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    Subject[patch 25/28] mm_for_maps: shift down_read(mmap_sem) to the caller
    2.6.27-stable review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Oleg Nesterov <oleg@redhat.com>

    commit 00f89d218523b9bf6b522349c039d5ac80aa536d upstream.

    mm_for_maps() takes ->mmap_sem after security checks, this looks
    strange and obfuscates the locking rules. Move this lock to its
    single caller, m_start().

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Acked-by: Serge Hallyn <serue@us.ibm.com>
    Signed-off-by: James Morris <jmorris@namei.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    fs/proc/base.c | 8 +++-----
    fs/proc/task_mmu.c | 1 +
    fs/proc/task_nommu.c | 1 +
    3 files changed, 5 insertions(+), 5 deletions(-)

    --- a/fs/proc/base.c
    +++ b/fs/proc/base.c
    @@ -240,9 +240,8 @@ static int check_mem_permission(struct t
    struct mm_struct *mm_for_maps(struct task_struct *task)
    {
    struct mm_struct *mm = get_task_mm(task);
    - if (!mm)
    - return NULL;
    - if (mm != current->mm) {
    +
    + if (mm && mm != current->mm) {
    /*
    * task->mm can be changed before security check,
    * in that case we must notice the change after.
    @@ -250,10 +249,9 @@ struct mm_struct *mm_for_maps(struct tas
    if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
    mm != task->mm) {
    mmput(mm);
    - return NULL;
    + mm = NULL;
    }
    }
    - down_read(&mm->mmap_sem);
    return mm;
    }

    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m,
    mm = mm_for_maps(priv->task);
    if (!mm)
    return NULL;
    + down_read(&mm->mmap_sem);

    tail_vma = get_gate_vma(priv->task);
    priv->tail_vma = tail_vma;
    --- a/fs/proc/task_nommu.c
    +++ b/fs/proc/task_nommu.c
    @@ -137,6 +137,7 @@ static void *m_start(struct seq_file *m,
    priv->task = NULL;
    return NULL;
    }
    + down_read(&mm->mmap_sem);

    /* start from the Nth VMA */
    for (vml = mm->context.vmlist; vml; vml = vml->next)



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