lkml.org 
[lkml]   [2019]   [Aug]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH v6 1/2] fork: extend clone3() to support setting a PID
    Date
    The main motivation to add set_tid to clone3() is CRIU.

    To restore a process with the same PID/TID CRIU currently uses
    /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
    ns_last_pid and then (quickly) does a clone(). This works most of the
    time, but it is racy. It is also slow as it requires multiple syscalls.

    Extending clone3() to support set_tid makes it possible restore a
    process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
    race free (as long as the desired PID/TID is available).

    This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
    on clone3() with set_tid as they are currently in place for ns_last_pid.

    Signed-off-by: Adrian Reber <areber@redhat.com>
    ---
    v2:
    - Removed (size < sizeof(struct clone_args)) as discussed with
    Christian and Dmitry
    - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
    - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)

    v3:
    - Return EEXIST if PID is already in use (Christian)
    - Drop CLONE_SET_TID (Christian and Oleg)
    - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
    - Handle different `struct clone_args` sizes (Dmitry)

    v4:
    - Rework struct size check with defines (Christian)
    - Reduce number of set_tid checks (Oleg)
    - Less parentheses and more robust code (Oleg)
    - Do ns_capable() on correct user_ns (Oleg, Christian)

    v5:
    - make set_tid checks earlier in alloc_pid() (Christian)
    - remove unnecessary comment and struct size check (Christian)

    v6:
    - remove CLONE_SET_TID from description (Christian)
    - add clone3() tests for different clone_args sizes (Christian)
    - move more set_tid checks to alloc_pid() (Oleg)
    - make all set_tid checks lockless (Oleg)
    ---
    include/linux/pid.h | 2 +-
    include/linux/sched/task.h | 1 +
    include/uapi/linux/sched.h | 1 +
    kernel/fork.c | 14 ++++++++++++--
    kernel/pid.c | 37 ++++++++++++++++++++++++++++++-------
    5 files changed, 45 insertions(+), 10 deletions(-)

    diff --git a/include/linux/pid.h b/include/linux/pid.h
    index 2a83e434db9d..052000db0ced 100644
    --- a/include/linux/pid.h
    +++ b/include/linux/pid.h
    @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
    extern struct pid *find_get_pid(int nr);
    extern struct pid *find_ge_pid(int nr, struct pid_namespace *);

    -extern struct pid *alloc_pid(struct pid_namespace *ns);
    +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
    extern void free_pid(struct pid *pid);
    extern void disable_pid_allocation(struct pid_namespace *ns);

    diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
    index 0497091e40c1..4f2a80564332 100644
    --- a/include/linux/sched/task.h
    +++ b/include/linux/sched/task.h
    @@ -26,6 +26,7 @@ struct kernel_clone_args {
    unsigned long stack;
    unsigned long stack_size;
    unsigned long tls;
    + pid_t set_tid;
    };

    /*
    diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
    index b3105ac1381a..e1ce103a2c47 100644
    --- a/include/uapi/linux/sched.h
    +++ b/include/uapi/linux/sched.h
    @@ -45,6 +45,7 @@ struct clone_args {
    __aligned_u64 stack;
    __aligned_u64 stack_size;
    __aligned_u64 tls;
    + __aligned_u64 set_tid;
    };

    /*
    diff --git a/kernel/fork.c b/kernel/fork.c
    index 2852d0e76ea3..8317d408a8d6 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -117,6 +117,11 @@
    */
    #define MAX_THREADS FUTEX_TID_MASK

    +/*
    + * For different sizes of struct clone_args
    + */
    +#define CLONE3_ARGS_SIZE_V0 64
    +
    /*
    * Protected counters by write_lock_irq(&tasklist_lock)
    */
    @@ -2031,7 +2036,7 @@ static __latent_entropy struct task_struct *copy_process(
    stackleak_task_init(p);

    if (pid != &init_struct_pid) {
    - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
    + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
    if (IS_ERR(pid)) {
    retval = PTR_ERR(pid);
    goto bad_fork_cleanup_thread;
    @@ -2535,9 +2540,13 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
    if (unlikely(size > PAGE_SIZE))
    return -E2BIG;

    - if (unlikely(size < sizeof(struct clone_args)))
    + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
    return -EINVAL;

    + if (size < sizeof(struct clone_args))
    + memset((void *)&args + size, 0,
    + sizeof(struct clone_args) - size);
    +
    if (unlikely(!access_ok(uargs, size)))
    return -EFAULT;

    @@ -2571,6 +2580,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
    .stack = args.stack,
    .stack_size = args.stack_size,
    .tls = args.tls,
    + .set_tid = args.set_tid,
    };

    return 0;
    diff --git a/kernel/pid.c b/kernel/pid.c
    index 0a9f2e437217..5cdab73b9094 100644
    --- a/kernel/pid.c
    +++ b/kernel/pid.c
    @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
    call_rcu(&pid->rcu, delayed_put_pid);
    }

    -struct pid *alloc_pid(struct pid_namespace *ns)
    +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
    {
    struct pid *pid;
    enum pid_type type;
    @@ -166,6 +166,16 @@ struct pid *alloc_pid(struct pid_namespace *ns)
    struct upid *upid;
    int retval = -ENOMEM;

    + if (set_tid) {
    + if (set_tid < 0 || set_tid >= pid_max)
    + return ERR_PTR(-EINVAL);
    + /* Also fail if a PID != 1 is requested and no PID 1 exists */
    + if (set_tid != 1 && !ns->child_reaper)
    + return ERR_PTR(-EINVAL);
    + if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
    + return ERR_PTR(-EPERM);
    + }
    +
    pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
    if (!pid)
    return ERR_PTR(retval);
    @@ -186,12 +196,25 @@ struct pid *alloc_pid(struct pid_namespace *ns)
    if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
    pid_min = RESERVED_PIDS;

    - /*
    - * Store a null pointer so find_pid_ns does not find
    - * a partially initialized PID (see below).
    - */
    - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
    - pid_max, GFP_ATOMIC);
    + if (set_tid) {
    + nr = idr_alloc(&tmp->idr, NULL, set_tid,
    + set_tid + 1, GFP_ATOMIC);
    + /*
    + * If ENOSPC is returned it means that the PID is
    + * alreay in use. Return EEXIST in that case.
    + */
    + if (nr == -ENOSPC)
    + nr = -EEXIST;
    + /* Only use set_tid for one PID namespace. */
    + set_tid = 0;
    + } else {
    + /*
    + * Store a null pointer so find_pid_ns does not find
    + * a partially initialized PID (see below).
    + */
    + nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
    + pid_max, GFP_ATOMIC);
    + }
    spin_unlock_irq(&pidmap_lock);
    idr_preload_end();

    --
    2.21.0
    \
     
     \ /
      Last update: 2019-08-12 22:10    [W:2.839 / U:0.124 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site