1. 根目录:
2. include
3. linux
4. highuid.h

#ifndef _LINUX_HIGHUID_H
#define _LINUX_HIGHUID_H

#include <linux/types.h>

/*
 * general notes:
 *
 * CONFIG_UID16 is defined if the given architecture needs to
 * support backwards compatibility for old system calls.
 *
 * kernel code should use uid_t and gid_t at all times when dealing with
 * kernel-private data.
 *
 * old_uid_t and old_gid_t should only be different if CONFIG_UID16 is
 * defined, else the platform should provide dummy typedefs for them
 * such that they are equivalent to __kernel_{u,g}id_t.
 *
 * uid16_t and gid16_t are used on all architectures. (when dealing
 * with structures hard coded to 16 bits, such as in filesystems)
 */


/*
 * This is the "overflow" UID and GID. They are used to signify uid/gid
 * overflow to old programs when they request uid/gid information but are
 * using the old 16 bit interfaces.
 * When you run a libc5 program, it will think that all highuid files or
 * processes are owned by this uid/gid.
 * The idea is that it's better to do so than possibly return 0 in lieu of
 * 65536, etc.
 */

extern int overflowuid;
extern int overflowgid;

extern void __bad_uid(void);
extern void __bad_gid(void);

#define DEFAULT_OVERFLOWUID	65534
#define DEFAULT_OVERFLOWGID	65534

#ifdef CONFIG_UID16

/* prevent uid mod 65536 effect by returning a default value for high UIDs */
#define high2lowuid(uid) ((uid) & ~0xFFFF ? (old_uid_t)overflowuid : (old_uid_t)(uid))
#define high2lowgid(gid) ((gid) & ~0xFFFF ? (old_gid_t)overflowgid : (old_gid_t)(gid))
/*
 * -1 is different in 16 bits than it is in 32 bits
 * these macros are used by chown(), setreuid(), ...,
 */
#define low2highuid(uid) ((uid) == (old_uid_t)-1 ? (uid_t)-1 : (uid_t)(uid))
#define low2highgid(gid) ((gid) == (old_gid_t)-1 ? (gid_t)-1 : (gid_t)(gid))

#define __convert_uid(size, uid) \
	(size >= sizeof(uid) ? (uid) : high2lowuid(uid))
#define __convert_gid(size, gid) \
	(size >= sizeof(gid) ? (gid) : high2lowgid(gid))
	

#else

#define __convert_uid(size, uid) (uid)
#define __convert_gid(size, gid) (gid)

#endif /* !CONFIG_UID16 */

/* uid/gid input should be always 32bit uid_t */
#define SET_UID(var, uid) do { (var) = __convert_uid(sizeof(var), (uid)); } while (0)
#define SET_GID(var, gid) do { (var) = __convert_gid(sizeof(var), (gid)); } while (0)

/*
 * Everything below this line is needed on all architectures, to deal with
 * filesystems that only store 16 bits of the UID/GID, etc.
 */

/*
 * This is the UID and GID that will get written to disk if a filesystem
 * only supports 16-bit UIDs and the kernel has a high UID/GID to write
 */
extern int fs_overflowuid;
extern int fs_overflowgid;

#define DEFAULT_FS_OVERFLOWUID	65534
#define DEFAULT_FS_OVERFLOWGID	65534

/*
 * Since these macros are used in architectures that only need limited
 * 16-bit UID back compatibility, we won't use old_uid_t and old_gid_t
 */
#define fs_high2lowuid(uid) ((uid) & ~0xFFFF ? (uid16_t)fs_overflowuid : (uid16_t)(uid))
#define fs_high2lowgid(gid) ((gid) & ~0xFFFF ? (gid16_t)fs_overflowgid : (gid16_t)(gid))

#define low_16_bits(x)	((x) & 0xFFFF)
#define high_16_bits(x)	(((x) & 0xFFFF0000) >> 16)

#endif /* _LINUX_HIGHUID_H */