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Document syscall -3

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DBJ314 2018-04-22 12:18:08 -04:00 committed by Elliot Nunn
parent 5f395530ab
commit 1b520d48ae
1 changed files with 8 additions and 4 deletions
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12
NanoKernel/NKInterrupts.s
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@ -2852,15 +2852,19 @@ IntSyscall ; OUTSIDE REFERER
cmpwi r0, -3
bne @not_minus_3
; sc -3:
; sc -3: used by FDP to go back to supervisor mode after instruction emulation.
; For security reasons, FDP goes into user mode when it emulates loads and stores.
; This is how it goes back into supervisor mode afterwards. During instruction emulation
; it has data paging on, but keeps instruction paging off. User Mode code never has instruction
; paging disabled, so this is not a privilege escalation.
; unset MSR_PR bit
mfspr r1, srr1
rlwinm. r0, r1, 26, 26, 27 ; nonsense code?
rlwinm. r0, r1, 26, 26, 27 ;move MSR_IR bit to sign bit (and a few others that don't matter)
_bclr r1, r1, 17
blt @dont_unset_pr ; r0 should never have bit 0 set
blt @not_in_FDP ; only do if MSR_IR = 0 (MSR_IR is sign bit, so it is < 0 if it is true)
mtspr srr1, r1
@dont_unset_pr
@not_in_FDP
; restore LR from SPRG2, r1 from SPRG1
mfsprg r1, 2