SETREGID(3POSIX)

💖 SETREGID — the POSIX Programmer's Manual tea 💖

okay bestie, welcome to SETREGID(3POSIX), straight out of the POSIX Programmer's Manual. before we dive in, a lil heads up from the PROLOG:

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

so like, if ur on Linux and something's giving different… check the Linux page, k? periodt.

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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 setregid() shall set the real and effective group IDs of the calling process. like, both of them. the whole group-ID situation. iconic.

if rgid is -1, the real group ID shall not be changed. and if egid is -1, the effective group ID shall not be changed. sneaky little sentinels, we love them.

the real and effective group IDs may be set to different values in the same call. yes, one call, two vibes.

only a process with appropriate privileges can set the real group ID and the effective group ID to any valid value. main character privileges only 🔥

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. that's the whole menu, no substitutions.

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. the way that one change ripples… 😭

any supplementary group IDs of the calling process remain unchanged. they're just there, chillin, untouched. ✨

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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, all-or-nothing, no half-finished fits. 💀

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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. bestie that number is not it. 🙈
• 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. permission? denied. 💅

the following sections are informative.

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🎀 EXAMPLES 🎀

None. literally nothing. moving on. 💖

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🦋 APPLICATION USAGE 🦋

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.

so yeah, -1 vs actual-value is a whole different storyline. iykyk. 🧚

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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. that's a glow-up for security, fr fr.

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(). no cap.

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🌸 FUTURE DIRECTIONS 🌸

None. the future is… nothing here. cute. 💜

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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>

go read those too bestie, they're the friend group. 🫶

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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. (aka the original is the main character, not this page.) 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!! brought to u by IEEE/The Open Group, 2017 — yes, the same SETREGID(3POSIX) u came in with. tysm for reading, ur doing amazing sweetie 💖✨🎀🦋