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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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