linux-kernel - [PATCH] posix-cpu-timers: Wider access to the thread CPU clocks

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Message-Id: <1278591752-9782-1-git-send-email-tomasz.buchert@inria.fr>
Date:	Thu,  8 Jul 2010 14:22:32 +0200
From:	Tomasz Buchert <tomasz.buchert@...ia.fr>
To:	linux-kernel@...r.kernel.org
Cc:	Tomasz Buchert <tomasz.buchert@...ia.fr>,
	Lucas Nussbaum <lucas.nussbaum@...ia.fr>
Subject: [PATCH] posix-cpu-timers: Wider access to the thread CPU clocks

Per-thread CPU clocks are so far accessible only within
the same thread group. This patch makes it possible to get
this information for any thread of the same user and for
a user with CAP_SYS_ADMIN capability.

same_thread_group is not used anymore because if the thread is in
the same thread group as 'current' then they will have the same
owner anyway.

Signed-off-by: Tomasz Buchert <tomasz.buchert@...ia.fr>
Signed-off-by: Lucas Nussbaum <lucas.nussbaum@...ia.fr>
---
 kernel/posix-cpu-timers.c |   33 ++++++++++++++++++++++++++-------
 1 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9829646..db7fce5 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -25,6 +25,24 @@ void update_rlimit_cpu(unsigned long rlim_new)
 	spin_unlock_irq(&current->sighand->siglock);
 }
 
+/*
+ * Check whether the CPU clock for task p is available to current.
+ * At least the RCU read lock must be held.
+ */
+static int clock_access_allowed(struct task_struct *p)
+{
+	const struct cred *cred, *pcred;
+
+	if (unlikely(capable(CAP_SYS_ADMIN)))
+		return 1;
+
+	cred = current_cred();
+	pcred = __task_cred(p);
+
+	return (cred->euid == pcred->euid ||
+		cred->euid == pcred->uid);
+}
+
 static int check_clock(const clockid_t which_clock)
 {
 	int error = 0;
@@ -40,7 +58,7 @@ static int check_clock(const clockid_t which_clock)
 	read_lock(&tasklist_lock);
 	p = find_task_by_vpid(pid);
 	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
-		   same_thread_group(p, current) : thread_group_leader(p))) {
+		   clock_access_allowed(p) : thread_group_leader(p))) {
 		error = -EINVAL;
 	}
 	read_unlock(&tasklist_lock);
@@ -357,16 +375,17 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
 		p = find_task_by_vpid(pid);
 		if (p) {
 			if (CPUCLOCK_PERTHREAD(which_clock)) {
-				if (same_thread_group(p, current)) {
-					error = cpu_clock_sample(which_clock,
-								 p, &rtn);
+				if (clock_access_allowed(p)) {
+					error =
+						cpu_clock_sample(
+							which_clock, p, &rtn);
 				}
 			} else {
 				read_lock(&tasklist_lock);
 				if (thread_group_leader(p) && p->sighand) {
 					error =
-					    cpu_clock_sample_group(which_clock,
-							           p, &rtn);
+						cpu_clock_sample_group(
+							which_clock, p, &rtn);
 				}
 				read_unlock(&tasklist_lock);
 			}
@@ -403,7 +422,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
 			p = current;
 		} else {
 			p = find_task_by_vpid(pid);
-			if (p && !same_thread_group(p, current))
+			if (p && !clock_access_allowed(p))
 				p = NULL;
 		}
 	} else {
-- 
1.6.3.3

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