Comment by frmdstryr
11 hours ago
After more searching I found this article https://www.gingerbill.org/article/2020/05/17/relative-point...
A far pointer sounds like the global based pointer described in that article. The far pointer Wikipedia article says they are problematic but doesn't give much reasoning as to why.
Far pointers are for accessing memory in different segments. They're basically obsolete now. They were necessary in older machines with limited sized pointers or address spaces.
GCC still supports `__seg_fs` and `__seg_gs`, which behave similar to `far` in the example on the wiki page, as the FS and GS segment registers are still valid in x86-64 and used for TLS. Clang uses attributes `address_space(257)` and `address_space(256)` for the same thing.
The `__based` pointer in MSVC exploits the addressing modes by pinning the base in eg: `[base+index*scale+displacement]`. It's unrelated to segmentation.
> Far pointers are for accessing memory in different segments. They're basically obsolete now.
Project CHERI would like to disagree.
That's Fat pointers, not Far pointers. A fat pointer is a pointer with some other associated data which is stored in the pointer itself - typically by widening the number of bits used to hold a pointer value. The addressable bits usually remain unchanged - the added bits contain the auxiliary data.
Segmentation isn't used. There's no separate registers to hold the bounds information in CHERI - the bounds are held in the pointer value, unlike for example, the now obsolete Intel MPX, which held bounds information in separate registers.
There's some similarity to segmentation because the CHERI pointer restricts which addresses can be accessed, but I wouldn't compare them to far pointers.
Most modern processors have a single linear virtual address space and don't use segmentation, and even where segment registers exist (eg, FS and GS on x86-64), they're only superficial "address spaces" - allocated sections of the process's linear virtual address space which could be accessed without segmentation registers if you knew the base address held in FS or GS.