include/linux/file.h: remove get_unused_fd() macro

include/linux/file.h: remove get_unused_fd() macro

Macro get_unused_fd() is used to allocate a file descriptor with default
flags.  Those default flags (0) don't enable close-on-exec.

This can be seen as an unsafe default: in most case close-on-exec should
be enabled to not leak file descriptor across exec().

It would be better to have a "safer" default set of flags, eg.  O_CLOEXEC
must be used to enable close-on-exec.

Instead this patch removes get_unused_fd() so that out of tree modules
won't be affect by a runtime behavor change which might introduce other
kind of bugs: it's better to catch the change at build time, making it
easier to fix.

Removing the macro will also promote use of get_unused_fd_flags() (or
anon_inode_getfd()) with flags provided by userspace.  Or, if flags cannot
be given by userspace, with flags set to O_CLOEXEC by default.

Signed-off-by: Yann Droneaud <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>