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It's a leaky abstraction over the underlying system call mechanism. Because the user stack lives in user space and accessing it from the kernel requires the sa
by caf 5y ago
It's a leaky abstraction over the underlying system call mechanism. Because the user stack lives in user space and accessing it from the kernel requires the same dance as any other access to user memory, system call arguments are instead passed in registers where they are immediately available to the kernel with no possibility of faults or TOCTTOU holes.
Here 'unsigned long' is just a convenient stand-in for "generic register-sized argument".
A "void * pointing to struct" requires a copy from user space. If a particular prctl() does need a struct, it can certainly stuff a pointer into one of those 'unsigned long' parameters (in the kernel environment, a pointer can be converted to and from an unsigned long without concern).
- dataflow 5y ago> Here 'unsigned long' is just a convenient stand-in for "generic register-sized argument". That wouldn't be register-sized on x86 would it?
- oynqr 5y agoThe minimum size for unsigned long is 4 bytes
- dataflow 5y agoOh I didn't realize its size varies on x86 and x64... I thought it's always 64-bit under compilers that target Linux! Today I learned...
- jamesfinlayson 5y agoI think the size can vary by compiler rather than just by architecture.
- dataflow 5y agoYeah it can but in practice systems have a canonical model in their official headers.
- tomsmeding 5y agoThere is a difference between 'long' and 'long long'. I believe the latter is always 64 bits on Linux.
- dataflow 5y agoI'm indeed aware of that one :-)
- cesarb 5y agoOn all Linux architectures, "unsigned long" and "long" are always register-sized. For 32-bit architectures, "long" is 32 bits (the same as "int" which is always 32 bits), and for 64-bit architectures, "long" is 64 bits (the same as "long long" which is always 64 bits). This is different from Windows, in which "long" is always 32 bits, even on 64-bit architectures (and you have to use "long long" if you want 64 bits). Therefore, Linux-only code (including the Linux kernel itself) commonly uses "long" or "unsigned long" to mean "register-sized" and "large enough to fit a pointer"; portable code normally uses "size_t" or "uintptr_t" for that.
- dataflow 5y agoThanks, yeah. Any idea why they still do this instead of switching to `uintptr_t`?
- dxuh 5y agoAfaik the kernel is built as C89 and uintptr_t was introduced in C99.
- caf 5y agoIt's not even just Linux - way back in the mid 90s there was an agreement among UNIX vendors to use the "LP64" model for 64 bit architectures, and ILP32 was already the de facto standard for 32 bit UNIX-likes, so "long can hold a pointer" is somewhat of a tradition in the POSIXy world.