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

Yann Droneaud
2013-07-02T16:39:24Z

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(-)

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1.8.3.1