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Subjectget_arg_page() && ptr_size accounting
Hi Kees,

I was thinking about backporting the commit 98da7d08850fb8bde
("fs/exec.c: account for argv/envp pointers"), but I am not sure
I understand it...

So get_arg_page() does

/*
* Since the stack will hold pointers to the strings, we
* must account for them as well.
*
* The size calculation is the entire vma while each arg page is
* built, so each time we get here it's calculating how far it
* is currently (rather than each call being just the newly
* added size from the arg page). As a result, we need to
* always add the entire size of the pointers, so that on the
* last call to get_arg_page() we'll actually have the entire
* correct size.
*/
ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
if (ptr_size > ULONG_MAX - size)
goto fail;
size += ptr_size;

OK, but
acct_arg_size(bprm, size / PAGE_SIZE);

after that doesn't look exactly right. This additional space will be used later
when the process already uses bprm->mm, right? so it shouldn't be accounted by
acct_arg_size().

Not to mention that ptr_size/PAGE_SIZE doesn't look right in any case...

In short. Am I totally confused or the patch below makes sense? This way we do
not need the fat comment.

Oleg.

--- x/fs/exec.c
+++ x/fs/exec.c
@@ -222,25 +222,17 @@ static struct page *get_arg_page(struct
unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
unsigned long ptr_size, limit;

+ acct_arg_size(bprm, size / PAGE_SIZE);
+
/*
* Since the stack will hold pointers to the strings, we
* must account for them as well.
- *
- * The size calculation is the entire vma while each arg page is
- * built, so each time we get here it's calculating how far it
- * is currently (rather than each call being just the newly
- * added size from the arg page). As a result, we need to
- * always add the entire size of the pointers, so that on the
- * last call to get_arg_page() we'll actually have the entire
- * correct size.
*/
ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
if (ptr_size > ULONG_MAX - size)
goto fail;
size += ptr_size;

- acct_arg_size(bprm, size / PAGE_SIZE);
-
/*
* We've historically supported up to 32 pages (ARG_MAX)
* of argument strings even with small stacks
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