Here `__end_of_permanent_fixed_addresses` is an element of the `fixed_addresses` enum and as I wrote above: Every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses`. `PAGE_SHIFT` determines the size of a page. For example size of the one page we can get with the `1 << PAGE_SHIFT` expression.
Here `__end_of_permanent_fixed_addresses` is an element of the `fixed_addresses` enum and as I wrote above, every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses`. `PAGE_SHIFT` determines the size of a page. For example size of the one page we can get with the `1 << PAGE_SHIFT` expression.
In our case we need to get the size of the fix-mapped area, but not only of one page, that's why we are using `__end_of_permanent_fixed_addresses` for getting the size of the fix-mapped area. The `__end_of_permanent_fixed_addresses` is the last index of the `fixed_addresses` enum or in other words the `__end_of_permanent_fixed_addresses` contains amount of pages in a fixed-mapped area. So if multiply value of the `__end_of_permanent_fixed_addresses` on a page size value we will get size of fix-mapped area. In my case it's a little more than `536` kilobytes. In your case it might be a different number, because the size depends on amount of the fix-mapped addresses which are depends on your kernel's configuration.
In our case we need to get the size of the fix-mapped area, but not only of one page, that's why we are using `__end_of_permanent_fixed_addresses` for getting the size of the fix-mapped area. The `__end_of_permanent_fixed_addresses` is the last index of the `fixed_addresses` enum or in other words the `__end_of_permanent_fixed_addresses` contains amount of pages in a fixed-mapped area. So if we multiply the value of the `__end_of_permanent_fixed_addresses` on a page size value we will get size of fix-mapped area. In my case it's a little more than `536` kilobytes. In your case it might be a different number, because the size depends on amount of the fix-mapped addresses which depends on your kernel configuration.
The second `FIXADDR_START` macro just subtracts the fix-mapped area size from the last address of the fix-mapped area to get its base virtual address. `FIXADDR_TOP` is a rounded up address from the base address of the [vsyscall](https://lwn.net/Articles/446528/) space:
@ -46,8 +46,8 @@ The second `FIXADDR_START` macro just subtracts the fix-mapped area size from th
The `fixed_addresses` enums are used as an index to get the virtual address by the `fix_to_virt` function. Implementation of this function is easy:
The `fixed_addresses` enums are used as indexes to get the virtual addresses by the `fix_to_virt` function. Implementation of this function is easy:
```C
static __always_inline unsigned long fix_to_virt(const unsigned int idx)
{
@ -65,7 +65,7 @@ first of all it checks that the index given for the `fixed_addresses` enum is no
Here we shift left the given index of a `fix-mapped` area on the `PAGE_SHIFT` which determines size of a page as I wrote above and subtract it from the `FIXADDR_TOP` which is the highest address of the `fix-mapped` area:
```
+-----------------+
+-----------------+
| PAGE 1 | FIXADDR_TOP (virt address)
| PAGE 2 |
| PAGE 3 |
@ -104,7 +104,7 @@ ioremap
The Linux kernel provides many different primitives to manage memory. For this moment we will touch `I/O memory`. Every device is controlled by reading/writing from/to its registers. For example a driver can turn off/on a device by writing to its registers or get the state of a device by reading from its registers. Besides registers, many devices have buffers where a driver can write something or read from there. As we know for this moment there are two ways to access device's registers and data buffers:
* through the I/O ports;
* mapping of the all registers to the memory address space;
* mapping of all the registers to the memory address space;
In the first case every control register of a device has a number of input and output port. A device driver can read from a port and write to it with two `in` and `out` instructions which we already saw. If you want to know about currently registered port regions, you can learn about them by accessing `/proc/ioports`:
@ -208,7 +208,7 @@ Here `RTC_IO_EXTENT` is the size of the memory region and it is `0x8`, `"rtc"` i
#define RTC_PORT(x) (0x70 + (x))
```
So with the `request_region(RTC_PORT(0), size, "rtc")` we register a memory region that starts at `0x70` and and has a size of `0x8`. Let's look at `/proc/ioports`:
