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Hello,
I am trying to make pmd level of the translation table as read only so that any writes in the pte entries should cause a permission fault.
Current steps which i am doing are:
1) inside kernel space, allocating 2 pointers (say p, q) and allocating memory using vmalloc (4k size to occupy entire page)
2) assigning int data to both (say p=10, q= 20)
3) setting APTable bit for readonly for both the pointers (By reading manual, bit 61 and 62 denotes APTable)
4) swapping pte value for both the pointer in translation table
5) printing value for both the pointer
At step 4, access permission fault is not occurring.
I tried flushing the TLB, still at point 4 fault does not occur, but fault is occurring at random places (sometimes after point 5 or in between any points)
Can anyone tell me possible reasons why this is happening or some solution to fix it?
thanks
This note is strange and maybe a reason for your findings:" The APTable[1:0] settings are combined with the translation table access permissions in the translation tables descriptors accessed in subsequent levels of lookup. They do not restrict or change the values entered in those descriptors."
Yes here is what i am confused.
Just below that line in the manual they say
"The VMSAv8-64 provides APTable[1:0] control only for stage 1 translations. The corresponding bits are RES 0 inthe stage 2 translation table descriptors."
so I assumed for stage 1 permission checks should be performed.
stage 2 is EL2 (AFAIK), so in your case it does not apply.
Yes