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Date: Thu, 25 May 2017 12:43:54 -0700
From: Kees Cook <keescook@...omium.org>
To: Manfred Spraul <manfred@...orfullife.com>
Cc: Michael Kerrisk <mtk.manpages@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
Davidlohr Bueso <dave@...olabs.net>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Fabian Frederick <fabf@...net.be>
Subject: Re: [PATCH 01/20] ipc/sem.c: remove sem_base, embed struct sem
On Thu, May 25, 2017 at 11:50 AM, Manfred Spraul
<manfred@...orfullife.com> wrote:
> sma->sem_base is initialized with
> sma->sem_base = (struct sem *) &sma[1];
>
> The current code has four problems:
> - There is an unnecessary pointer dereference - sem_base is not needed.
> - Alignment for struct sem only works by chance.
> - The current code causes false positive for static code analysis.
> - This is a cast between different non-void types, which the future
> randstruct GCC plugin warns on.
>
> And, as bonus, the code size gets smaller:
>
> Before:
> 0 .text 00003770
> After:
> 0 .text 0000374e
>
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Acked-by: Kees Cook <keescook@...omium.org>
-Kees
> ---
> include/linux/sem.h | 22 +++++++++++++-
> ipc/sem.c | 88 +++++++++++++++++++++--------------------------------
> 2 files changed, 55 insertions(+), 55 deletions(-)
>
> diff --git a/include/linux/sem.h b/include/linux/sem.h
> index 9edec92..9db1409 100644
> --- a/include/linux/sem.h
> +++ b/include/linux/sem.h
> @@ -8,11 +8,29 @@
>
> struct task_struct;
>
> +/* One semaphore structure for each semaphore in the system. */
> +struct sem {
> + int semval; /* current value */
> + /*
> + * PID of the process that last modified the semaphore. For
> + * Linux, specifically these are:
> + * - semop
> + * - semctl, via SETVAL and SETALL.
> + * - at task exit when performing undo adjustments (see exit_sem).
> + */
> + int sempid;
> + spinlock_t lock; /* spinlock for fine-grained semtimedop */
> + struct list_head pending_alter; /* pending single-sop operations */
> + /* that alter the semaphore */
> + struct list_head pending_const; /* pending single-sop operations */
> + /* that do not alter the semaphore*/
> + time_t sem_otime; /* candidate for sem_otime */
> +} ____cacheline_aligned_in_smp;
> +
> /* One sem_array data structure for each set of semaphores in the system. */
> struct sem_array {
> struct kern_ipc_perm sem_perm; /* permissions .. see ipc.h */
> time_t sem_ctime; /* last change time */
> - struct sem *sem_base; /* ptr to first semaphore in array */
> struct list_head pending_alter; /* pending operations */
> /* that alter the array */
> struct list_head pending_const; /* pending complex operations */
> @@ -21,6 +39,8 @@ struct sem_array {
> int sem_nsems; /* no. of semaphores in array */
> int complex_count; /* pending complex operations */
> unsigned int use_global_lock;/* >0: global lock required */
> +
> + struct sem sems[];
> };
>
> #ifdef CONFIG_SYSVIPC
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 947dc23..fff8337 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -87,24 +87,6 @@
> #include <linux/uaccess.h>
> #include "util.h"
>
> -/* One semaphore structure for each semaphore in the system. */
> -struct sem {
> - int semval; /* current value */
> - /*
> - * PID of the process that last modified the semaphore. For
> - * Linux, specifically these are:
> - * - semop
> - * - semctl, via SETVAL and SETALL.
> - * - at task exit when performing undo adjustments (see exit_sem).
> - */
> - int sempid;
> - spinlock_t lock; /* spinlock for fine-grained semtimedop */
> - struct list_head pending_alter; /* pending single-sop operations */
> - /* that alter the semaphore */
> - struct list_head pending_const; /* pending single-sop operations */
> - /* that do not alter the semaphore*/
> - time_t sem_otime; /* candidate for sem_otime */
> -} ____cacheline_aligned_in_smp;
>
> /* One queue for each sleeping process in the system. */
> struct sem_queue {
> @@ -175,7 +157,7 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
> * sem_array.sem_undo
> *
> * b) global or semaphore sem_lock() for read/write:
> - * sem_array.sem_base[i].pending_{const,alter}:
> + * sem_array.sems[i].pending_{const,alter}:
> *
> * c) special:
> * sem_undo_list.list_proc:
> @@ -250,7 +232,7 @@ static void unmerge_queues(struct sem_array *sma)
> */
> list_for_each_entry_safe(q, tq, &sma->pending_alter, list) {
> struct sem *curr;
> - curr = &sma->sem_base[q->sops[0].sem_num];
> + curr = &sma->sems[q->sops[0].sem_num];
>
> list_add_tail(&q->list, &curr->pending_alter);
> }
> @@ -270,7 +252,7 @@ static void merge_queues(struct sem_array *sma)
> {
> int i;
> for (i = 0; i < sma->sem_nsems; i++) {
> - struct sem *sem = sma->sem_base + i;
> + struct sem *sem = &sma->sems[i];
>
> list_splice_init(&sem->pending_alter, &sma->pending_alter);
> }
> @@ -306,7 +288,7 @@ static void complexmode_enter(struct sem_array *sma)
> sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
>
> for (i = 0; i < sma->sem_nsems; i++) {
> - sem = sma->sem_base + i;
> + sem = &sma->sems[i];
> spin_lock(&sem->lock);
> spin_unlock(&sem->lock);
> }
> @@ -366,7 +348,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
> *
> * Both facts are tracked by use_global_mode.
