Messages in this thread | | | From | Nicolai Stange <> | Subject | [PATCH v3 0/7] fix debugfs file removal races | Date | Sun, 14 Feb 2016 02:31:08 +0100 |
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Original v2 thread is here:
http://lkml.kernel.org/g/87fux3memd.fsf@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d_fsdata have now been replaced by completely unbarriered d_delete() and d_unlinked() calls in debugfs_use_file_start() and debugfs_remove() (all in [1/7] now).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, there is no need to run DocBook on debugfs.h: do not modify Documentation/DocBook/filesystems.tmpl anymore. - In the commit message, encourage new users of debugfs to prefer DEFINE_DEBUGFS_ATTRIBUTE() and friends over DEFINE_SIMPLE_ATTRIBUTE().
Nicolai Stange (7): debugfs: prevent access to possibly dead file_operations at file open debugfs: prevent access to removed files' private data debugfs: add support for self-protecting attribute file fops debugfs: unproxify attribute files created through debugfs_create_XXX() debugfs: unproxify files created through debugfs_create_bool() debugfs: unproxify files created through debugfs_create_blob() debugfs: unproxify files created through debugfs_create_u32_array()
fs/debugfs/file.c | 437 +++++++++++++++++---- fs/debugfs/inode.c | 101 ++++- fs/debugfs/internal.h | 26 ++ include/linux/debugfs.h | 47 ++- lib/Kconfig.debug | 1 + .../api/debugfs/debugfs_simple_attr.cocci | 68 ++++ 6 files changed, 592 insertions(+), 88 deletions(-) create mode 100644 fs/debugfs/internal.h create mode 100644 scripts/coccinelle/api/debugfs/debugfs_simple_attr.cocci
-- 2.7.1
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