uid_wrapper (1) - Linux Manuals

uid_wrapper: A wrapper to fake privilege separation


uid_wrapper - A wrapper to fake privilege separation


LD_PRELOAD=libuid_wrapper.so UID_WRAPPER=1 UID_WRAPPER_ROOT=1 ./myapplication


• Allows uid switching as a normal user.

• Start any application making it believe it is running as root.

• Support for user/group changing in the local thread using the syscalls (like glibc).

• More precisely this library intercepts seteuid and related calls, and simulates them in a manner similar to the nss_wrapper and socket_wrapper libraries.

Some projects like a file server need privilege separation to be able to switch to the connection user and do file operations. uid_wrapper convincingly lies to the application letting it believe it is operating as root and even switching between UIDs and GIDs as needed.



If you load the uid_wrapper and enable it with setting UID_WRAPPER=1 all setuid and setgid will work, even as a normal user.


It is possible to start your application as fake root with setting UID_WRAPPER_ROOT=1.


If you need to see what is going on in uid_wrapper itself or try to find a bug, you can enable logging support in uid_wrapper if you built it with debug symbols.

• 0 = ERROR


• 2 = DEBUG

• 3 = TRACE


This environment variable can be used to tell uid_wrapper to let geteuid() return the real (instead of the faked) UID of the user who started the process with uid_wrapper.

uid_t uid;

setenv("UID_WRAPPER_MYUID", "1", 1);
uid = geteuid();


$ LD_PRELOAD=libuid_wrapper.so UID_WRAPPER=1 UID_WRAPPER_ROOT=1 id
uid=0(root) gid=0(root) 0(root)


If you need to write code that behaves differently depending on whether uid_wrapper is enabled or not, for example in cases where you have to file permissions, you can predefine the uid_wrapper_enabled() function in your project as follows:

bool uid_wrapper_enabled(void)
    return false;

Since uid_wrapper overloads this function if enabled, you can use it in your code to detect uid_wrapper.