> */
> - sem = sma->sem_base + sops->sem_num;
> + sem = &sma->sems[sops->sem_num];
>
> /*
> * Initial check for use_global_lock. Just an optimization,
> @@ -421,7 +403,7 @@ static inline void sem_unlock(struct sem_array *sma, int locknum)
> complexmode_tryleave(sma);
> ipc_unlock_object(&sma->sem_perm);
> } else {
> - struct sem *sem = sma->sem_base + locknum;
> + struct sem *sem = &sma->sems[locknum];
> spin_unlock(&sem->lock);
> }
> }
> @@ -487,7 +469,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
> if (ns->used_sems + nsems > ns->sc_semmns)
> return -ENOSPC;
>
> - size = sizeof(*sma) + nsems * sizeof(struct sem);
> + size = sizeof(*sma) + nsems * sizeof(sma->sems[0]);
> sma = ipc_rcu_alloc(size);
> if (!sma)
> return -ENOMEM;
> @@ -504,12 +486,10 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
> return retval;
> }
>
> - sma->sem_base = (struct sem *) &sma[1];
> -
> for (i = 0; i < nsems; i++) {
> - INIT_LIST_HEAD(&sma->sem_base[i].pending_alter);
> - INIT_LIST_HEAD(&sma->sem_base[i].pending_const);
> - spin_lock_init(&sma->sem_base[i].lock);
> + INIT_LIST_HEAD(&sma->sems[i].pending_alter);
> + INIT_LIST_HEAD(&sma->sems[i].pending_const);
> + spin_lock_init(&sma->sems[i].lock);
> }
>
> sma->complex_count = 0;
> @@ -612,7 +592,7 @@ static int perform_atomic_semop_slow(struct sem_array *sma, struct sem_queue *q)
> un = q->undo;
>
> for (sop = sops; sop < sops + nsops; sop++) {
> - curr = sma->sem_base + sop->sem_num;
> + curr = &sma->sems[sop->sem_num];
> sem_op = sop->sem_op;
> result = curr->semval;
>
> @@ -639,7 +619,7 @@ static int perform_atomic_semop_slow(struct sem_array *sma, struct sem_queue *q)
> sop--;
> pid = q->pid;
> while (sop >= sops) {
> - sma->sem_base[sop->sem_num].sempid = pid;
> + sma->sems[sop->sem_num].sempid = pid;
> sop--;
> }
>
> @@ -661,7 +641,7 @@ static int perform_atomic_semop_slow(struct sem_array *sma, struct sem_queue *q)
> sop--;
> while (sop >= sops) {
> sem_op = sop->sem_op;
> - sma->sem_base[sop->sem_num].semval -= sem_op;
> + sma->sems[sop->sem_num].semval -= sem_op;
> if (sop->sem_flg & SEM_UNDO)
> un->semadj[sop->sem_num] += sem_op;
> sop--;
> @@ -692,7 +672,7 @@ static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
> * until the operations can go through.
> */
> for (sop = sops; sop < sops + nsops; sop++) {
> - curr = sma->sem_base + sop->sem_num;
> + curr = &sma->sems[sop->sem_num];
> sem_op = sop->sem_op;
> result = curr->semval;
>
> @@ -716,7 +696,7 @@ static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
> }
>
> for (sop = sops; sop < sops + nsops; sop++) {
> - curr = sma->sem_base + sop->sem_num;
> + curr = &sma->sems[sop->sem_num];
> sem_op = sop->sem_op;
> result = curr->semval;
>
> @@ -815,7 +795,7 @@ static int wake_const_ops(struct sem_array *sma, int semnum,
> if (semnum == -1)
> pending_list = &sma->pending_const;
> else
> - pending_list = &sma->sem_base[semnum].pending_const;
> + pending_list = &sma->sems[semnum].pending_const;
>
> list_for_each_entry_safe(q, tmp, pending_list, list) {
> int error = perform_atomic_semop(sma, q);
> @@ -856,7 +836,7 @@ static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
> for (i = 0; i < nsops; i++) {
> int num = sops[i].sem_num;
>
> - if (sma->sem_base[num].semval == 0) {
> + if (sma->sems[num].semval == 0) {
> got_zero = 1;
> semop_completed |= wake_const_ops(sma, num, wake_q);
> }
> @@ -867,7 +847,7 @@ static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
> * Assume all were changed.
