[gPXE-devel] [PATCH] Run COM32 programs with a valid IDT

Daniel Verkamp daniel at drv.nu
Thu Jul 1 11:26:04 EDT 2010


This looks good to me after a first reading; I'm happy with it
assuming it has been tested (sounds like it has if mboot.c32 now
works).

CC'd hpa: if you could take a glance and see if it looks okay, that
would be helpful too.

As a side note, it might be nice if an IDT for COM32 would be
mentioned as part of the interface in the syslinux documentation. I
mostly followed doc/comboot.txt in the initial implementation, so I
missed this entirely.

Thanks,
-- Daniel Verkamp

On Tue, Jun 29, 2010 at 12:23 AM, Stefan Hajnoczi <stefanha at gmail.com> wrote:
> CCed Daniel Verkamp, who wrote the COMBOOT implementation.
>
> On Tue, Jun 29, 2010 at 2:18 AM, Geoff Lywood <glywood at vmware.com> wrote:
>> COM32 binaries generally expect to run with interrupts enabled. Syslinux does
>> so, and COM32 programs will execute cli/sti pairs when running a critical
>> section, to provide mutual exclusion against BIOS interrupt handlers.
>> Previously, under gPXE, the IDT was not valid, so any interrupt (e.g. a timer
>> tick) would generally cause the machine to triple fault.
>>
>> This change introduces code to:
>> - Create a valid IDT at the same location that syslinux uses
>> - Create an "interrupt jump buffer", which contains small pieces of code that
>>  simply record the vector number and jump to a common handler
>> - Thunk down to real mode and execute the BIOS's interrupt handler whenever
>>  an interrupt is received in a COM32 program
>> - Switch IDTs and enable/disable interrupts when context switching to and from
>>  COM32 binaries
>>
>> Testing done:
>> - Booted VMware ESX using a COM32 multiboot loader (mboot.c32)
>> - Built with GDBSERIAL enabled, and tested breakpoints on int22 and com32_irq
>> - Put the following code in a COM32 program:
>>    asm volatile ( "sti" );
>>    while ( 1 );
>>  Before this change, the machine would triple fault immediately. After this
>>  change, it hangs as expected. Under Bochs, it is possible to see the
>>  interrupt handler run, and the current time in the BIOS data area gets
>>  incremented.
>>
>> I have a feeling that Outlook is going to mangle this patch, so I'm attaching it as well.
>>
>> VMware, Inc. is providing this patch to you under the terms of the GPL version 2 and any later version. This patch is provided as is, with no warranties or support. VMware disclaims all liability in connection with the use/inability to use this patch. Any use of the attached is considered acceptance of the above.
>>
>> Signed-off-by: Geoff Lywood <glywood at vmware.com>
>> ---
>>  src/arch/i386/image/com32.c                      |   73 +++++++++++++++++++++-
>>  src/arch/i386/include/comboot.h                  |   45 +++++++++++++
>>  src/arch/i386/interface/syslinux/com32_call.c    |   17 +++++
>>  src/arch/i386/interface/syslinux/com32_wrapper.S |   28 ++++++++
>>  4 files changed, 162 insertions(+), 1 deletions(-)
>>
>> diff --git a/src/arch/i386/image/com32.c b/src/arch/i386/image/com32.c
>> index 6ab347c..0f01027 100644
>> --- a/src/arch/i386/image/com32.c
>> +++ b/src/arch/i386/image/com32.c
>> @@ -42,6 +42,13 @@ FILE_LICENCE ( GPL2_OR_LATER );
>>
>>  struct image_type com32_image_type __image_type ( PROBE_NORMAL );
>>
>> +struct idt_register com32_external_idtr = {
>> +       .limit = COM32_NUM_IDT_ENTRIES * sizeof ( struct idt_descriptor ) - 1,
>> +       .base = COM32_IDT
>> +};
>> +
>> +struct idt_register com32_internal_idtr;
>> +
>>  /**
>>  * Execute COMBOOT image
>>  *
>> @@ -89,9 +96,12 @@ static int com32_exec ( struct image *image ) {
>>                unregister_image ( image );
>>
>>                __asm__ __volatile__ (
>> +                       "sidt com32_internal_idtr\n\t"
>> +                       "lidt com32_external_idtr\n\t"         /* Set up IDT */
>>                        "movl %%esp, (com32_internal_esp)\n\t" /* Save internal virtual address space ESP */
>>                        "movl (com32_external_esp), %%esp\n\t" /* Switch to COM32 ESP (top of available memory) */
>>                        "call _virt_to_phys\n\t"               /* Switch to flat physical address space */
>> +                       "sti\n\t"                              /* Enable interrupts */