So with the `request_region(RTC_PORT(0), size, "rtc")` we register a memory region that starts at `0x70` and has a size of `0x8`. Let's look at `/proc/ioports`:
```
~$ sudo cat /proc/ioports | grep rtc
@ -499,7 +499,7 @@ static inline void __native_flush_tlb_single(unsigned long addr)
}
```
or call `__flush_tlb` which just updates the `cr3` register as we have seen. After this step execution of the `__early_set_fixmap` function is finished and we can go back to the `__early_ioremap` implementation. When we have set up the fixmap area for the given address, we need to save the base virtual address of the I/O Re-mapped area in the `prev_map` using the `slot` index:
or call `__flush_tlb` which just updates the `cr3` register as we have seen. After this step execution of the `__early_set_fixmap` function is finished and we can go back to the `__early_ioremap` implementation. When we have set up the fixmap area for the given address, we need to save the base virtual address of the I/O remapped area in the `prev_map` using the `slot` index:
can show us lists of currently registered port regions used for input or output communication with a device. All memory-mapped I/O addresses are not used by the kernel directly. So, before the Linux kernel can use such memory, it must map it to the virtual memory space which is the main purpose of the `ioremap` mechanism. Note that we saw only early `ioremap` in the previous [part](https://0xax.gitbook.io/linux-insides/summary/mm/linux-mm-2). Soon we will look at the implementation of the non-early `ioremap` function. But before this we must learn other things, like a different types of memory allocators and etc., because in other way it will be very difficult to understand it.
can show us lists of currently registered port regions used for input or output communication with a device. All memory-mapped I/O addresses are not used by the kernel directly. So, before the Linux kernel can use such memory, it must map it to the virtual memory space which is the main purpose of the `ioremap` mechanism. Note that we saw only early `ioremap` in the previous [part](https://0xax.gitbook.io/linux-insides/summary/mm/linux-mm-2). Soon we will look at the implementation of the non-early `ioremap` function. But before this we must learn other things, like different types of memory allocators and etc., because otherwise it will be very difficult to understand it.
So, before we will move on to the non-early [memory management](https://en.wikipedia.org/wiki/Memory_management) of the Linux kernel, we will see some mechanisms which provide special abilities for [debugging](https://en.wikipedia.org/wiki/Debugging), check of [memory leaks](https://en.wikipedia.org/wiki/Memory_leak), memory control and etc. It will be easier to understand how memory management arranged in the Linux kernel after learning of all of these things.
@ -90,7 +90,7 @@ Here we allocate memory for the `A` structure and tries to print value of the `a
gcc test.c -o test
```
The [compiler](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) will not show us warning that `a` filed is not uninitialized. But if we will run this program with [valgrind](https://en.wikipedia.org/wiki/Valgrind) tool, we will see the following output:
The [compiler](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) will not show us warning that `a` field is not uninitialized. But if we will run this program with [valgrind](https://en.wikipedia.org/wiki/Valgrind) tool, we will see the following output:
```
~$ valgrind --leak-check=yes ./test
@ -98,12 +98,12 @@ The [compiler](https://en.wikipedia.org/wiki/GNU_Compiler_Collection) will not s
==28469== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==28469== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==28469== Command: ./test
==28469==
==28469==
==28469== Conditional jump or move depends on uninitialised value(s)
==28469== at 0x4E820EA: vfprintf (in /usr/lib64/libc-2.22.so)
==28469== by 0x4E88D48: printf (in /usr/lib64/libc-2.22.so)
==28469== by 0x4005B9: main (in /home/alex/test)
==28469==
==28469==
==28469== Use of uninitialised value of size 8
==28469== at 0x4E7E0BB: _itoa_word (in /usr/lib64/libc-2.22.so)
==28469== by 0x4E8262F: vfprintf (in /usr/lib64/libc-2.22.so)
@ -122,7 +122,7 @@ To enable this mechanism in the Linux kernel, you need to enable the `CONFIG_KME
So, there is no anything which is specific for other architectures.