> */
> for (i = 0; i < sma->sem_nsems; i++) {
> - if (sma->sem_base[i].semval == 0) {
> + if (sma->sems[i].semval == 0) {
> got_zero = 1;
> semop_completed |= wake_const_ops(sma, i, wake_q);
> }
> @@ -909,7 +889,7 @@ static int update_queue(struct sem_array *sma, int semnum, struct wake_q_head *w
> if (semnum == -1)
> pending_list = &sma->pending_alter;
> else
> - pending_list = &sma->sem_base[semnum].pending_alter;
> + pending_list = &sma->sems[semnum].pending_alter;
>
> again:
> list_for_each_entry_safe(q, tmp, pending_list, list) {
> @@ -922,7 +902,7 @@ static int update_queue(struct sem_array *sma, int semnum, struct wake_q_head *w
> * be in the per semaphore pending queue, and decrements
> * cannot be successful if the value is already 0.
> */
> - if (semnum != -1 && sma->sem_base[semnum].semval == 0)
> + if (semnum != -1 && sma->sems[semnum].semval == 0)
> break;
>
> error = perform_atomic_semop(sma, q);
> @@ -959,9 +939,9 @@ static int update_queue(struct sem_array *sma, int semnum, struct wake_q_head *w
> static void set_semotime(struct sem_array *sma, struct sembuf *sops)
> {
> if (sops == NULL) {
> - sma->sem_base[0].sem_otime = get_seconds();
> + sma->sems[0].sem_otime = get_seconds();
> } else {
> - sma->sem_base[sops[0].sem_num].sem_otime =
> + sma->sems[sops[0].sem_num].sem_otime =
> get_seconds();
> }
> }
> @@ -1067,9 +1047,9 @@ static int count_semcnt(struct sem_array *sma, ushort semnum,
> semcnt = 0;
> /* First: check the simple operations. They are easy to evaluate */
> if (count_zero)
> - l = &sma->sem_base[semnum].pending_const;
> + l = &sma->sems[semnum].pending_const;
> else
> - l = &sma->sem_base[semnum].pending_alter;
> + l = &sma->sems[semnum].pending_alter;
>
> list_for_each_entry(q, l, list) {
> /* all task on a per-semaphore list sleep on exactly
> @@ -1124,7 +1104,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
> wake_up_sem_queue_prepare(q, -EIDRM, &wake_q);
> }
> for (i = 0; i < sma->sem_nsems; i++) {
> - struct sem *sem = sma->sem_base + i;
> + struct sem *sem = &sma->sems[i];
> list_for_each_entry_safe(q, tq, &sem->pending_const, list) {
> unlink_queue(sma, q);
> wake_up_sem_queue_prepare(q, -EIDRM, &wake_q);
> @@ -1174,9 +1154,9 @@ static time_t get_semotime(struct sem_array *sma)
> int i;
> time_t res;
>
> - res = sma->sem_base[0].sem_otime;
> + res = sma->sems[0].sem_otime;
> for (i = 1; i < sma->sem_nsems; i++) {
> - time_t to = sma->sem_base[i].sem_otime;
> + time_t to = sma->sems[i].sem_otime;
>
> if (to > res)
> res = to;
> @@ -1325,7 +1305,7 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
> return -EIDRM;
> }
>
> - curr = &sma->sem_base[semnum];
> + curr = &sma->sems[semnum];
>
> ipc_assert_locked_object(&sma->sem_perm);
> list_for_each_entry(un, &sma->list_id, list_id)
> @@ -1402,7 +1382,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
> }
> }
> for (i = 0; i < sma->sem_nsems; i++)
> - sem_io[i] = sma->sem_base[i].semval;
> + sem_io[i] = sma->sems[i].semval;
> sem_unlock(sma, -1);
> rcu_read_unlock();
> err = 0;
> @@ -1450,8 +1430,8 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
> }
>
> for (i = 0; i < nsems; i++) {
> - sma->sem_base[i].semval = sem_io[i];
> - sma->sem_base[i].sempid = task_tgid_vnr(current);
> + sma->sems[i].semval = sem_io[i];
> + sma->sems[i].sempid = task_tgid_vnr(current);
> }
>
> ipc_assert_locked_object(&sma->sem_perm);
> @@ -1476,7 +1456,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
> err = -EIDRM;
> goto out_unlock;
> }
> - curr = &sma->sem_base[semnum];
> + curr = &sma->sems[semnum];
>
> switch (cmd) {
> case GETVAL:
> @@ -1932,7 +1912,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
> */
> if (nsops == 1) {
> struct sem *curr;
> - curr = &sma->sem_base[sops->sem_num];
> + curr = &sma->sems[sops->sem_num];
>
> if (alter) {
> if (sma->complex_count) {
> @@ -2146,7 +2126,7 @@ void exit_sem(struct task_struct *tsk)
>
> /* perform adjustments registered in un */
> for (i = 0; i < sma->sem_nsems; i++) {
> - struct sem *semaphore = &sma->sem_base[i];
> + struct sem *semaphore = &sma->sems[i];
> if (un->semadj[i]) {
> semaphore->semval += un->semadj[i];
> /*
> --
> 2.9.3
>
--
Kees Cook
Pixel Security
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