>>                        "pushl %0\n\t"                         /* Pointer to CDECL helper function */
>>                        "pushl %1\n\t"                         /* Pointer to FAR call helper function */
>>                        "pushl %2\n\t"                         /* Size of low memory bounce buffer */
>> @@ -100,7 +110,9 @@ static int com32_exec ( struct image *image ) {
>>                        "pushl %5\n\t"                         /* Pointer to the command line arguments */
>>                        "pushl $6\n\t"                         /* Number of additional arguments */
>>                        "call *%6\n\t"                         /* Execute image */
>> -                       "call _phys_to_virt\n\t"               /* Switch back to internal virtual address space */
>> +                       "cli\n\t"                              /* Disable interrupts */
>> +                       "call _phys_to_virt\n\t"               /* Switch back to internal virtual address space */
>> +                       "lidt com32_internal_idtr\n\t"         /* Switch back to internal IDT (for debugging) */
>>                        "movl (com32_internal_esp), %%esp\n\t" /* Switch back to internal stack */
>>                :
>>                :
>> @@ -216,6 +228,60 @@ static int comboot_load_image ( struct image *image ) {
>>  }
>>
>>  /**
>> + * Prepare COM32 Interrupt Descriptor Table and Interrupt Jump Buffer
>> + * @v image            COM32 image
>> + * @ret rc             Return status code
>> + */
>> +static int comboot_prepare_idt ( struct image * image ) {
>> +       struct idt_descriptor *idt;
>> +       struct ijb_entry *ijb;
>> +       physaddr_t com32_irq_wrapper_phys;
>> +       userptr_t buffer;
>> +       size_t memsz;
>> +       int i, rc;
>> +
>> +       buffer = phys_to_user ( COM32_IDT );
>> +       memsz = COM32_NUM_IDT_ENTRIES * sizeof ( struct idt_descriptor );
>> +       rc = prep_segment ( buffer, memsz, memsz );
>> +       if ( rc != 0 ) {
>> +               DBGC ( image, "COM32 %p: could not prepare IDT segment: %s\n",
>> +                      image, strerror ( rc ) );
>> +               return rc;
>> +       }
>> +
>> +       buffer = phys_to_user ( COM32_IJB );
>> +       memsz = COM32_NUM_IDT_ENTRIES * sizeof ( struct ijb_entry );
>> +       rc = prep_segment ( buffer, memsz, memsz );
>> +       if ( rc != 0 ) {
>> +               DBGC ( image, "COM32 %p: could not prepare IJB segment: %s\n",
>> +                      image, strerror ( rc ) );
>> +               return rc;
>> +       }
>> +
>> +       idt = phys_to_virt ( COM32_IDT );
>> +       ijb = phys_to_virt ( COM32_IJB );
>> +       com32_irq_wrapper_phys = virt_to_phys ( com32_irq_wrapper );
>> +
>> +       for ( i = 0; i < COM32_NUM_IDT_ENTRIES; i++ ) {
>> +               uint32_t ijb_address = virt_to_phys ( &ijb[i] );
>> +
>> +               idt[i].offset_low = ijb_address & 0xFFFF;
>> +               idt[i].selector = PHYSICAL_CS;
>> +               idt[i].flags = IDT_INTERRUPT_GATE_FLAGS;
>> +               idt[i].offset_high = ijb_address >> 16;
>> +
>> +               ijb[i].pusha_instruction = IJB_PUSHA;
>> +               ijb[i].mov_instruction = IJB_MOV_AL_IMM8;
>> +               ijb[i].mov_value = i;
>> +               ijb[i].jump_instruction = IJB_JMP_REL32;
>> +               ijb[i].jump_destination = com32_irq_wrapper_phys -
>> +                       virt_to_phys ( &ijb[i + 1] );
>> +       }
>> +
>> +       return 0;
>> +}
>> +
>> +/**
>>  * Prepare COM32 low memory bounce buffer
>>  * @v image            COM32 image
>>  * @ret rc             Return status code
>> @@ -269,6 +335,11 @@ static int com32_load ( struct image *image ) {
>>                return rc;
>>        }
>>
>> +       /* Set up IDT */
>> +       if ( ( rc = comboot_prepare_idt ( image ) ) != 0 ) {
>> +               return rc;
>> +       }
>> +
>>        /* Prepare bounce buffer segment */
>>        if ( ( rc = comboot_prepare_bounce_buffer ( image ) ) != 0 ) {
>>                return rc;
>> diff --git a/src/arch/i386/include/comboot.h b/src/arch/i386/include/comboot.h
>> index 1232f0a..ce6990d 100644
>> --- a/src/arch/i386/include/comboot.h
>> +++ b/src/arch/i386/include/comboot.h
>> @@ -13,6 +13,50 @@ FILE_LICENCE ( GPL2_OR_LATER );
>>  #include <setjmp.h>
>>  #include <gpxe/in.h>
>>
>> +/** Descriptor in a 32-bit IDT */
