Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Shaobo He]

Good catch. But if we agree on that memory management functions are those 
specified by the C standard, would it be OK to ignore so-called use after free 
or double free bugs for the kernel as C standard does not apply to kfree?

On 2/28/19 4:33 PM, Bart Van Assche wrote:
> On Thu, 2019-02-28 at 16:18 -0700, Shaobo He wrote:
> > I can't afford a pdf version of the C standard. So I looked at the draft version
> > used in the link I put in the commit message. It says (in 6.2.4:2),
> >
> > ```
> > The lifetime of an object is the portion of program execution during which
> > storage is guaranteed to be reserved for it. An object exists, has a constant
> > address, and retains its last-stored value throughout its lifetime. If an object
> > is referred to outside of its lifetime, the behavior is undefined. The value of
> > a pointer becomes indeterminate when the object it points to (or just past)
> > reaches the end of its lifetime.
> > ```
> > I couldn't find the definition of lifetime over a dynamically allocated object
> > in the draft of C standard. I refer to this link
> > (https://en.cppreference.com/w/c/language/lifetime) which suggests that the
> > lifetime of an allocated object ends after the deallocation function is called
> > upon it.
> >
> > I think maybe the more problematic issue is that the value of a freed pointer is
> > intermediate.
>
> In another section of the same draft I found the following:
>
> J.2 Undefined behavior [ ... ] The value of a pointer that refers to space
> deallocated by a call to the free or realloc function is used (7.22.3).
>
> Since the C standard explicitly refers to free() and realloc(), does that
> mean that that statement about undefined behavior does not apply to munmap()
> (for user space code) nor to kfree() (for kernel code)?
>
> Bart.