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SETFSGID - reference manual online

Set group identity used for filesystem checks.

Chapter
2013-08-08
SETFSGID(2)                         Linux Programmer's Manual                         SETFSGID(2)

NAME setfsgid - set group identity used for filesystem checks
SYNOPSIS #include <sys/fsuid.h> int setfsgid(uid_t fsgid);
DESCRIPTION The system call setfsgid() changes the value of the caller's filesystem group ID—the group ID that the Linux kernel uses to check for all accesses to the filesystem. Normally, the value of the filesystem group ID will shadow the value of the effective group ID. In fact, whenever the effective group ID is changed, the filesystem group ID will also be changed to the new value of the effective group ID. Explicit calls to setfsuid(2) and setfsgid() are usually used only by programs such as the Linux NFS server that need to change what user and group ID is used for file access with‐ out a corresponding change in the real and effective user and group IDs. A change in the normal user IDs for a program such as the NFS server is a security hole that can expose it to unwanted signals. (But see below.) setfsgid() will succeed only if the caller is the superuser or if fsgid matches either the caller's real group ID, effective group ID, saved set-group-ID, or current the filesystem user ID.
RETURN VALUE On both success and failure, this call returns the previous filesystem group ID of the caller.
VERSIONS This system call is present in Linux since version 1.2.
CONFORMING TO setfsgid() is Linux-specific and should not be used in programs intended to be portable.
NOTES When glibc determines that the argument is not a valid group ID, it will return -1 and set errno to EINVAL without attempting the system call. Note that at the time this system call was introduced, a process could send a signal to a process with the same effective user ID. Today signal permission handling is slightly different. See setfsuid(2) for a discussion of why the use of both setfsuid(2) and setfs‐ gid() is nowadays unneeded. The original Linux setfsgid() system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added setfsgid32() supporting 32-bit IDs. The glibc setfsgid() wrapper function transparently deals with the variation across kernel versions.
BUGS No error indications of any kind are returned to the caller, and the fact that both suc‐ cessful and unsuccessful calls return the same value makes it impossible to directly determine whether the call succeeded or failed. Instead, the caller must resort to look‐ ing at the return value from a further call such as setfsgid(-1) (which will always fail), in order to determine if a preceding call to setfsgid() changed the filesystem group ID. At the very least, EPERM should be returned when the call fails (because the caller lacks the CAP_SETGID capability).
SEE ALSO kill(2), setfsuid(2), capabilities(7), credentials(7)
COLOPHON This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-08-08 SETFSGID(2)
This manual Reference Other manuals
setfsgid(2) referred by credentials(7) | path_resolution(7) | setfsuid(2) | setresuid(2) | syscalls(2) | user_namespaces(7)
refer to capabilities(7) | credentials(7) | kill(2) | setfsuid(2)