UCRED(9)

💖 UCRED(9) — user credential functions, but make it make sense 💖

okay so this is the Kernel Developer's Manual page for UCRED(9), and it's all about user credentials in the kernel. Yes bestie, we're getting into struct ucred lore. Buckle up 🎀

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🌸 NAME 🌸

ucred, crget, crhold, crfree, crcopy, crdup, cru2x — functions related to user credentials

✨ SYNOPSIS ✨

#include <sys/param.h>
#include <sys/ucred.h>

struct ucred *
crget(void);

struct ucred *
crhold(struct ucred *cr);

void
crfree(struct ucred *cr);

void
crcopy(struct ucred *dest, struct ucred *src);

struct ucred *
crcopysafe(struct proc *p, struct ucred *cr);

struct ucred *
crdup(struct ucred *cr);

void
crsetgroups(struct ucred *cr, int ngrp, gid_t *groups);

void
crsetgroups_and_egid(struct ucred *cr, int ngrp, gid_t *groups,
    gid_t default_egid);

void
cru2x(struct ucred *cr, struct xucred *xcr);

💅 DESCRIPTION 💅

okay so the whole ucred family is here to manage user credential structures (struct ucred) inside the kernel. It's giving gatekeeping but for permissions, and honestly iconic 👑

• crget() — allocates memory for a new structure, sets its reference count to 1, and initializes its lock. Fresh, new, untouched vibes ✨
• crhold() — increases the reference count on the credential. Like adding another bestie to the group chat 🫶
• crfree() — decreases the reference count on the credential. If the count drops to 0, the storage for the structure is freed. Periodt 💀
• crcopy() — copies the contents of the source (template) credential into the destination template. The uidinfo structure within the destination is referenced by calling uihold(9).
• crcopysafe() — copies the current credential associated with the process p into the newly allocated credential cr. The process lock on p must be held and will be dropped and reacquired as needed to allocate group storage space in cr. Lock drama, but make it safe 💅
• crdup() — allocates memory for a new structure and copies the contents of cr into it. The actual copying is performed by crcopy().
• crsetgroups() — sets the cr_groups and cr_ngroups variables representing the supplementary groups, allocating space as needed. It also truncates the group list to the current maximum number of groups.
• crsetgroups_and_egid() — similar, but interprets separately the first group of groups as the effective GID to set, only setting the subsequent groups as supplementary ones. It will use default_egid as the new effective GID if groups is empty. No other mechanism should be used to modify the cr_groups array. That's the rule, bestie — no side-door edits 🚪
• cru2x() — converts a ucred structure to an xucred structure. That is, it copies data from cr to xcr; it ignores fields in the former that are not present in the latter (e.g., cr_uidinfo), and appropriately sets fields in the latter that are not present in the former (e.g., cr_version). Glow-up conversion energy 🌟

🫶 RETURN VALUES 🫶

crget(), crhold(), crdup(), and crcopysafe() all return a pointer to a ucred structure.

🧚 USAGE NOTES 🧚

okay this is the part where the kernel-dev lore gets real, so listen up bestie 🧚

As of FreeBSD 5.0, the ucred structure contains extensible fields. This means that the correct protocol must always be followed to create a fresh and writable credential structure: new credentials must always be derived from existing credentials using crget(), crcopy(), and crcopysafe().

In the common case, credentials required for access control decisions are used in a read-only manner. In these circumstances, the thread credential td_ucred should be used, as it requires no locking to access safely, and remains stable for the duration of the call even in the face of a multi-threaded application changing the process credentials from another thread.

During a process credential update, the process lock must be held across check and update, to prevent race conditions. The process credential, td->td_proc->p_ucred, must be used both for check and update. If a process credential is updated during a system call and checks against the thread credential are to be made later during the same system call, the thread credential must also be refreshed from the process credential so as to prevent use of a stale value. To avoid this scenario, it is recommended that system calls updating the process credential be designed to avoid other authorization functions.

If temporarily elevated privileges are required for a thread, the thread credential can be replaced for the duration of an activity, or for the remainder of the system call. However, as a thread credential is often shared, appropriate care should be taken to make sure modifications are made to a writable credential through the use of crget() and crcopy().

Caution should be exercised when checking authorization for a thread or process to perform an operation on another thread or process. As a result of temporary elevation, the target thread credential should never be used as the target credential in an access control decision: the process credential associated with the thread, td->td_proc->p_ucred, should be used instead. For example, p_candebug(9) accepts a target process, not a target thread, for access control purposes.

bestie this is the "do not use the wrong credential" warning. it is not a drill 😭

🔥 SEE ALSO 🔥

• uihold(9)

👑 AUTHORS 👑

This manual page was written by Chad David <davidc@acns.ab.ca>.

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okay bye bestie 🫶 — this page is from the Debian version, dated August 29, 2025. UCRED(9) out. 💖