triple fault when use data from bss

[TheLittleWho]

Hello everyone!

After many headaches, I managed to make the paging to work partially. I have only one problem, when I access uninitialized global variables I receive triple fault, but if I used only initialized global variables then everything is good. I think I could initialize all the global variables, but I want to know the cause of the problem. Partially, I know the problem, the global variables which are put into .data segment are loaded correct while those from the bss segment are not.
Note that the kernel is mapped in higher half (3GiB). I disassembled the .elf file and the code is:

Code: Select all

mov    (%esp),%ecx
add    $0x6167,%ecx

lea    0x4dc4(%ecx),%eax // here it's loaded the initialized global variable
movl   $0x0,(%eax)

mov    -0x38(%ecx),%eax // here it's loaded the UNinitialized global variable
movl   $0x5,(%eax)

Note that the value from eax is exactly physical address of the variable (should be the virtual address...)

here it's the code where are accessing the global variables: https://github.com/EnachescuAlin/PhiOS/ ... main.c#L27
here it's entry function: https://github.com/EnachescuAlin/PhiOS/ ... ot32.s#L29
here it's linker.ld: https://github.com/EnachescuAlin/PhiOS/ ... inker32.ld

[LtG]

A bit more background would be useful, like who/what loaded the executable into memory, is this in kernel or userspace? etc..

[MichaelFarthing]

Hello everyone!

Code: Select all

mov    (%esp),%ecx
add    $0x6167,%ecx

lea    0x4dc4(%ecx),%eax // here it's loaded the initialized global variable
movl   $0x0,(%eax)

mov    -0x38(%ecx),%eax // here it's loaded the UNinitialized global variable
movl   $0x5,(%eax)

Why are you using a load effective address instruction (lea) for initialised and a move (mov) instruction for uninitialised?

EDIT: Ignore what I say below - I missed the brackets round the esp

It also looks odd to me that you are calculating an address relative to the stack pointer - that's (a) unusual and (b) likely to be impossibly unmaintainable.

[TheLittleWho]

A bit more background would be useful, like who/what loaded the executable into memory, is this in kernel or userspace? etc..

I'm using grub and it's in kernel space. Let me know what else you want to know.

[TheLittleWho]

Hello everyone!

Code: Select all

mov    (%esp),%ecx
add    $0x6167,%ecx

lea    0x4dc4(%ecx),%eax // here it's loaded the initialized global variable
movl   $0x0,(%eax)

mov    -0x38(%ecx),%eax // here it's loaded the UNinitialized global variable
movl   $0x5,(%eax)

Why are you using a load effective address instruction (lea) for initialised and a move (mov) instruction for uninitialised?
It also looks odd to me that you are calculating an address relative to the stack pointer - that's (a) unusual and (b) likely to be impossibly unmaintainable.

That code is generated by the compiler, is not written by me. I'm using gcc cross compiler from here: http://wiki.osdev.org/GCC_Cross-Compiler
These are the flags used for compiler and linker:
set(CMAKE_C_FLAGS " ${CMAKE_C_FLAGS} -std=gnu11 -ffreestanding -O2 -Wall -Wextra -fpic")
set(CMAKE_LINKER_FLAGS "-T ${LINKER_FILE} -O2 -nostdlib -z max-page-size=0x1000")

[LtG]

A bit more background would be useful, like who/what loaded the executable into memory, is this in kernel or userspace? etc..

I'm using grub and it's in kernel space. Let me know what else you want to know.

I was hoping for a bit more than that. You did notice the "etc"?

Is the issue that:
- Grub loads your elf (kernel/bootstrap) and accessing the .bss in that causes a #TF
- Grub has loaded your kernel ages ago and now you have a userspace process running, but it does a syscall and that somehow causes a #TF
- etc

Explain the actual problem. Otherwise we're just guessing at what the problem is, let alone how to solve it. It would be useful to provide such a detailed description in the first place, not shifting the burden to "us" to ask you specific questions about 10 different things..

[TheLittleWho]

So, grub call my entry function and I initialized the following "components": vga, gdt, tss, idt, pic, pit, pma
For each "component" I called an init function which (mostly) initialized the global variables used by that component. (in this moment everything is good).
Then I initialized the PD with the PTs and pages used in this moment (the kernel is mapped at 0xC0000000), I disabled the interrupts and enabled the paging then I did absolute jump in the higher half and I moved the virtual address of the kernel stack in esp and called kernel_main() which is a function written in C.
Here I can't used global variables which wasn't initialized in code (.bss segment), but I can use global variables which was initialized (.data segment) also I can use local variables.

I written an program (on linux) in C and I disassembled it and I saw that the global variables (both initialized and uninitialized) are accessed relative to rip register while in my .elf file they are accessed relative to esp.

