notes/Enable_MPX at master · Yellow-Pay/notes · GitHub

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Enable Intel MPX
	1. Rebuild the kernel with option to enable Intel MPX
		Make menuconfig -- enable Intel_MPX....
	2. Rebuild GCC
	    Using --enable-libmpx to configure the compiler.

	To enable MPX related instruction or instrument in program.
	"-mmpx -fcheck-pointer-bounds" is need to be added to compile option string.
	Other options could be find in
	https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler#Compiler_options


	Sometimes we do not need checking instructions instrumented to all the pointer access(read/write). Because it causes a large overhead. We just want to insert checking code into where we prefer by inline assemble. So we need to enable the MPX manually.

	When using gcc's compile option to enable MPX, gcc just link the program with two libs: libmpx.so and libmpxwrappers.so
	When the libs are loaded, MPX is enabled at the beginning. Look into the code we will get to know how to enable MPX manually.


Enable the Intel MPX manually.

	1. Check the CPUID to see if the CPU supports Intel MPX.
		(Refer to Intel Developer Manual)
			Also can use xgetbv(0) to get XCR0's value.
	2. Allocate a memory region for Bound Directory.
		(2G for 64 bit system, refer to Intel Developer Manual)
	3. Set the register bndcfgu/s using xrstor/xsave instruction.
		Some of these procedures must be done in kernel mode, so a new syscall should be added.

Add a syscall:
	https://medium.com/@ssreehari/implementing-a-system-call-in-linux-kernel-4-7-1-6f98250a8c38#.1rbmp72vw This tutorial is simple and easy to understand.
	If MPX is not enabled(enabled bit of register bndcfgu/s is not set), all instructions related to MPX(BNDMOV, BNDSTX etc.) are considered as NOP.
	For Bndstx: bndstx bnd_reg, M. store the bnd_reg's bound to BndTable. M ---- (ptr, index, 0)

Attention: Every bound directory entry(BDE) has a valid bit(0x1), it needs to be check before access the bndstx.
			(This is only mentioned in the pseudo code of bndstx of Intel Developer Manual, and not mentioned at all in MPX introdution part.)

bndstx will use ptr to get the bndtable entry and store the upperbound and lowerbound in bnd_reg to the BTE. and store index to the BTE's pointer value BTE's reserve is always 0. (For current systems)