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Documentation: describe credential APIs and pseudo-fs ownership

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Document setreuid/setregid, getresuid/getresgid, and how inode_reserve
assigns owner and group for pseudo-filesystem objects.

Signed-off-by: Abhishek Mishra <mishra.abhishek2808@gmail.com>
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Abhishek Mishra 2026-07-16 18:55:41 +00:00 • committed by Xiang Xiao
parent 282d4cb0e9
commit ddf38abe45
1 changed files with 29 additions and 0 deletions
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Documentation/implementation/user_identity.rst
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@ -59,6 +59,24 @@ the saved ID. Otherwise the function returns ``-1`` with ``errno`` set to
This implements the standard POSIX pattern of temporarily dropping privileges
with ``seteuid()`` or ``setegid()`` and later restoring them to the saved value.
``setreuid()`` and ``setregid()``
---------------------------------
These functions set the real and/or effective IDs in a single call. When the
effective ID is zero, any requested real and effective values may be assigned
and the saved set-ID is updated accordingly. When the effective ID is
non-zero, each requested value must equal the current effective ID, saved
set-ID, or (for the effective argument only) the real ID; otherwise the call
returns ``-1`` with ``errno`` set to ``EPERM``. When the real ID is changed,
or the effective ID is changed to a value not equal to the real ID, the saved
set-ID is set to the new effective ID.
``getresuid()`` and ``getresgid()``
-----------------------------------
These functions return the real, effective, and saved set-IDs for the calling
task group. Any output pointer may be ``NULL`` if that ID is not needed.
Configuration
=============
@ -69,3 +87,14 @@ Configuration
``CONFIG_FS_PERMISSION``
Enables filesystem ownership and permission enforcement. Requires
``CONFIG_SCHED_USER_IDENTITY``.
Pseudo-Filesystem Ownership
===========================
When ``CONFIG_PSEUDOFS_ATTRIBUTES`` and ``CONFIG_SCHED_USER_IDENTITY`` are both
enabled, ``inode_alloc()`` assigns ``i_owner`` and ``i_group`` from the
caller's effective credentials. This covers
message queues (``mq_open()``), named semaphores (``sem_open()``), shared
memory objects (``shm_open()``), FIFOs (``mkfifo()``), and pseudo-files
created through the same inode reservation path. Open-time permission checks
use ``inode_checkopenperm()`` (or ``inode_checkperm()`` for message queues).