[simeonz]

I written an program (on linux) in C and I disassembled it and I saw that the global variables (both initialized and uninitialized) are accessed relative to rip register while in my .elf file they are accessed relative to esp.

This esp approach is for x86, while the rip approach is for x64. There is no eip relative addressing in the 32-bit instruction set, so the return address is taken from the stack and used as base for code-relative accesses.

So, grub call my entry function and I initialized the following "components": vga, gdt, tss, idt, pic, pit, pma
For each "component" I called an init function which (mostly) initialized the global variables used by that component. (in this moment everything is good).
Then I initialized the PD with the PTs and pages used in this moment (the kernel is mapped at 0xC0000000), I disabled the interrupts and enabled the paging then I did absolute jump in the higher half and I moved the virtual address of the kernel stack in esp and called kernel_main() which is a function written in C.
Here I can't used global variables which wasn't initialized in code (.bss segment), but I can use global variables which was initialized (.data segment) also I can use local variables.

How much of the physical memory do you map at 0xC0000000. I am speaking slightly on a limb, but since the .bss is not part of the file, I suspect that you may have taken something related to the file size rather then the address space size when deciding how much memory to map.

[LtG]

Initially you said: "when I access uninitialized global variables I receive triple fault".

What's the cause of the triple fault? Faulting #PF, which causes #DF which causes #TF?

Why does the initial #PF happen? Is it caused by using .bss and the .bss has invalid values instead of zeroes or is the access itself causing #PF which would mean your PD/PT's is at fault..?

[TheLittleWho]

How much of the physical memory do you map at 0xC0000000. I am speaking slightly on a limb, but since the .bss is not part of the file, I suspect that you may have taken something related to the file size rather then the address space size when deciding how much memory to map.

I have 2 variables in linker.ld for kernel begin and kernel end. Before enabling paging, if i get a pointer to a variable from .bss and after enabling paging add 0xBFF00000 to pointer and use it, it's ok. So, bss is mapped virtual, my problem is that when I access the global variable from bss, it's accessed physical address instead of virtual address...

[TheLittleWho]

Initially you said: "when I access uninitialized global variables I receive triple fault".

What's the cause of the triple fault? Faulting #PF, which causes #DF which causes #TF?

Why does the initial #PF happen? Is it caused by using .bss and the .bss has invalid values instead of zeroes or is the access itself causing #PF which would mean your PD/PT's is at fault..?

It's a page fault -> double fault -> triple fault... I said in the first post, when access a variable from bss, her physical address is loaded, instead of the virtual address. I put in the first post 6 lines of asm code on which I took from .elf file. I want to know why for the variables from .data the address it's loaded some way and for the variables from .bss the address it's loaded differently. For variables from bss it's accessed physical address instead of virtual address, why?

[xenos]

I have 2 variables in linker.ld for kernel begin and kernel end. Before enabling paging, if i get a pointer to a variable from .bss and after enabling paging add 0xBFF00000 to pointer and use it, it's ok. So, bss is mapped virtual, my problem is that when I access the global variable from bss, it's accessed physical address instead of virtual address...

In that case it would be helpful to see your linker script and the output of "your-target-objdump -x yourkernel" (replace "your-target" with your target triplet and "yourkernel" with your kernel file name), to see where symbols actually end up in the file.

[TheLittleWho]

The linker script and the dump are shown below.

Code: Select all

bin/x86_32/phios.elf:     file format elf32-i386
bin/x86_32/phios.elf
architecture: i386, flags 0x00000112:
EXEC_P, HAS_SYMS, D_PAGED
start address 0x00100018

Program Header:
    LOAD off    0x00001000 vaddr 0x00100000 paddr 0x00100000 align 2**12
         filesz 0x00005530 memsz 0x00005530 flags r-x
    LOAD off    0x00007000 vaddr 0x00106000 paddr 0x00106000 align 2**12
         filesz 0x00000248 memsz 0x0000a020 flags rw-

