@ -37,7 +37,7 @@ Addresses of each of the interrupt handlers are maintained in a special location
BUG_ON((unsigned)n > 0xFF);
```
You can find this check within the Linux kernel source code related to interrupt setup (eg. The `set_intr_gate`, `void set_system_intr_gate` in [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/desc.h)). First 32 vector numbers from `0` to `31` are reserved by the processor and used for the processing of architecture-defined exceptions and interrupts. You can find the table with the description of these vector numbers in the second part of the Linux kernel initialization process - [Early interrupt and exception handling](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-2.html). Vector numbers from `32` to `255` are designated as user-defined interrupts and are not reserved by the processor. These interrupts are generally assigned to external I/O devices to enable those devices to send interrupts to the processor.
You can find this check within the Linux kernel source code related to interrupt setup (eg. The `set_intr_gate`, `void set_system_intr_gate` in [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/desc.h)). The first 32 vector numbers from `0` to `31` are reserved by the processor and used for the processing of architecture-defined exceptions and interrupts. You can find the table with the description of these vector numbers in the second part of the Linux kernel initialization process - [Early interrupt and exception handling](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-2.html). Vector numbers from `32` to `255` are designated as user-defined interrupts and are not reserved by the processor. These interrupts are generally assigned to external I/O devices to enable those devices to send interrupts to the processor.
Now let's talk about the types of interrupts. Broadly speaking, we can split interrupts into 2 major classes:
@ -154,12 +154,12 @@ static void setup_idt(void)
}
```
from the [arch/x86/boot/pm.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pm.c). The `Interrupt Descriptor table` can be located anywhere in the linear address space and the base address of it must be aligned on an 8-byte boundary on `x86` or 16-byte boundary on `x86_64`. Base address of the `IDT` is stored in the special register - `IDTR`. There are two instructions on `x86`-compatible processors to modify the `IDTR` register:
from the [arch/x86/boot/pm.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pm.c). The `Interrupt Descriptor table` can be located anywhere in the linear address space and the base address of it must be aligned on an 8-byte boundary on `x86` or 16-byte boundary on `x86_64`. The base address of the `IDT` is stored in the special register - `IDTR`. There are two instructions on `x86`-compatible processors to modify the `IDTR` register:
* `LIDT`
* `SIDT`
The first instruction `LIDT` is used to load the base-address of the `IDT` i.e. the specified operand into the `IDTR`. The second instruction `SIDT` is used to read and store the contents of the `IDTR` into the specified operand. The `IDTR` register is 48-bits on the `x86` and contains following information:
The first instruction `LIDT` is used to load the base-address of the `IDT` i.e. the specified operand into the `IDTR`. The second instruction `SIDT` is used to read and store the contents of the `IDTR` into the specified operand. The `IDTR` register is 48-bits on the `x86` and contains the following information:
The `PAGE_SIZE` is `4096`-bytes and the `THREAD_SIZE_ORDER` depends on the `KASAN_STACK_ORDER`. As we can see, the `KASAN_STACK` depends on the `CONFIG_KASAN` kernel configuration parameter and equals to the:
The `PAGE_SIZE` is `4096`-bytes and the `THREAD_SIZE_ORDER` depends on the `KASAN_STACK_ORDER`. As we can see, the `KASAN_STACK` depends on the `CONFIG_KASAN` kernel configuration parameter and is defined as:
```C
#ifdef CONFIG_KASAN
@ -304,9 +304,9 @@ union irq_stack_union {
};
```
The first `irq_stack` field is a 16 kilobytes array. Also you can see that `irq_stack_union` contains structure with the two fields:
The first `irq_stack` field is a 16 kilobytes array. Also you can see that `irq_stack_union` contains a structure with the two fields:
* `gs_base` - The `gs` register always points to the bottom of the `irqstack` union. On the `x86_64`, the `gs` register is shared by per-cpu area and stack canary (more about `per-cpu` variables you can read in the special [part](http://0xax.gitbooks.io/linux-insides/content/Concepts/per-cpu.html)). All per-cpu symbols are zero based and the `gs` points to the base of per-cpu area. You already know that [segmented memory model](http://en.wikipedia.org/wiki/Memory_segmentation) is abolished in the long mode, but we can set base address for the two segment registers - `fs` and `gs` with the [Model specific registers](http://en.wikipedia.org/wiki/Model-specific_register) and these registers can be still be used as address registers. If you remember the first [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-1.html) of the Linux kernel initialization process, you can remember that we have set the `gs` register:
