SETFSUID(2) Linux Programmer's Manual SETFSUID(2)
setfsuid - set user identity used for filesystem checks
int setfsuid(uid_t fsuid);
The system call setfsuid() changes the value of the caller's filesystem
user ID--the user ID that the Linux kernel uses to check for all
accesses to the filesystem. Normally, the value of the filesystem user
ID will shadow the value of the effective user ID. In fact, whenever
the effective user ID is changed, the filesystem user ID will also be
changed to the new value of the effective user ID.
Explicit calls to setfsuid() and setfsgid(2) 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 without 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.)
setfsuid() will succeed only if the caller is the superuser or if fsuid
matches either the caller's real user ID, effective user ID, saved set-
user-ID, or current filesystem user ID.
On both success and failure, this call returns the previous filesystem
user ID of the caller.
This system call is present in Linux since version 1.2.
setfsuid() is Linux-specific and should not be used in programs
intended to be portable.
At the time when this system call was introduced, one process could
send a signal to another process with the same effective user ID. This
meant that if a privileged process changed its effective user ID for
the purpose of file permission checking, then it could become vulnera-
ble to receiving signals sent by another (unprivileged) process with
the same user ID. The filesystem user ID attribute was thus added to
allow a process to change its user ID for the purposes of file permis-
sion checking without at the same time becoming vulnerable to receiving
unwanted signals. Since Linux 2.0, signal permission handling is dif-
ferent (see kill(2)), with the result that a process change can change
its effective user ID without being vulnerable to receiving signals
from unwanted processes. Thus, setfsuid() is nowadays unneeded and
should be avoided in new applications (likewise for setfsgid(2)).
The original Linux setfsuid() system call supported only 16-bit user
IDs. Subsequently, Linux 2.4 added setfsuid32() supporting 32-bit IDs.
The glibc setfsuid() 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 deter-
mines that the argument can't be passed to the kernel without integer
truncation (because the kernel is old and does not support 32-bit user
IDs), they 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 setfsuid(-1) (which will always fail), in
order to determine if a preceding call to setfsuid() changed the
filesystem user ID. At the very least, EPERM should be returned when
the call fails (because the caller lacks the CAP_SETUID capability).
kill(2), setfsgid(2), capabilities(7), credentials(7)
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