>> +struct idt_descriptor {
>> +       uint16_t offset_low;
>> +       uint16_t selector;
>> +       uint16_t flags;
>> +       uint16_t offset_high;
>> +} PACKED;
>> +
>> +/** Operand for the LIDT instruction */
>> +struct idt_register {
>> +       uint16_t limit;
>> +       uint32_t base;
>> +} PACKED;
>> +
>> +/** Entry in the interrupt jump buffer */
>> +struct ijb_entry {
>> +       uint8_t pusha_instruction;
>> +       uint8_t mov_instruction;
>> +       uint8_t mov_value;
>> +       uint8_t jump_instruction;
>> +       uint32_t jump_destination;
>> +} PACKED;
>> +
>> +/** The x86 opcode for "pushal" */
>> +#define IJB_PUSHA 0x60
>> +
>> +/** The x86 opcode for "movb $imm8,%al" */
>> +#define IJB_MOV_AL_IMM8 0xB0
>> +
>> +/** The x86 opcode for "jmp rel32" */
>> +#define IJB_JMP_REL32 0xE9
>> +
>> +/** Flags that specify a 32-bit interrupt gate with DPL=0 */
>> +#define IDT_INTERRUPT_GATE_FLAGS 0x8E00
>> +
>> +/** Address of COM32 interrupt descriptor table */
>> +#define COM32_IDT 0x100000
>> +
>> +/** Number of entries in a fully populated IDT */
>> +#define COM32_NUM_IDT_ENTRIES 256
>> +
>> +/** Address of COM32 interrupt jump buffer */
>> +#define COM32_IJB 0x100800
>> +
>>  /** Segment used for COMBOOT PSP and image */
>>  #define COMBOOT_PSP_SEG 0x07C0
>>
>> @@ -109,6 +153,7 @@ extern void unhook_comboot_interrupts ( );
>>  extern void com32_intcall_wrapper ( );
>>  extern void com32_farcall_wrapper ( );
>>  extern void com32_cfarcall_wrapper ( );
>> +extern void com32_irq_wrapper ( );
>>
>>  /* Resolve a hostname to an (IPv4) address */
>>  extern int comboot_resolv ( const char *name, struct in_addr *address );
>> diff --git a/src/arch/i386/interface/syslinux/com32_call.c b/src/arch/i386/interface/syslinux/com32_call.c
>> index d2c3f91..d271cc7 100644
>> --- a/src/arch/i386/interface/syslinux/com32_call.c
>> +++ b/src/arch/i386/interface/syslinux/com32_call.c
>> @@ -188,3 +188,20 @@ int __asmcall com32_cfarcall ( uint32_t proc, physaddr_t stack, size_t stacksz )
>>
>>        return eax;
>>  }
>> +
>> +/**
>> + * IRQ handler
>> + */
>> +void __asmcall com32_irq ( uint32_t vector ) {
>> +       uint32_t *ivt_entry = phys_to_virt( vector * 4 );
>> +
>> +       __asm__ __volatile__ (
>> +               REAL_CODE ( "pushfw\n\t"
>> +                           "pushw %%cs\n\t"
>> +                           "pushw $com32_irq_return\n\t"
>> +                           "pushl %0\n\t"
>> +                           "retf\n"
>> +                           "com32_irq_return:\n\t" )
>> +               : /* no outputs */
>> +               : "r" ( *ivt_entry ) );
>> +}
>> diff --git a/src/arch/i386/interface/syslinux/com32_wrapper.S b/src/arch/i386/interface/syslinux/com32_wrapper.S
>> index 5c5bd13..84d1571 100644
>> --- a/src/arch/i386/interface/syslinux/com32_wrapper.S
>> +++ b/src/arch/i386/interface/syslinux/com32_wrapper.S
>> @@ -22,6 +22,26 @@ FILE_LICENCE ( GPL2_OR_LATER )
>>        .arch i386
>>        .code32
>>
>> +       /*
>> +        * This code is entered after running the following sequence out of
>> +        * the interrupt jump buffer:
>> +        *
>> +        * pushal
>> +        * movb $vector, %al
>> +        * jmp com32_irq_wrapper
>> +        */
>> +
>> +       .globl com32_irq_wrapper
>> +com32_irq_wrapper:
>> +
>> +       movzxb %al,%eax
>> +       pushl %eax
>> +       movl $com32_irq, %eax
>> +       call com32_wrapper
>> +       popl %eax
>> +       popal
>> +       iret
>> +
>>        .globl com32_farcall_wrapper
>>  com32_farcall_wrapper:
>>
>> @@ -43,10 +63,14 @@ com32_intcall_wrapper:
>>        /*jmp com32_wrapper*/ /* fall through */
>>
>>  com32_wrapper:
>> +       cli
>>
>>        /* Switch to internal virtual address space */
>>        call _phys_to_virt
>>
>> +       /* Switch to internal IDT (if we have one for debugging) */
>> +       lidt com32_internal_idtr
>> +
>>        mov %eax, (com32_helper_function)
>>
>>        /* Save external COM32 stack pointer */
>> @@ -74,9 +98,13 @@ com32_wrapper:
>>        movl %esp, (com32_internal_esp)
>>        movl (com32_external_esp), %esp
>>
>> +       /* Switch to com32 IDT */
>> +       lidt com32_external_idtr
>> +
>>        /* Switch to external flat physical address space */
>>        call _virt_to_phys
>>
>> +       sti
>>        ret
>>


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