[PATCH 00/13] Getting rid of get_unused_fd()

[PATCH 00/13] Getting rid of get_unused_fd()

[Yann Droneaud]

Hi,

Macro get_unused_fd() is a shortcut to call function get_unused_fd_flags(),
to allocate a file descriptor.

The macro use 0 as flags, so the file descriptor is created
without O_CLOEXEC flag.

This can be seen as an unsafe default eg. in most case O_CLOEXEC
must be used to not leak file descriptor across exec().

Newer kernel code should use anon_inode_getfd() or get_unused_fd_flags()
with flags provided by userspace. If flags cannot be given by userspace,
O_CLOEXEC must be the default flag.

Using O_CLOEXEC by default allows userspace to choose, without race,
if the file descriptor is going to be inherited across exec().

They are two ways to achieve this:

- makes get_unused_fd() use O_CLOEXEC by default

  It's difficult to get it right: every code using of get_unused_fd()
  must take this change into account and be fixed as soon as
  macro get_unused_fd() do the switch. Non updated code will have
  unexpected behavor and it's likely going to break API contract.

- remove get_unused_fd()

  It's going to break some out of tree, not yet upstream kernel code,
  but it's easy to notice and fix. Anyway, newer code should use
  anon_inode_getfd() or get_unused_fd_flags().

The latter option was choosen to ensure no unexpected behavor
for out of tree, not yet upstream code. Removing the macro is the safest
choice: it's better to break build than trying to make get_unused_fd()
use O_CLOEXEC by default and get all user of get_unused_fd() update.

Additionnaly, removing the macro is not going to break modules ABI.

In linux-next tag 20130702, they're currently:

- 15 calls to get_unused_fd_flags()
     not counting get_unused_fd() and anon_inode_getfd()
- 14 calls to get_unused_fd()
- 11 calls to anon_inode_getfd()

The following patchset try to convert all calls to get_unused_fd()
to get_unused_fd_flags(0) before removing get_unused_fd() macro.

Without get_unused_fd() macro, more subsystems are likely to use
anon_inode_getfd() and be teached to provide an API that let userspace
choose the opening flags of the file descriptor.

Yann Droneaud (13):
  ia64: use get_unused_fd_flags(0) instead of get_unused_fd()
  ppc/cell: use get_unused_fd_flags(0) instead of get_unused_fd()
  infiniband: use get_unused_fd_flags(0) instead of get_unused_fd()
  android/sw_sync: use get_unused_fd_flags(0) instead of get_unused_fd()
  android/sync: use get_unused_fd_flags(0) instead of get_unused_fd()
  vfio: use get_unused_fd_flags(0) instead of get_unused_fd()
  binfmt_misc: use get_unused_fd_flags(0) instead of get_unused_fd()
  file: use get_unused_fd_flags(0) instead of get_unused_fd()
  fanotify: use get_unused_fd_flags(0) instead of get_unused_fd()
  xfs: use get_unused_fd_flags(0) instead of get_unused_fd()
  events: use get_unused_fd_flags(0) instead of get_unused_fd()
  sctp: use get_unused_fd_flags(0) instead of get_unused_fd()
  file: remove get_unused_fd()

 arch/ia64/kernel/perfmon.c                | 2 +-
 arch/powerpc/platforms/cell/spufs/inode.c | 4 ++--
 drivers/infiniband/core/uverbs_cmd.c      | 4 ++--
 drivers/staging/android/sw_sync.c         | 2 +-
 drivers/staging/android/sync.c            | 2 +-
 drivers/vfio/vfio.c                       | 2 +-
 fs/binfmt_misc.c                          | 2 +-
 fs/file.c                                 | 2 +-
 fs/notify/fanotify/fanotify_user.c        | 2 +-
 fs/xfs/xfs_ioctl.c                        | 2 +-
 include/linux/file.h                      | 1 -
 kernel/events/core.c                      | 2 +-
 net/sctp/socket.c                         | 2 +-
 13 files changed, 14 insertions(+), 15 deletions(-)

-- 
1.8.3.1

[PATCH 01/13] ia64: use get_unused_fd_flags(0) instead of get_unused_fd()

[Yann Droneaud]

Macro get_unused_fd() is used to allocate a file descriptor with
default flags. Those default flags (0) can be "unsafe":
O_CLOEXEC must be used by default to not leak file descriptor
across exec().

Instead of macro get_unused_fd(), functions anon_inode_getfd()
or get_unused_fd_flags() should be used with flags given by userspace.
If not possible, flags should be set to O_CLOEXEC to provide userspace
with a default safe behavor.

In a further patch, get_unused_fd() will be removed so that
new code start using anon_inode_getfd() or get_unused_fd_flags()
with correct flags.

This patch replaces calls to get_unused_fd() with equivalent call to
get_unused_fd_flags(0) to preserve current behavor for existing code.

The hard coded flag value (0) should be reviewed on a per-subsystem basis,
and, if possible, set to O_CLOEXEC.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
---
 arch/ia64/kernel/perfmon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/ia64/kernel/perfmon.c b/arch/ia64/kernel/perfmon.c
index 5a9ff1c..64757c1 100644
--- a/arch/ia64/kernel/perfmon.c
+++ b/arch/ia64/kernel/perfmon.c
@@ -2668,7 +2668,7 @@ pfm_context_create(pfm_context_t *ctx, void *arg, int count, struct pt_regs *reg
 
 	ret = -ENOMEM;
 
-	fd = get_unused_fd();
+	fd = get_unused_fd_flags(0);
 	if (fd < 0)
 		return fd;
 
-- 
1.8.3.1