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

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Message-ID: <1551396788.31902.213.camel@acm.org>
Date:   Thu, 28 Feb 2019 15:33:08 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Shaobo He <shaobo@...utah.edu>, linux-rdma@...r.kernel.org
Cc:     Steve Wise <swise@...lsio.com>, Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cxgb4: fix undefined behavior in mem.c

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.