diff options
Diffstat (limited to 'arch/x86/kernel/entry_32.S')
| -rw-r--r-- | arch/x86/kernel/entry_32.S | 49 |
1 files changed, 34 insertions, 15 deletions
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S index d7d1c7d..848f73f 100644 --- a/arch/x86/kernel/entry_32.S +++ b/arch/x86/kernel/entry_32.S @@ -588,24 +588,34 @@ ldt_ss: jne restore_nocheck #endif - /* If returning to userspace with 16bit stack, - * try to fix the higher word of ESP, as the CPU - * won't restore it. - * This is an "official" bug of all the x86-compatible - * CPUs, which we can try to work around to make - * dosemu and wine happy. */ - movl PT_OLDESP(%esp), %eax - movl %esp, %edx - call patch_espfix_desc +/* + * Setup and switch to ESPFIX stack + * + * We're returning to userspace with a 16 bit stack. The CPU will not + * restore the high word of ESP for us on executing iret... This is an + * "official" bug of all the x86-compatible CPUs, which we can work + * around to make dosemu and wine happy. We do this by preloading the + * high word of ESP with the high word of the userspace ESP while + * compensating for the offset by changing to the ESPFIX segment with + * a base address that matches for the difference. + */ + mov %esp, %edx /* load kernel esp */ + mov PT_OLDESP(%esp), %eax /* load userspace esp */ + mov %dx, %ax /* eax: new kernel esp */ + sub %eax, %edx /* offset (low word is 0) */ + PER_CPU(gdt_page, %ebx) + shr $16, %edx + mov %dl, GDT_ENTRY_ESPFIX_SS * 8 + 4(%ebx) /* bits 16..23 */ + mov %dh, GDT_ENTRY_ESPFIX_SS * 8 + 7(%ebx) /* bits 24..31 */ pushl $__ESPFIX_SS CFI_ADJUST_CFA_OFFSET 4 - pushl %eax + push %eax /* new kernel esp */ CFI_ADJUST_CFA_OFFSET 4 /* Disable interrupts, but do not irqtrace this section: we * will soon execute iret and the tracer was already set to * the irqstate after the iret */ DISABLE_INTERRUPTS(CLBR_EAX) - lss (%esp), %esp + lss (%esp), %esp /* switch to espfix segment */ CFI_ADJUST_CFA_OFFSET -8 jmp restore_nocheck CFI_ENDPROC @@ -718,15 +728,24 @@ PTREGSCALL(vm86) PTREGSCALL(vm86old) .macro FIXUP_ESPFIX_STACK - /* since we are on a wrong stack, we cant make it a C code :( */ +/* + * Switch back for ESPFIX stack to the normal zerobased stack + * + * We can't call C functions using the ESPFIX stack. This code reads + * the high word of the segment base from the GDT and swiches to the + * normal stack and adjusts ESP with the matching offset. + */ + /* fixup the stack */ PER_CPU(gdt_page, %ebx) - GET_DESC_BASE(GDT_ENTRY_ESPFIX_SS, %ebx, %eax, %ax, %al, %ah) - addl %esp, %eax + mov GDT_ENTRY_ESPFIX_SS * 8 + 4(%ebx), %al /* bits 16..23 */ + mov GDT_ENTRY_ESPFIX_SS * 8 + 7(%ebx), %ah /* bits 24..31 */ + shl $16, %eax + addl %esp, %eax /* the adjusted stack pointer */ pushl $__KERNEL_DS CFI_ADJUST_CFA_OFFSET 4 pushl %eax CFI_ADJUST_CFA_OFFSET 4 - lss (%esp), %esp + lss (%esp), %esp /* switch to the normal stack segment */ CFI_ADJUST_CFA_OFFSET -8 .endm .macro UNWIND_ESPFIX_STACK |
