1. 07 Nov, 2017 5 commits
    • Greg Kroah-Hartman's avatar
      debugfs: Remove redundant license text · 2b2d8788
      Greg Kroah-Hartman authored
      Now that the SPDX tag is in all debugfs files, that identifies the
      license in a specific and legally-defined manner.  So the extra GPL text
      wording can be removed as it is no longer needed at all.
      This is done on a quest to remove the 700+ different ways that files in
      the kernel describe the GPL license text.  And there's unneeded stuff
      like the address (sometimes incorrect) for the FSF which is never
      No copyright headers or other non-license-description text was removed.
      Cc: Nicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    • Greg Kroah-Hartman's avatar
      debugfs: add SPDX identifiers to all debugfs files · 3bce94fd
      Greg Kroah-Hartman authored
      It's good to have SPDX identifiers in all files to make it easier to
      audit the kernel tree for correct licenses.
      Update the debugfs files files with the correct SPDX license identifier
      based on the license text in the file itself.  The SPDX identifier is a
      legally binding shorthand, which can be used instead of the full boiler
      plate text.
      This work is based on a script and data from Thomas Gleixner, Philippe
      Ombredanne, and Kate Stewart.
      Cc: Thomas Gleixner <tglx@linutronix.de>
      Cc: Kate Stewart <kstewart@linuxfoundation.org>
      Cc: Philippe Ombredanne <pombredanne@nexb.com>
      Cc: Nicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    • Nicolai Stange's avatar
      debugfs: defer debugfs_fsdata allocation to first usage · 7d39bc50
      Nicolai Stange authored
      Currently, __debugfs_create_file allocates one struct debugfs_fsdata
      instance for every file created. However, there are potentially many
      debugfs file around, most of which are never touched by userspace.
      Thus, defer the allocations to the first usage, i.e. to the first
      A dentry's ->d_fsdata starts out to point to the "real", user provided
      fops. After a debugfs_fsdata instance has been allocated (and the real
      fops pointer has been moved over into its ->real_fops member),
      ->d_fsdata is changed to point to it from then on. The two cases are
      distinguished by setting BIT(0) for the real fops case.
      struct debugfs_fsdata's foremost purpose is to track active users and to
      make debugfs_remove() block until they are done. Since no debugfs_fsdata
      instance means no active users, make debugfs_remove() return immediately
      in this case.
      Take care of possible races between debugfs_file_get() and
      debugfs_remove(): either debugfs_remove() must see a debugfs_fsdata
      instance and thus wait for possible active users or debugfs_file_get() must
      see a dead dentry and return immediately.
      Make a dentry's ->d_release(), i.e. debugfs_release_dentry(), check whether
      ->d_fsdata is actually a debugfs_fsdata instance before kfree()ing it.
      Similarly, make debugfs_real_fops() check whether ->d_fsdata is actually
      a debugfs_fsdata instance before returning it, otherwise emit a warning.
      The set of possible error codes returned from debugfs_file_get() has grown
      from -EIO to -EIO and -ENOMEM. Make open_proxy_open() and full_proxy_open()
      pass the -ENOMEM onwards to their callers.
      Signed-off-by: default avatarNicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    • Nicolai Stange's avatar
      debugfs: implement per-file removal protection · e9117a5a
      Nicolai Stange authored
      Since commit 49d200de ("debugfs: prevent access to removed files'
      private data"), accesses to a file's private data are protected from
      concurrent removal by covering all file_operations with a SRCU read section
      and sychronizing with those before returning from debugfs_remove() by means
      of synchronize_srcu().
      As pointed out by Johannes Berg, there are debugfs files with forever
      blocking file_operations. Their corresponding SRCU read side sections would
      block any debugfs_remove() forever as well, even unrelated ones. This
      results in a livelock. Because a remover can't cancel any indefinite
      blocking within foreign files, this is a problem.
      Resolve this by introducing support for more granular protection on a
      per-file basis.
