3 ms·

Why do you think NULL isn't a pointer? It most definitely is a pointer, it's the kernel the limits a ptr to 0 from being used, in fact you can use it if mmap_mi

Why do you think NULL isn't a pointer? It most definitely is a pointer, it's the kernel the limits a ptr to 0 from being used, in fact you can use it if mmap_min_addr is set to 0.

deathanatos 9y ago
The point is that there are two very fundamentally different things that can be stored in a C pointer: 1. A valid address to an object of some type. 2. Null. One of these can be dereferenced, the other cannot; the valid operations are not the same, because they are not the same "type". Now, C of course will happily store both the address of an object and "NULL" in the same "type", and that's the problem. There are a great many places in code in C where one would like to take a pointer that must contain an object's address; that is, a non-null pointer. The type system offered by C has no way to indicate this, and so the compiler cannot catch passing NULL to such a function. (And I honestly would bet that the kernel limits it more due to C, than C uses 0 because the kernel limits it. That is, yes, the kernel limits allocating address 0, but the arrow of causality is the other way around.)
geofft 9y ago
NULL is not a pointer—what does it point to? 0x00000000 is a pointer, sure, and it points to the memory at address zero. But that's not the same concept as NULL, even if it happens to have the same representation. If malloc(32) returns NULL, it doesn't mean the memory between 0x00000000 and 0x00000020 is available for me to use. false also has the same representation, but no one would claim that false is a pointer. mmap_min_addr exists because C is unable to distinguish NULL and a pointer to address zero, and the Linux kernel is written in C, and too much code wrongly treats NULL as a pointer to 0x00000000 and attempts to read or execute the contents of memory there. If code did not confuse the two, mmap_min_addr would not need to exist. And it is relatively recent; it was added as a security measure in June 2007 for Linux 2.6.23 in https://github.com/torvalds/linux/commit/ed0321895182ffb6ecf210e066d87911b270d587 https://github.com/torvalds/linux/commit/ed0321895182ffb6ecf... , about 40 years after the invention of NULL.

Why do you think NULL isn't a pointer? It most definitely is a pointer, it's the kernel the limits a ptr to 0 from being used, in fact you can use it if mmap_min_addr is set to 0.

deathanatos 9y ago

The point is that there are two very fundamentally different things that can be stored in a C pointer: 1. A valid address to an object of some type. 2. Null. One of these can be dereferenced, the other cannot; the valid operations are not the same, because they are not the same "type". Now, C of course will happily store both the address of an object and "NULL" in the same "type", and that's the problem. There are a great many places in code in C where one would like to take a pointer that must contain an object's address; that is, a non-null pointer. The type system offered by C has no way to indicate this, and so the compiler cannot catch passing NULL to such a function. (And I honestly would bet that the kernel limits it more due to C, than C uses 0 because the kernel limits it. That is, yes, the kernel limits allocating address 0, but the arrow of causality is the other way around.)

geofft 9y ago

NULL is not a pointer—what does it point to? 0x00000000 is a pointer, sure, and it points to the memory at address zero. But that's not the same concept as NULL, even if it happens to have the same representation. If malloc(32) returns NULL, it doesn't mean the memory between 0x00000000 and 0x00000020 is available for me to use. false also has the same representation, but no one would claim that false is a pointer. mmap_min_addr exists because C is unable to distinguish NULL and a pointer to address zero, and the Linux kernel is written in C, and too much code wrongly treats NULL as a pointer to 0x00000000 and attempts to read or execute the contents of memory there. If code did not confuse the two, mmap_min_addr would not need to exist. And it is relatively recent; it was added as a security measure in June 2007 for Linux 2.6.23 in https://github.com/torvalds/linux/commit/ed0321895182ffb6ecf210e066d87911b270d587 https://github.com/torvalds/linux/commit/ed0321895182ffb6ecf... , about 40 years after the invention of NULL.