问题
I need to use two bits of the PTE to store a custom "state" value that my kernel module will use when intercepting page protection faults.
I am developing on a Galaxy Nexus which has an ARM Cortex A9 (ARM v7, I believe). Linux kernel version 3.0.31. The Linux PTE definitions are as follows (from arch/arm/include/asm/pgtable.h:
/*
* "Linux" PTE definitions.
*
* We keep two sets of PTEs - the hardware and the linux version.
* This allows greater flexibility in the way we map the Linux bits
* onto the hardware tables, and allows us to have YOUNG and DIRTY
* bits.
*
* The PTE table pointer refers to the hardware entries; the "Linux"
* entries are stored 1024 bytes below.
*/
#define L_PTE_PRESENT (_AT(pteval_t, 1) << 0)
#define L_PTE_YOUNG (_AT(pteval_t, 1) << 1)
#define L_PTE_FILE (_AT(pteval_t, 1) << 2) /* only when !PRESENT */
#define L_PTE_DIRTY (_AT(pteval_t, 1) << 6)
#define L_PTE_RDONLY (_AT(pteval_t, 1) << 7)
#define L_PTE_USER (_AT(pteval_t, 1) << 8)
#define L_PTE_XN (_AT(pteval_t, 1) << 9)
#define L_PTE_SHARED (_AT(pteval_t, 1) << 10) /* shared(v6), coherent(xsc3) */
Just by looking at this list of definitions, it appears that bits 3,4,5 are available, as well as bits 11 and up. However, I know that the 20 most significant bits [31:12] are used for the page number (PFN, I believe), so I can't use any of those.
Can I freely use bits [5:3] or will that create problems? I've spent hours searching for the answer to this but I can only find documentation on how Linux uses PTE bits for x86 architecture.
UPDATE:
I have compiled a list of what I believe each PTE bit to be.
bit 0 PRESENT
bit 1 YOUNG
bit 2 MEMORY TYPES 0 B FILE (only when not PRESENT)
bit 3 MEMORY TYPES 1 C
bit 4 AP0
bit 5 AP1
bit 6 DIRTY
bit 7 RD_ONLY
bit 8 USER
bit 9 XN
bit 10 SHARED
bit 11 EXT_NG (no idea what this is)
bit 12 |---|
... |PFN|
bit 31 |---|
Unfortunately I don't see a way to specify no read or write permissions, but somehow mmap with PROT_NONE still seems to work. I know how to specify read or R/W permissions, but I still need to know how to set a page to have no permissions.
回答1:
You can try to clear the present bit in the pte, will that force a page fault?
来源：https://stackoverflow.com/questions/16085976/arm-linux-page-table-entry-format-unused-bits