So, there isn't anything which is specific for other architectures.
Ok, so we know that `kmemcheck` provides mechanism to check usage of `uninitialized memory` in the Linux kernel and how to enable it. How it does these checks? When the Linux kernel tries to allocate some memory i.e. something is called like this:
@ -148,7 +148,7 @@ Ok, so we know that `kmemcheck` provides mechanism to check usage of `uninitiali
or in other words somebody wants to access a [page](https://en.wikipedia.org/wiki/Page_%28computer_memory%29), a [page fault](https://en.wikipedia.org/wiki/Page_fault) exception is generated. This is achieved by the fact that the `kmemcheck` marks memory pages as `non-present` (more about this you can read in the special part which is devoted to [Paging](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-1)). If a `page fault` exception is occurred, the exception handler knows about it and in a case when the `kmemcheck` is enabled it transfers control to it. After the `kmemcheck` will finish its checks, the page will be marked as `present` and the interrupted code will be able to continue execution. There is little subtlety in this chain. When the first instruction of interrupted code will be executed, the `kmemcheck` will mark the page as `non-present` again. In this way next access to memory will be caught again.
or in other words somebody wants to access a [page](https://en.wikipedia.org/wiki/Page_%28computer_memory%29), a [page fault](https://en.wikipedia.org/wiki/Page_fault) exception is generated. This is achieved by the fact that the `kmemcheck` marks memory pages as `non-present` (more about this you can read in the special part which is devoted to [Paging](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-1)). If a `page fault` exception occurred, the exception handler knows about it and in a case when the `kmemcheck` is enabled it transfers control to it. After the `kmemcheck` will finish its checks, the page will be marked as `present` and the interrupted code will be able to continue execution. There is little subtlety in this chain. When the first instruction of interrupted code will be executed, the `kmemcheck` will mark the page as `non-present` again. In this way next access to memory will be caught again.
We just considered the `kmemcheck` mechanism from theoretical side. Now let's consider how it is implemented in the Linux kernel.
@ -215,9 +215,9 @@ if (!kmemcheck_selftest()) {
printk(KERN_INFO "kmemcheck: Initialized\n");
```
and return with the `EINVAL` if this check is failed. The `kmemcheck_selftest` function checks sizes of different memory access related [opcodes](https://en.wikipedia.org/wiki/Opcode) like `rep movsb`, `movzwq` and etc. If sizes of opcodes are equal to expected sizes, the `kmemcheck_selftest` will return `true` and `false`in other way.
and return with the `EINVAL` if this check is failed. The `kmemcheck_selftest` function checks sizes of different memory access related [opcodes](https://en.wikipedia.org/wiki/Opcode) like `rep movsb`, `movzwq` and etc. If sizes of opcodes are equal to expected sizes, the `kmemcheck_selftest` will return `true` and `false`otherwise.
So, here we check that the if `kmemcheck` is enabled and the `SLAB_NOTRACK` bit is not set in flags we set `non-present` bit for the just allocated page. The `SLAB_NOTRACK` bit tell us to not track uninitialized memory. Additionally we check if a cache object has constructor (details will be considered in next parts) we mark allocated page as uninitialized or unallocated in other way. The `kmemcheck_alloc_shadow` function is defined in the [mm/kmemcheck.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/mm/kmemcheck.c) source code file and does following things:
So, here we check that the if `kmemcheck` is enabled and the `SLAB_NOTRACK` bit is not set in flags we set `non-present` bit for the just allocated page. The `SLAB_NOTRACK` bit tell us to not track uninitialized memory. Additionally we check if a cache object has constructor (details will be considered in next parts) we mark allocated page as uninitialized or unallocated otherwise. The `kmemcheck_alloc_shadow` function is defined in the [mm/kmemcheck.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/mm/kmemcheck.c) source code file and does following things:
```C
void kmemcheck_alloc_shadow(struct page *page, int order, gfp_t flags, int node)
Here we go through all pages and and tries to get `page table entry` for each page. If this operation was successful, we unset present bit and set hidden bit in each page. In the end we flush [translation lookaside buffer](https://en.wikipedia.org/wiki/Translation_lookaside_buffer), because some pages was changed. From this point allocated pages are tracked by the `kmemcheck`. Now, as `present` bit is unset, the [page fault](https://en.wikipedia.org/wiki/Page_fault) execution will be occurred right after the `kmalloc` will return pointer to allocated space and a code will try to access this memory.