      This is implemented by introducing an  'active_users' refcount_t to the
      per-file struct debugfs_fsdata state. At file creation time, it is set to
      one and a debugfs_remove() will drop that initial reference. The new
      debugfs_file_get() and debugfs_file_put(), intended to be used in place of
      former debugfs_use_file_start() and debugfs_use_file_finish(), increment
      and decrement it respectively. Once the count drops to zero,
      debugfs_file_put() will signal a completion which is possibly being waited
      for from debugfs_remove().
      Thus, as long as there is a debugfs_file_get() not yet matched by a
      corresponding debugfs_file_put() around, debugfs_remove() will block.
      Actual users of debugfs_use_file_start() and -finish() will get converted
      to the new debugfs_file_get() and debugfs_file_put() by followup patches.
      Fixes: 49d200de ("debugfs: prevent access to removed files' private data")
      Reported-by: default avatarJohannes Berg <johannes@sipsolutions.net>
      Signed-off-by: default avatarNicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    • Nicolai Stange's avatar
      debugfs: add support for more elaborate ->d_fsdata · 7c8d4698
      Nicolai Stange authored
      Currently, the user provided fops, "real_fops", are stored directly into
      In order to be able to store more per-file state and thus prepare for more
      granular file removal protection, wrap the real_fops into a dynamically
      allocated container struct, debugfs_fsdata.
      A struct debugfs_fsdata gets allocated at file creation and freed from the
      newly intoduced ->d_release().
      Finally, move the implementation of debugfs_real_fops() out of the public
      debugfs header such that struct debugfs_fsdata's declaration can be kept
      Signed-off-by: default avatarNicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
  2. 31 Aug, 2016 1 commit
  3. 12 Apr, 2016 2 commits
    • Nicolai Stange's avatar
      debugfs: prevent access to removed files' private data · 49d200de
      Nicolai Stange authored
      Upon return of debugfs_remove()/debugfs_remove_recursive(), it might
      still be attempted to access associated private file data through
      previously opened struct file objects. If that data has been freed by
      the caller of debugfs_remove*() in the meanwhile, the reading/writing
      process would either encounter a fault or, if the memory address in
      question has been reassigned again, unrelated data structures could get
      However, since debugfs files are seldomly removed, usually from module
      exit handlers only, the impact is very low.
      Currently, there are ~1000 call sites of debugfs_create_file() spread
      throughout the whole tree and touching all of those struct file_operations
      in order to make them file removal aware by means of checking the result of
      debugfs_use_file_start() from within their methods is unfeasible.
      Instead, wrap the struct file_operations by a lifetime managing proxy at
      file open:
      - In debugfs_create_file(), the original fops handed in has got stashed
        away in ->d_fsdata already.
      - In debugfs_create_file(), install a proxy file_operations factory,
        debugfs_full_proxy_file_operations, at ->i_fop.
      This proxy factory has got an ->open() method only. It carries out some
      lifetime checks and if successful, dynamically allocates and sets up a new
      struct file_operations proxy at ->f_op. Afterwards, it forwards to the
      ->open() of the original struct file_operations in ->d_fsdata, if any.
      The dynamically set up proxy at ->f_op has got a lifetime managing wrapper
      set for each of the methods defined in the original struct file_operations
      in ->d_fsdata.
      Its ->release()er frees the proxy again and forwards to the original
      ->release(), if any.
      In order not to mislead the VFS layer, it is strictly necessary to leave
      those fields blank in the proxy that have been NULL in the original
      struct file_operations also, i.e. aren't supported. This is why there is a
      need for dynamically allocated proxies. The choice made not to allocate a
      proxy instance for every dentry at file creation, but for every
      struct file object instantiated thereof is justified by the expected usage
      pattern of debugfs, namely that in general very few files get opened more
      than once at a time.
      The wrapper methods set in the struct file_operations implement lifetime
      managing by means of the SRCU protection facilities already in place for
      They set up a SRCU read side critical section and check whether the dentry
      is still alive by means of debugfs_use_file_start(). If so, they forward
      the call to the original struct file_operation stored in ->d_fsdata, still
      under the protection of the SRCU read side critical section.
