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Date: Mon, 26 Apr 2010 11:58:49 +0100 From: David Howells <dhowells@...hat.com> To: linux-security-module@...r.kernel.org Cc: dhowells@...hat.com, serue@...ibm.com, paulmck@...ibm.com, linux-kernel@...r.kernel.org Subject: [PATCH] CRED: Holding a spinlock does not imply the holding of RCU read lock From: Serge E. Hallyn <serue@...ibm.com> Change the credentials documentation to make it clear that the RCU read lock must be explicitly held when accessing credentials pointers in some other task than current. Holding a spinlock does not implicitly hold the RCU read lock. Signed-off-by: Serge Hallyn <serue@...ibm.com> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> Signed-off-by: David Howells <dhowells@...hat.com> --- Documentation/credentials.txt | 14 +++++--------- 1 files changed, 5 insertions(+), 9 deletions(-) diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt index df03169..a2db352 100644 --- a/Documentation/credentials.txt +++ b/Documentation/credentials.txt @@ -408,9 +408,6 @@ This should be used inside the RCU read lock, as in the following example: ... } -A function need not get RCU read lock to use __task_cred() if it is holding a -spinlock at the time as this implicitly holds the RCU read lock. - Should it be necessary to hold another task's credentials for a long period of time, and possibly to sleep whilst doing so, then the caller should get a reference on them using: @@ -426,17 +423,16 @@ credentials, hiding the RCU magic from the caller: uid_t task_uid(task) Task's real UID uid_t task_euid(task) Task's effective UID -If the caller is holding a spinlock or the RCU read lock at the time anyway, -then: +If the caller is holding the RCU read lock at the time anyway, then: __task_cred(task)->uid __task_cred(task)->euid should be used instead. Similarly, if multiple aspects of a task's credentials -need to be accessed, RCU read lock or a spinlock should be used, __task_cred() -called, the result stored in a temporary pointer and then the credential -aspects called from that before dropping the lock. This prevents the -potentially expensive RCU magic from being invoked multiple times. +need to be accessed, RCU read lock should be used, __task_cred() called, the +result stored in a temporary pointer and then the credential aspects called +from that before dropping the lock. This prevents the potentially expensive +RCU magic from being invoked multiple times. Should some other single aspect of another task's credentials need to be accessed, then this can be used: -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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