Here we go through all pages and try to get `page table entry` for each page. If this operation was successful, we unset present bit and set hidden bit in each page. In the end we flush [translation lookaside buffer](https://en.wikipedia.org/wiki/Translation_lookaside_buffer), because some pages was changed. From this point allocated pages are tracked by the `kmemcheck`. Now, as `present` bit is unset, the [page fault](https://en.wikipedia.org/wiki/Page_fault) execution will be occurred right after the `kmalloc` will return pointer to allocated space and a code will try to access this memory.
As you may remember from the [second part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-2) of the Linux kernel initialization chapter, the `page fault` handler is located in the [arch/x86/mm/fault.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/fault.c) source code file and represented by the `do_page_fault` function. We can see following check from the beginning of the `do_page_fault` function:
@ -296,7 +296,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
}
```
The `kmemcheck_active` gets `kmemcheck_context` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) structure and return the result of comparison of the `balance` field of this structure with zero:
The `kmemcheck_active` gets `kmemcheck_context` [per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) structure and returns the result of comparison of the `balance` field of this structure with zero:
```
bool kmemcheck_active(struct pt_regs *regs)
@ -314,7 +314,7 @@ if (kmemcheck_fault(regs, address, error_code))
return;
```
First of all the `kmemcheck_fault` function checks that the fault was occurred by the correct reason. At first we check the [flags register](https://en.wikipedia.org/wiki/FLAGS_register) and check that we are in normal kernel mode:
First of all the `kmemcheck_fault` function checks that the fault occurred by the correct reason. At first we check the [flags register](https://en.wikipedia.org/wiki/FLAGS_register) and check that we are in normal kernel mode:
```C
if (regs->flags & X86_VM_MASK)
@ -323,7 +323,7 @@ if (regs->cs != __KERNEL_CS)
return false;
```
If these checks wasn't successful we return from the `kmemcheck_fault` function as it was not `kmemcheck` related page fault. After this we try to lookup a `page table entry` related to the faulted address and if we can't find it we return:
If these checks weren't successful we return from the `kmemcheck_fault` function as it was not `kmemcheck` related page fault. After this we try to lookup a `page table entry` related to the faulted address and if we can't find it we return:
```C
pte = kmemcheck_pte_lookup(address);
@ -331,7 +331,7 @@ if (!pte)
return false;
```
Last two steps of the `kmemcheck_fault` function is to call the `kmemcheck_access` function which check access to the given page and show addresses again by setting present bit in the given page. The `kmemcheck_access` function does all main job. It check current instruction which caused a page fault. If it will find an error, the context of this error will be saved by `kmemcheck` to the ring queue:
Last two steps of the `kmemcheck_fault` function is to call the `kmemcheck_access` function which check access to the given page and show addresses again by setting present bit in the given page. The `kmemcheck_access` function does all main job. It checks current instruction which caused a page fault. If it finds an error, the context of this error will be saved by `kmemcheck` to the ring queue:
which runs the `do_wakeup` function from the [arch/x86/mm/kmemcheck/error.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/kmemcheck/error.c) source code file when it will be scheduled to run.
which runs the `do_wakeup` function from the [arch/x86/mm/kmemcheck/error.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/mm/kmemcheck/error.c) source code file when it is scheduled to run.
The `do_wakeup` function will call the `kmemcheck_error_recall` function which will print errors collected by `kmemcheck`. As we already saw the:
Renaud Germain
2 years ago