Intel architecture ia-32 User Manual
4-34 Vol. 3A
PROTECTION
4.10.2
Checking Read/Write Rights (VERR and VERW
Instructions)
Instructions)
When the processor accesses any code or data segment it checks the read/write privileges
assigned to the segment to verify that the intended read or write operation is allowed. Software
can check read/write rights using the VERR (verify for reading) and VERW (verify for writing)
instructions. Both these instructions specify the segment selector for the segment being checked.
The instructions then perform the following operations:
assigned to the segment to verify that the intended read or write operation is allowed. Software
can check read/write rights using the VERR (verify for reading) and VERW (verify for writing)
instructions. Both these instructions specify the segment selector for the segment being checked.
The instructions then perform the following operations:
1.
Check that the segment selector is not null.
2.
Checks that the segment selector points to a segment descriptor that is within the descriptor
table limit (GDT or LDT).
table limit (GDT or LDT).
3.
Checks that the segment descriptor is a code or data-segment descriptor type.
4.
If the segment is not a conforming code segment, checks if the segment descriptor is
visible at the CPL (that is, if the CPL and the RPL of the segment selector are less than or
equal to the DPL).
visible at the CPL (that is, if the CPL and the RPL of the segment selector are less than or
equal to the DPL).
5.
Checks that the segment is readable (for the VERR instruction) or writable (for the
VERW) instruction.
VERW) instruction.
The VERR instruction sets the ZF flag in the EFLAGS register if the segment is visible at the
CPL and readable; the VERW sets the ZF flag if the segment is visible and writable. (Code
segments are never writable.) The ZF flag is cleared if any of these checks fail.
CPL and readable; the VERW sets the ZF flag if the segment is visible and writable. (Code
segments are never writable.) The ZF flag is cleared if any of these checks fail.
4.10.3
Checking That the Pointer Offset Is Within Limits (LSL
Instruction)
Instruction)
When the processor accesses any segment it performs a limit check to insure that the offset is
within the limit of the segment. Software can perform this limit check using the LSL (load
segment limit) instruction. Like the LAR instruction, the LSL instruction specifies the segment
selector for the segment descriptor whose limit is to be checked and a destination register. The
instruction then performs the following operations:
within the limit of the segment. Software can perform this limit check using the LSL (load
segment limit) instruction. Like the LAR instruction, the LSL instruction specifies the segment
selector for the segment descriptor whose limit is to be checked and a destination register. The
instruction then performs the following operations:
1.
Check that the segment selector is not null.
2.
Checks that the segment selector points to a segment descriptor that is within the descriptor
table limit (GDT or LDT).
table limit (GDT or LDT).
3.
Checks that the segment descriptor is a code, data, LDT, or TSS segment-descriptor type.
4.
If the segment is not a conforming code segment, checks if the segment descriptor is
visible at the CPL (that is, if the CPL and the RPL of the segment selector less than or
equal to the DPL).
visible at the CPL (that is, if the CPL and the RPL of the segment selector less than or
equal to the DPL).