From 964987738b3fe557cb1ee37acb4e7f85e29b7cea Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Mon, 27 Apr 2020 07:55:07 -0500 Subject: [PATCH] posix-cpu-timers: Replace __get_task_for_clock with pid_for_clock Now that the codes store references to pids instead of referendes to tasks. Looking up a task for a clock instead of looking up a struct pid makes the code more difficult to verify it is correct than necessary. In posix_cpu_timers_create get_task_pid can race with release_task for threads and return a NULL pid. As put_pid and cpu_timer_task_rcu handle NULL pids just fine the code works without problems but it is an extra case to consider and keep in mind while verifying and modifying the code. There are races with de_thread to consider that only don't apply because thread clocks are only allowed for threads in the same thread_group. So instead of leaving a burden for people making modification to the code in the future return a rcu protected struct pid for the clock instead. The logic for __get_task_for_pid and lookup_task has been folded into the new function pid_for_clock with the only change being the logic has been modified from working on a task to working on a pid that will be returned. In posix_cpu_clock_get instead of calling pid_for_clock checking the result and then calling pid_task to get the task. The result of pid_for_clock is fed directly into pid_task. This is safe because pid_task handles NULL pids. As such an extra error check was unnecessary. Instead of hiding the flag that enables the special clock_gettime handling, I have made the 3 callers just pass the flag in themselves. That is less code and seems just as simple to work with as the wrapper functions. Historically the clock_gettime special case of allowing a process clock to be found by the thread id did not even exist [33ab0fec3352] but Thomas Gleixner reports that he has found code that uses that functionality [55e8c8eb2c7b]. Link: https://lkml.kernel.org/r/87zhaxqkwa.fsf@nanos.tec.linutronix.de/ Ref: 33ab0fec3352 ("posix-timers: Consolidate posix_cpu_clock_get()") Ref: 55e8c8eb2c7b ("posix-cpu-timers: Store a reference to a pid not a task") Signed-off-by: "Eric W. Biederman" --- kernel/time/posix-cpu-timers.c | 81 ++++++++++++++-------------------- 1 file changed, 33 insertions(+), 48 deletions(-) diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c index 42f673974d71..165117996ea0 100644 --- a/kernel/time/posix-cpu-timers.c +++ b/kernel/time/posix-cpu-timers.c @@ -47,59 +47,44 @@ void update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new) /* * Functions for validating access to tasks. */ -static struct task_struct *lookup_task(const pid_t pid, bool thread, - bool gettime) +static struct pid *pid_for_clock(const clockid_t clock, bool gettime) { - struct task_struct *p; + const bool thread = !!CPUCLOCK_PERTHREAD(clock); + const pid_t upid = CPUCLOCK_PID(clock); + struct pid *pid; + + if (CPUCLOCK_WHICH(clock) >= CPUCLOCK_MAX) + return NULL; /* * If the encoded PID is 0, then the timer is targeted at current * or the process to which current belongs. */ + if (upid == 0) + return thread ? task_pid(current) : task_tgid(current); + + pid = find_vpid(upid); if (!pid) - return thread ? current : current->group_leader; - - p = find_task_by_vpid(pid); - if (!p) - return p; - - if (thread) - return same_thread_group(p, current) ? p : NULL; - - /* - * For clock_gettime(PROCESS) the task does not need to be - * the actual group leader. task->signal gives - * access to the group's clock. - */ - if (gettime && (p == current)) - return p; - - /* - * For processes require that p is group leader. - */ - return thread_group_leader(p) ? p : NULL; -} - -static struct task_struct *__get_task_for_clock(const clockid_t clock, - bool gettime) -{ - const bool thread = !!CPUCLOCK_PERTHREAD(clock); - const pid_t pid = CPUCLOCK_PID(clock); - - if (CPUCLOCK_WHICH(clock) >= CPUCLOCK_MAX) return NULL; - return lookup_task(pid, thread, gettime); -} + if (thread) { + struct task_struct *tsk = pid_task(pid, PIDTYPE_PID); + return (tsk && same_thread_group(tsk, current)) ? pid : NULL; + } -static inline struct task_struct *get_task_for_clock(const clockid_t clock) -{ - return __get_task_for_clock(clock, false); -} + /* + * For clock_gettime(PROCESS) allow finding the process by + * with the pid of the current task. The code needs the tgid + * of the process so that pid_task(pid, PIDTYPE_TGID) can be + * used to find the process. + */ + if (gettime && (pid == task_pid(current))) + return task_tgid(current); -static inline struct task_struct *get_task_for_clock_get(const clockid_t clock) -{ - return __get_task_for_clock(clock, true); + /* + * For processes require that pid identifies a process. + */ + return pid_has_task(pid, PIDTYPE_TGID) ? pid : NULL; } static inline int validate_clock_permissions(const clockid_t clock) @@ -107,7 +92,7 @@ static inline int validate_clock_permissions(const clockid_t clock) int ret; rcu_read_lock(); - ret = __get_task_for_clock(clock, false) ? 0 : -EINVAL; + ret = pid_for_clock(clock, false) ? 0 : -EINVAL; rcu_read_unlock(); return ret; @@ -369,7 +354,7 @@ static int posix_cpu_clock_get(const clockid_t clock, struct timespec64 *tp) u64 t; rcu_read_lock(); - tsk = get_task_for_clock_get(clock); + tsk = pid_task(pid_for_clock(clock, true), clock_pid_type(clock)); if (!tsk) { rcu_read_unlock(); return -EINVAL; @@ -392,18 +377,18 @@ static int posix_cpu_clock_get(const clockid_t clock, struct timespec64 *tp) */ static int posix_cpu_timer_create(struct k_itimer *new_timer) { - struct task_struct *p; + struct pid *pid; rcu_read_lock(); - p = get_task_for_clock(new_timer->it_clock); - if (!p) { + pid = pid_for_clock(new_timer->it_clock, false); + if (!pid) { rcu_read_unlock(); return -EINVAL; } new_timer->kclock = &clock_posix_cpu; timerqueue_init(&new_timer->it.cpu.node); - new_timer->it.cpu.pid = get_task_pid(p, clock_pid_type(new_timer->it_clock)); + new_timer->it.cpu.pid = get_pid(pid); rcu_read_unlock(); return 0; }