Messages in this thread | | | Subject | Re: [PATCH] cxgb4: fix undefined behavior in mem.c | From | Shaobo He <> | Date | Thu, 28 Feb 2019 16:57:06 -0700 |
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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. >
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