open_proxy_file_operations" to distinguish it from the full proxy
introduced in [2/7].
[2/7] ("debugfs: prevent access to removed files' private data")
- This one has changed completely: instead of providing file
removal-safe fops helpers to opt-into at the debugfs users, the
original struct file_operations get completely and
unconditionally proxied now.
[3-7/7]
New. Opt-out from the full proxying introduced in [2/7] for some
special case struct file_operations provided by debugfs itself.
Changes v1 -> v2:
[1/2] ("debugfs: prevent access to possibly dead file_operations at file
open")
- Resolve trivial diff conflict in debugfs_remove_recursive():
in the meanwhile, an unrelated 'mutex_unlock(...)' had been rewritten to
'inode_unlock(...)' which broke the diff's context.
- Introduce the fs/debugfs/internal.h header and move the declarations of
debugfs_noop_file_operations, debugfs_proxy_file_operations and
debugfs_rcu from include/linux/debugfs.h thereinto. Include this header
from file.c and inode.c.
- Add a word about the new internal header to the commit message.
- Move the inclusion of linux/srcu.h from include/linux/debugfs.h
into file.c and inode.c respectively.
[2/2] ("debugfs: prevent access to removed files' private data")
- Move the definitions of debugfs_file_use_data_start() and
debugfs_file_use_data_finish() from include/linux/debugfs.h to
file.c. Export them and keep their declarations in debugfs.h,
- In order to be able to attach proper __acquires() and __releases() tags
to the decalarations of debugfs_file_use_data_*() in debugfs.h,
move the debugfs_srcu declaration from internal.h into debugfs.h.
- Since the definitions as well as the docstrings of
debugfs_file_use_data_*() have been moved into file.c,