* `gs_base` - The `gs` register always points to the bottom of the `irqstack` union. On the `x86_64`, the `gs` register is shared by per-cpu area and stack canary (more about `per-cpu` variables you can read in the special [part](http://0xax.gitbooks.io/linux-insides/content/Concepts/per-cpu.html)). All per-cpu symbols are zero based and the `gs` points to the base of the per-cpu area. You already know that [segmented memory model](http://en.wikipedia.org/wiki/Memory_segmentation) is abolished in the long mode, but we can set the base address for the two segment registers - `fs` and `gs` with the [Model specific registers](http://en.wikipedia.org/wiki/Model-specific_register) and these registers can be still be used as address registers. If you remember the first [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-1.html) of the Linux kernel initialization process, you can remember that we have set the `gs` register:
```assembly
movl $MSR_GS_BASE,%ecx
@ -323,7 +323,7 @@ GLOBAL(initial_gs)
```
* `stack_canary` - [Stack canary](http://en.wikipedia.org/wiki/Stack_buffer_overflow#Stack_canaries) for the interrupt stack is a `stack protector`
to verify that the stack hasn't been overwritten. Note that `gs_base` is an 40 bytes array. `GCC` requires that stack canary will be on the fixed offset from the base of the `gs` and its value must be `40` for the `x86_64` and `20` for the `x86`.
to verify that the stack hasn't been overwritten. Note that `gs_base` is a 40 bytes array. `GCC` requires that stack canary will be on the fixed offset from the base of the `gs` and its value must be `40` for the `x86_64` and `20` for the `x86`.
The `irq_stack_union` is the first datum in the `percpu` area, we can see it in the `System.map`:
@ -343,7 +343,7 @@ We can see its definition in the code:
Now, its time to look at the initialization of the `irq_stack_union`. Besides the `irq_stack_union` definition, we can see the definition of the following per-cpu variables in the [arch/x86/include/asm/processor.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/processor.h):
Now, it's time to look at the initialization of the `irq_stack_union`. Besides the `irq_stack_union` definition, we can see the definition of the following per-cpu variables in the [arch/x86/include/asm/processor.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/processor.h):
```C
DECLARE_PER_CPU(char *, irq_stack_ptr);
@ -374,7 +374,7 @@ for_each_possible_cpu(cpu) {
}
```
Here we go over all the CPUs on-by-one and setup `irq_stack_ptr`. This turns out to be equal to the top of the interrupt stack minus `64`. Why `64`?TODO [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/common.c) source code file is following:
Here we go over all the CPUs one-by-one and setup `irq_stack_ptr`. This turns out to be equal to the top of the interrupt stack minus `64`. Why `64`?TODO [arch/x86/kernel/cpu/common.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/common.c) source code file is following:
and as we already know `gs` register points to the bottom of the interrupt stack:
and as we already know the `gs` register points to the bottom of the interrupt stack:
```assembly
movl $MSR_GS_BASE,%ecx
@ -399,7 +399,7 @@ and as we already know `gs` register points to the bottom of the interrupt stack
.quad INIT_PER_CPU_VAR(irq_stack_union)
```
Here we can see the `wrmsr` instruction which loads the data from `edx:eax` into the [Model specific register](http://en.wikipedia.org/wiki/Model-specific_register) pointed by the `ecx` register. In our case model specific register is `MSR_GS_BASE` which contains the base address of the memory segment pointed by the `gs` register. `edx:eax` points to the address of the `initial_gs` which is the base address of our `irq_stack_union`.
Here we can see the `wrmsr` instruction which loads the data from `edx:eax` into the [Model specific register](http://en.wikipedia.org/wiki/Model-specific_register) pointed by the `ecx` register. In our case the model specific register is `MSR_GS_BASE` which contains the base address of the memory segment pointed by the `gs` register. `edx:eax` points to the address of the `initial_gs` which is the base address of our `irq_stack_union`.
We already know that `x86_64` has a feature called `Interrupt Stack Table` or `IST` and this feature provides the ability to switch to a new stack for events non-maskable interrupt, double fault and etc... There can be up to seven `IST` entries per-cpu. Some of them are:
Dave Willmer
9 years ago