As a side note, just to compel the differences in architectures, a certain OS developed and maintained by a company known for their three-letter acronym name and often referred to as a large primary color has a most-fasicnating NULL determination. In accordance with their OS, a "valid" pointer must be placed on a bit boundary within that address space.
This, btw, causes fascinating side effects for structs, packed or not, that house pointers. Anyway, tucked away in a per-process dedicated page is a bitmap that assigns one bit for every valid location in a process address space where a valid pointer can lay. ALL opcodes on their hardware and OS that can generate and return a valid memory address and assign it to a pointer will set the bit that represents the memory address where that pointer the target pointer is located.
Its something you have to see to believe. I can't even imagine the housekeeping done to maintain that bit map, especially when copying pointer values or freeing dynamic memory. On CPU which support virtual mermory, a page fault exception will be usually issued if you try to read at memory address 0x0. The OS page fault handler will be invoked, the OS will then decide that the page is invalid and aborts your program.
As the C Standard says dereferencing a null pointer is undefined, if the compiler is able to detect at compile time or even runtime that your are dereferencing a null pointer it can do whatever it wants, like aborting the program with a verbose error message.
C99, 6. Assuming the CPU supports memory protection and you're using an OS that enables it , the CPU will check that attempted access before it happens though. In a typical case, address 0 won't be mapped to any physical address. In this case, the CPU will generate an access violation exception. For one fairly typical example, Microsoft Windows leaves the first 4 megabytes un-mapped, so any address in that range will normally result in an access violation.
On an older CPU or an older operating system that doesn't enable the CPUs protection features the attempted write will often succeed. In small or medium model with bit addresses for data most compilers would write some known pattern to the first few bytes of the data segment, and when the program ended, they'd check to see if that pattern remained intact and do something to indicate that you'd written via a NULL pointer if it failed. In compact or large model bit data addresses they'd generally just write to address zero without warning.
I imagine that this is platform and compiler dependent. The NULL pointer could be implemented by using a NULL page, in which case you'd have a page fault, or it could be below the segment limit for an expand-down segment, in which case you'd have a segmentation fault. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow.
Learn more. Ask Question. Asked 9 years, 1 month ago. Active 15 days ago. Viewed 23k times. Let's say there is a pointer and we initialize it with NULL. Alexey Frunze TJD Linux. Take any CPU. The language says that dereferencing a null pointer results in undefined behavior. What your given processor and OS will do about it is implementation specific. There exists no "in general" — Ed S. The point is that CPUs have different mechanism to deal with this.
In some cases that's no mechanism. Pointer members in structs are not checked. Does not guess that return values from malloc , strchr , etc.
Checks for use of null pointers rule partially covered. Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Key here explains table format and definitions. Key here for mapping notes. EXPC is a common consequence of ignoring function return values, but it is a distinct error, and can occur in other scenarios too. I say "theoretical" because I have not successfully produced strings of this length in testing.
Ah, gotcha. That makes sense. I think you cover that in a different rule. It also reinforces the notion to the reader that any time you see arithmetic in an allocation expression, you need to think about corner-cases. A common memory-leak idiom, is reallocating storage and assigning its address to a pointer that already points to allocated storage.
The correct idiom is to only allocate storage if the pointer is currently NULL. But no where in that particular idiom would a NULL pointer necessarily be deferenced.
The article easily misleads the reader into believeing that ensuring pointer validity boils down to checking for pointer being not equal to NULL.
Unfortunately the problem is much more complex, and generally unsolvable within standard C. Consider the following example:. There's no way f can check whether x points into valid memory or not. Using platform-specific means e. IMHO, the rule title should be changed to something less general. Note that it doesn't know how to check for non-heap, non-stack. Many platforms can support testing for those also.
The idea is not to guarantee validity, but to catch a substantial number of problems that could occur. The final NCCE is actually more insidious than it seems at first.
Because null pointer dereferencing is UB, the if! One could argue that all code examples would be redundant with the first pair. In this case, the difference is the assumption that malloc always returns non-null for the second NCCE, whereas the first NCCE has the malloc abstracted away. I suggest that this topic needs to include calloc and realloc Refer to Linux man pages online for more enlightenment about malloc , and friends.
I believe that dereferencing NULL should not crash the system, should not allow a write to a NULL pointer area, but should always set errno, If I am a hacker, could I trap a null failure that would force a memory dump. Could I capture, and I would be able to glean much security information from the dump? The null pointer check for writing or dereferencing should be a compiler flag or library setting. Believing that dereferencing NULL shouldn't crash the system doesn't make it true.
I guess you could write a proposal to modify the C Standard, but our coding standard is meant to provide guidance for the existing language. Solution 1, it looks like, today's solution tomorrow's problem. Then we hit memcpy with length 0. When length is zero, it is probably unusable condition for this function. There are other problems with this code, as is noted in the rule. But passing 0 to memcpy is not one of them.
The standard will simply copy 0 bytes C11, S7. That interpretation of the standard is not supported universally. See C17 7. Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow:. If an argument to a function has an invalid value such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified or a type after default argument promotion not expected by a function with a variable number of arguments, the behavior is undefined.
The issue is: memcpy and friends do not explicitly state that a null pointer is a valid pointer value, even if the number of bytes to copy is 0. Isn't easier just to check valid range of length? I doubt that "length" of zero is a valid parameter, and although there no copy, but we see memory allocation.
It looks like a logic bug, which can cause a memory leaking. A non-null but invalid pointer passed to memcpy can indeed cause undefined behavior, but that is not the issue in the noncompliant code And the compliant solution guarantees that the pointer will be valid if the code calls memcpy.
Best to cite C11 s7. The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined. In contrast, the case of passing 0 bytes to malloc is addressed in C Your assertion is not backed by the wording in the standard, nor by common implementer understanding.
Running a program that contains a NULL pointer dereference generates an immediate segmentation fault error. When the memory analysis feature detects this type of error, it traps these errors for any of the following functions if error detection is enabled when they are called within your program:. The memory analysis feature doesn't trap errors for the following functions when they are called:. Enabling error detection for a NULL pointer dereference.
In the IDE, you can expect the message for this type of memory error to include the following types of information and detail:.
0コメント