SETFSGID(2)                 Linux Programmer's Manual                SETFSGID(2)

       setfsgid - set group identity used for filesystem checks

       #include <sys/fsuid.h>

       int setfsgid(gid_t fsgid);

       On Linux, a process has both a filesystem group ID and an effective group
       ID.  The (Linux-specific) filesystem group ID is used for permissions
       checking when accessing filesystem objects, while the effective group ID
       is used for some other kinds of permissions checks (see credentials(7)).

       Normally, the value of the process's filesystem group ID is the same as
       the value of its effective group ID.  This is so, because whenever a
       process's effective group ID is changed, the kernel also changes the
       filesystem group ID to be the same as the new value of the effective
       group ID.  A process can cause the value of its filesystem group ID to
       diverge from its effective group ID by using setfsgid() to change its
       filesystem group ID to the value given in fsgid.

       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.

       On both success and failure, this call returns the previous filesystem
       group ID of the caller.

       This system call is present in Linux since version 1.2.

       setfsgid() is Linux-specific and should not be used in programs intended
       to be portable.

       The filesystem group ID concept and the setfsgid() system call were
       invented for historical reasons that are no longer applicable on modern
       Linux kernels.  See setfsuid(2) for a discussion of why the use of both
       setfsuid(2) and setfsgid() 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.

   C library/kernel differences
       In glibc 2.15 and earlier, when the wrapper for this system call
       determines that the argument can't be passed to the kernel without
       integer truncation (because the kernel is old and does not support 32-bit
       group IDs), it will return -1 and set errno to EINVAL without attempting
       the system call.

       No error indications of any kind are returned to the caller, and the fact
       that both successful 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 looking 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).

       kill(2), setfsuid(2), capabilities(7), credentials(7)

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       latest version of this page, can be found at

Linux                              2021-03-22                        SETFSGID(2)