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MFC 256293:

Sanitize the %eflags returned by BIOS routines.  Some BIOS routines enter
protected mode and may leave protected-mode-specific flags like PSL_NT set
when they return to real mode.  This can cause a fault when BTX re-enters
protected mode after the BIOS mode returns.

Approved by:	re (gjb)
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1 parent e8958f2 commit 9f3020013a19321c4f393376c9191ac458efee60 @bsdjhb bsdjhb committed Oct 15, 2013
Showing with 5 additions and 2 deletions.
  1. +5 −2 sys/boot/i386/btx/btx/btx.S
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7 sys/boot/i386/btx/btx/btx.S
@@ -41,6 +41,8 @@
.set PSL_RESERVED_DEFAULT,0x00000002
.set PSL_T,0x00000100 # Trap flag
.set PSL_I,0x00000200 # Interrupt enable flag
+ .set PSL_D,0x00000400 # String instruction direction
+ .set PSL_NT,0x00004000 # Nested task flag
.set PSL_VM,0x00020000 # Virtual 8086 mode flag
.set PSL_AC,0x00040000 # Alignment check flag
/*
@@ -611,8 +613,8 @@ rret_tramp: movw $MEM_ESPR-0x08,%sp # Reset stack pointer
pushl %ds # regs
pushl %es
pushfl # Save %eflags
- cli # Disable interrupts
- std # String ops dec
+ pushl $PSL_RESERVED_DEFAULT|PSL_D # Use clean %eflags with
+ popfl # string ops dec
xorw %ax,%ax # Reset seg
movw %ax,%ds # regs
movw %ax,%es # (%ss is already 0)
@@ -675,6 +677,7 @@ rret_tramp.1: xorl %ecx,%ecx # Zero
testl $V86F_FLAGS,%edx # User wants flags?
jz rret_tramp.3 # No
movl MEM_ESPR-0x3c,%eax # Read real mode flags
+ andl $~(PSL_T|PSL_NT),%eax # Clear unsafe flags
movw %ax,-0x08(%esi) # Update user flags (low 16)
/*
* Return to the user task

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