SETREGID(3P)

💖 SETREGID(3P) — POSIX Programmer's Manual 💖

okay bestie, so this is SETREGID(3P) from the POSIX Programmer's Manual, and we're about to get into it. 💅

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🌸 PROLOG 🌸

ngl this part is a little disclaimer-y but it matters: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so like, go consult the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may not even be implemented on Linux at all. 🙈

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✨ NAME ✨

setregid — set real and effective group IDs 🫶

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🎀 SYNOPSIS 🎀

#include <unistd.h>

int setregid(gid_t rgid, gid_t egid);

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👑 DESCRIPTION 👑

So the setregid() function shall set the real and effective group IDs of the calling process. iconic behavior, honestly.

Here's the tea on the -1 thing, because it's giving "magic value" energy: if rgid is -1, the real group ID shall not be changed; if egid is -1, the effective group ID shall not be changed.

And yes, the real and effective group IDs may be set to different values in the same call. one call, two values, no cap.

Now the privilege part — pay attention bestie:

• Only a process with appropriate privileges can set the real group ID and the effective group ID to any valid value.
• A non-privileged process can set either the real group ID to the saved set-group-ID from one of the exec family of functions, or the effective group ID to the saved set-group-ID or the real group ID.

And here's the part that lowkey runs the whole show: if the real group ID is being set (rgid is not -1), or the effective group ID is being set to a value not equal to the real group ID, then the saved set-group-ID of the current process shall be set equal to the new effective group ID. 👑

Also, any supplementary group IDs of the calling process remain unchanged. they are simply not part of this conversation. 💅

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🦋 RETURN VALUE 🦋

Upon successful completion, 0 shall be returned. slay. 🎉

Otherwise, -1 shall be returned and errno set to indicate the error, and neither of the group IDs are changed. so like, it's all-or-nothing, no half-finished chaos. 😭

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💀 ERRORS 💀

The setregid() function shall fail if:

• EINVAL — The value of the rgid or egid argument is invalid or out-of-range.
• EPERM — The process does not have appropriate privileges and a change other than changing the real group ID to the saved set-group-ID, or changing the effective group ID to the real group ID or the saved set-group-ID, was requested.

The following sections are informative.

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🌈 EXAMPLES 🌈

None.

(yeah. that's it. nothing here. we move. 🙈)

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🍓 APPLICATION USAGE 🍓

okay this one is a whole scenario, stay with me:

If a non-privileged set-group-ID process sets its effective group ID to its real group ID, it can only set its effective group ID back to the previous value if rgid was -1 in the setregid() call, since the saved-group-ID is not changed in that case.

If rgid was equal to the real group ID in the setregid() call, then the saved set-group-ID will also have been changed to the real user ID.

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🧚 RATIONALE 🧚

Earlier versions of this standard did not specify whether the saved set-group-ID was affected by setregid() calls. This version specifies common existing practice that constitutes an important security feature. 🔥

The ability to set both the effective group ID and saved set-group-ID to be the same as the real group ID means that any security weakness in code that is executed after that point cannot result in malicious code being executed with the previous effective group ID. obsessed with the safety of it all, honestly.

Privileged applications could already do this using just setgid(), but for non-privileged applications the only standard method available is to use this feature of setregid(). 💖

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⭐ FUTURE DIRECTIONS ⭐

None.

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💜 SEE ALSO 💜

exec, getegid(), geteuid(), getgid(), getuid(), setegid(), seteuid(), setgid(), setreuid(), setuid()

The Base Definitions volume of POSIX.1‐2017, <unistd.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page bestie, no notes 💅 ok bye, this was IEEE/The Open Group, 2017 — the source, the version, and the date, all wrapped up with a bow 🎀🫶