linux-kernel - [PATCH] Documentation: security/credentials.rst: explain need to sort group

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Message-ID: <87wp10dlgk.fsf@notabene.neil.brown.name>
Date:   Wed, 03 Jan 2018 08:01:15 +1100
From:   NeilBrown <neilb@...e.com>
To:     Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Cc:     dhowells@...hat.com,
        Thiago Rafael Becker <thiago.becker@...il.com>,
        viro@...iv.linux.org.uk, schwidefsky@...ibm.com,
        willy@...radead.org, bfields@...ldses.org,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] Documentation: security/credentials.rst: explain need to sort
 group_list


This patch updates the documentation with the observations that led
to commit bdcf0a423ea1 ("kernel: make groups_sort calling a
responsibility group_info allocators") and the new behaviour required.
Specifically that groups_sort() should be called on a new group_list
before set_groups() or set_current_groups() is called.

Signed-off-by: NeilBrown <neilb@...e.com>
---
 Documentation/security/credentials.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/security/credentials.rst b/Documentation/security/credentials.rst
index 66a2e24939d8..5d659e3e52ad 100644
--- a/Documentation/security/credentials.rst
+++ b/Documentation/security/credentials.rst
@@ -451,6 +451,13 @@ checks and hooks done.  Both the current and the proposed sets of credentials
 are available for this purpose as current_cred() will return the current set
 still at this point.
 
+When replacing the group list, the new list must be sorted before it
+is added to the credential, as a binary search is used to test for
+membership.  In practice, this means ``groups_sort()`` should be
+called before ``set_groups()`` or ``set_current_groups()``.
+``groups_sort()`` must not be called on a ``struct group_list`` which
+is shared as it may permute elements as part of the sorting process
+even if the array is already sorted.
 
 When the credential set is ready, it should be committed to the current process
 by calling::
-- 
2.14.0.rc0.dirty


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