On 16.03.2015 17:07, Oleg Nesterov wrote:
On 03/16, Konstantin Khlebnikov wrote:
+/**
+ * set_mm_exe_file - change a reference to the mm's executable file
+ *
+ * This changes mm's executale file (shown as symlink /proc/[pid]/exe).
+ *
+ * Main users are mmput(), sys_execve() and sys_prctl(PR_SET_MM_MAP/EXE_FILE).
+ * Callers prevent concurrent invocations: in mmput() nobody alive left,
+ * in execve task is single-threaded, prctl holds mmap_sem exclusively.
+ */
void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
{
+ struct file *old_exe_file = rcu_dereference_protected(mm->exe_file,
+ !atomic_read(&mm->mm_users) || current->in_execve ||
+ lock_is_held(&mm->mmap_sem));
+
if (new_exe_file)
get_file(new_exe_file);
- if (mm->exe_file)
- fput(mm->exe_file);
- mm->exe_file = new_exe_file;
+ rcu_assign_pointer(mm->exe_file, new_exe_file);
+ if (old_exe_file)
+ fput(old_exe_file);
}
Yes, I think this is correct, __fput() does call_rcu(file_free_rcu). And
much better than the new lock ;)
Acked-by: Oleg Nesterov <o...@redhat.com>
So I think the patch is fine, but personally I dislike the "prctl holds
mmap_sem exclusively" and rcu_dereference_protected().
I mean, I think we can do another cleanup on top of this change.
1. set_mm_exe_file() should be called by exit/exec only, so
it should use
rcu_dereference_protected(mm->exe_file,
atomic_read(&mm->mm_users) <= 1);
2. prctl() should not use it, it can do
get_file(new_exe);
old_exe = xchg(&mm->exe_file);
if (old_exe)
fput(old_exe);
I think smp_mb() is required before xchg() or
probably this stuff should be hidden inside yet another magic RCU macro
( with two screens of comments =)
3. and we can remove down_write(mmap_sem) from prctl paths.
Actually we can do this even without xchg() above, but we might
want to kill MMF_EXE_FILE_CHANGED and test_and_set_bit() check.
What do you think?
Oleg.
--
Konstantin
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