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         000049c8  00100000  00100000  00001000  2**12
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .text.__x86.get_pc_thunk.cx 00000004  001049c8  001049c8  000059c8  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .text.__x86.get_pc_thunk.bx 00000004  001049cc  001049cc  000059cc  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  3 .rodata       00000058  00105000  00105000  00006000  2**12
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .rodata.str1.1 00000330  00105058  00105058  00006058  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  5 .rodata.str1.4 000001a8  00105388  00105388  00006388  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  6 .data         00000159  00106000  00106000  00007000  2**12
                  CONTENTS, ALLOC, LOAD, DATA
  7 .got          000000e0  0010615c  0010615c  0000715c  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  8 .got.plt      0000000c  0010623c  0010623c  0000723c  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  9 .bss          00009020  00107000  00107000  00007248  2**12
                  ALLOC
SYMBOL TABLE:
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001049cc l    d  .text.__x86.get_pc_thunk.bx	00000000 .text.__x86.get_pc_thunk.bx
00105000 l    d  .rodata	00000000 .rodata
00105058 l    d  .rodata.str1.1	00000000 .rodata.str1.1
00105388 l    d  .rodata.str1.4	00000000 .rodata.str1.4
00106000 l    d  .data	00000000 .data
0010615c l    d  .got	00000000 .got
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00107000 l    d  .bss	00000000 .bss
00000000 l    df *ABS*	00000000 CMakeFiles/phios.elf.dir/kernel/src/arch/x86/boot32.s.o
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0010b000 l       .bss	00000000 stack_top
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00000000 l    df *ABS*	00000000 phi_main.c
00000000 l    df *ABS*	00000000 paa.c
0010b004 l     O .bss	00000004 g_placementAddress
00000000 l    df *ABS*	00000000 bitmap_pma.c
00000000 l    df *ABS*	00000000 pmm.c
00000000 l    df *ABS*	00000000 asm_io.c
00000000 l    df *ABS*	00000000 gdt32.c
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00000000 l    df *ABS*	00000000 handlers32.c
00000000 l    df *ABS*	00000000 pit.c
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00100000 g       .text	00000000 linker_textStart
00103491 g     F .text	00000000 isr_32_30
00103469 g     F .text	00000000 isr_32_26
00110008 g     O .bss	00000004 g_shiftLayout

Code: Select all

KERNEL_VMA = 0x100000;
KERNEL_PMA = 0x100000;

OUTPUT_FORMAT("elf32-i386")
ENTRY(_start32)
SECTIONS
{
    /* Place kernel at 1 MiB */
    . = KERNEL_PMA;

    linker_kernelStart = .;

    /* Read-Only sections */
    .text BLOCK(4K) : ALIGN(4K)
    {
        linker_textStart = .;

        CMakeFiles/phios.elf.dir/kernel/src/arch/x86/boot32.s.o (.text)
        *(EXCLUDE_FILE(CMakeFiles/phios.elf.dir/kernel/src/arch/x86/boot32.s.o) .text)

        linker_textEnd = .;
    }

    .rodata BLOCK(4K) : ALIGN(4K)
    {
        linker_rodataStart = .;

    	*(.rodata)

        linker_rodataEnd = .;
    }

    /* Read-Write sections */
    .data BLOCK(4K) : ALIGN(4K)
    {
        linker_dataStart = .;

        *(.data)

        linker_dataEnd = .;
    }

    .bss BLOCK(4K) : ALIGN(4K)
    {
        linker_bssStart = .;

        *(.bss)
        *(COMMON)

        linker_bssEnd = .;
    }

    linker_kernelEnd = .;

    /* Discard useless sections */ 
    /DISCARD/ :
    {
        *(.note)
        *(.comment)
        *(.eh_frame) /* Add this if C++ support is needed */
    }
}

[simeonz]

So, bss is mapped virtual, my problem is that when I access the global variable from bss, it's accessed physical address instead of virtual address...

Apparently, you have to pass "-pic" or "-pie" option to the final ld. I am confused by this, actually.

The linker can bypass GOT when the output is ET_EXEC. The original relocation, which is R_386_GOT32, is semantically converted to R_386_GOTOFF. Then it is preapplied and elided, which is possible, because the relative virtual layout is fixed in ET_EXEC and the offset of any variable from GOT is constant. Additionally a mov opcode in the relocated instruction has to be converted converted to lea opcode during link time. This is where the lea instruction in the assembly comes from. You can check the intermediate object file and see that it is still a mov instruction before the optimization.

The problem in your case is that the optimization is used only for .data, whilst for .bss the GOT entries are computed by the linker in terms of the fixed virtual addresses, thus causing the .bss variables to use GOT, but with fixed entries (and no corresponding R_386_GLOB_DAT relocations as with "-shared"). As a final resort you could have used "-q" to keep the relocations and processed them before switching paging on. Anyway, passing "-pic" to ld suddenly triggers GOT elision for both .data and .bss.

I also couldn't reproduce the relative addressing technique from your code. That is, with pic, normally the get_pc_thunk function is used to retrieve the current eip, because the return address is not a fixed quantity. I tried calling a static function and expected some kind of call graph analysis to show that it is called from one place, but the compiler still uses get_pc_thunk.

[xenos]

First of all, your KERNEL_VMA should rather be 0xC0000000, not 0x100000. Further, to have your sections at a different virtual and physical location, you will need to give the physical address using the AT directive. See this linker script for an example:
http://wiki.osdev.org/Higher_Half_x86_B ... #linker.ld