Re: [PATCH 1/2] task_struct cleanup: move binfmt field to mm_struct

Oleg Nesterov wrote:
(add Rusty)

On 07/27, Hiroshi Shimamoto wrote:
void set_binfmt(struct linux_binfmt *new)
{
- if (current->binfmt)
- module_put(current->binfmt->module);
+ struct mm_struct *mm = current->mm;
+
+ BUG_ON(!mm);
+ if (mm->binfmt)

I am not sure we need this BUG_ON() above. If mm == NULL we will
have the same debug info after the crash.

Ah, right. It will crash at accessing mm->binfmt and we'll get the
same information. I didn't think seriously.
BTW, now I noticed that it looks similar the security issue with
NULL page mapping. If the kernel has the page mapping to NULL,
it causes unstable behavior, no?


@@ -619,6 +621,9 @@ struct mm_struct *dup_mm(struct task_struct *tsk)
mm->hiwater_rss = get_mm_rss(mm);
mm->hiwater_vm = mm->total_vm;

+ if (mm->binfmt && !try_module_get(mm->binfmt->module))
+ goto free_pt;

free_pt does mmput() which calls module_put(mm->binfmt->module). This
is not right when try_module_get() fails. Hmm, the same if dup_mmap()
fails, we are doing an extra module_put().

Good catch, should avoid over putting binfmt->module.


Perhaps we can use __module_get() again, current->mm->binfmt holds a
reference. But this is a user-visible change, currently fork() can't
succeed if ->module->state == MODULE_STATE_GOING.

So I think we need another small change:

free_pt:
+ mm->binfmt = NULL;
mmput(mm);

looks good for me.
Will send an update.

thanks,
Hiroshi

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