      This SRCU read side critical section prevents any pending debugfs_remove()
      and friends to return to their callers. Since a file's private data must
      only be freed after the return of debugfs_remove(), the ongoing proxied
      call is guarded against any file removal race.
      If, on the other hand, the initial call to debugfs_use_file_start() detects
      that the dentry is dead, the wrapper simply returns -EIO and does not
      forward the call. Note that the ->poll() wrapper is special in that its
      signature does not allow for the return of arbitrary -EXXX values and thus,
      POLLHUP is returned here.
      In order not to pollute debugfs with wrapper definitions that aren't ever
      needed, I chose not to define a wrapper for every struct file_operations
      method possible. Instead, a wrapper is defined only for the subset of
      methods which are actually set by any debugfs users.
      Currently, these are:
      The ->release() wrapper is special in that it does not protect the original
      ->release() in any way from dead files in order not to leak resources.
      Thus, any ->release() handed to debugfs must implement file lifetime
      management manually, if needed.
      For only 33 out of a total of 434 releasers handed in to debugfs, it could
      not be verified immediately whether they access data structures that might
      have been freed upon a debugfs_remove() return in the meanwhile.
      Export debugfs_use_file_start() and debugfs_use_file_finish() in order to
      allow any ->release() to manually implement file lifetime management.
      For a set of common cases of struct file_operations implemented by the
      debugfs_core itself, future patches will incorporate file lifetime
      management directly within those in order to allow for their unproxied
      operation. Rename the original, non-proxying "debugfs_create_file()" to
      "debugfs_create_file_unsafe()" and keep it for future internal use by
      debugfs itself. Factor out code common to both into the new
      Signed-off-by: default avatarNicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    • Nicolai Stange's avatar
      debugfs: prevent access to possibly dead file_operations at file open · 9fd4dcec
      Nicolai Stange authored
      Nothing prevents a dentry found by path lookup before a return of
      __debugfs_remove() to actually get opened after that return. Now, after
      the return of __debugfs_remove(), there are no guarantees whatsoever
      regarding the memory the corresponding inode's file_operations object
      had been kept in.
      Since __debugfs_remove() is seldomly invoked, usually from module exit
      handlers only, the race is hard to trigger and the impact is very low.
      A discussion of the problem outlined above as well as a suggested
      solution can be found in the (sub-)thread rooted at
        ("Yet another pipe related oops.")
      Basically, Greg KH suggests to introduce an intermediate fops and
      Al Viro points out that a pointer to the original ones may be stored in
      Follow this line of reasoning:
      - Add SRCU as a reverse dependency of DEBUG_FS.
      - Introduce a srcu_struct object for the debugfs subsystem.
      - In debugfs_create_file(), store a pointer to the original
        file_operations object in ->d_fsdata.
      - Make debugfs_remove() and debugfs_remove_recursive() wait for a
        SRCU grace period after the dentry has been delete()'d and before they
        return to their callers.
      - Introduce an intermediate file_operations object named
        "debugfs_open_proxy_file_operations". It's ->open() functions checks,
        under the protection of a SRCU read lock, whether the dentry is still
        alive, i.e. has not been d_delete()'d and if so, tries to acquire a
        reference on the owning module.
        On success, it sets the file object's ->f_op to the original
        file_operations and forwards the ongoing open() call to the original
      - For clarity, rename the former debugfs_file_operations to
        debugfs_noop_file_operations -- they are in no way canonical.
      The choice of SRCU over "normal" RCU is justified by the fact, that the
      former may also be used to protect ->i_private data from going away
      during the execution of a file's readers and writers which may (and do)
      Finally, introduce the fs/debugfs/internal.h header containing some
      declarations internal to the debugfs implementation.
      Signed-off-by: default avatarNicolai Stange <nicstange@gmail.com>
      Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>