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1 | Kernel stacks on x86-64 bit |
2 | --------------------------- | |
3 | ||
352f7bae AK |
4 | Most of the text from Keith Owens, hacked by AK |
5 | ||
6 | x86_64 page size (PAGE_SIZE) is 4K. | |
7 | ||
8 | Like all other architectures, x86_64 has a kernel stack for every | |
9 | active thread. These thread stacks are THREAD_SIZE (2*PAGE_SIZE) big. | |
10 | These stacks contain useful data as long as a thread is alive or a | |
11 | zombie. While the thread is in user space the kernel stack is empty | |
12 | except for the thread_info structure at the bottom. | |
13 | ||
14 | In addition to the per thread stacks, there are specialized stacks | |
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15 | associated with each CPU. These stacks are only used while the kernel |
16 | is in control on that CPU; when a CPU returns to user space the | |
17 | specialized stacks contain no useful data. The main CPU stacks are: | |
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18 | |
19 | * Interrupt stack. IRQSTACKSIZE | |
20 | ||
21 | Used for external hardware interrupts. If this is the first external | |
22 | hardware interrupt (i.e. not a nested hardware interrupt) then the | |
23 | kernel switches from the current task to the interrupt stack. Like | |
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24 | the split thread and interrupt stacks on i386, this gives more room |
25 | for kernel interrupt processing without having to increase the size | |
26 | of every per thread stack. | |
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27 | |
28 | The interrupt stack is also used when processing a softirq. | |
29 | ||
30 | Switching to the kernel interrupt stack is done by software based on a | |
31 | per CPU interrupt nest counter. This is needed because x86-64 "IST" | |
32 | hardware stacks cannot nest without races. | |
33 | ||
34 | x86_64 also has a feature which is not available on i386, the ability | |
35 | to automatically switch to a new stack for designated events such as | |
36 | double fault or NMI, which makes it easier to handle these unusual | |
37 | events on x86_64. This feature is called the Interrupt Stack Table | |
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38 | (IST). There can be up to 7 IST entries per CPU. The IST code is an |
39 | index into the Task State Segment (TSS). The IST entries in the TSS | |
40 | point to dedicated stacks; each stack can be a different size. | |
352f7bae | 41 | |
57d30772 | 42 | An IST is selected by a non-zero value in the IST field of an |
352f7bae AK |
43 | interrupt-gate descriptor. When an interrupt occurs and the hardware |
44 | loads such a descriptor, the hardware automatically sets the new stack | |
45 | pointer based on the IST value, then invokes the interrupt handler. If | |
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46 | the interrupt came from user mode, then the interrupt handler prologue |
47 | will switch back to the per-thread stack. If software wants to allow | |
48 | nested IST interrupts then the handler must adjust the IST values on | |
49 | entry to and exit from the interrupt handler. (This is occasionally | |
50 | done, e.g. for debug exceptions.) | |
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51 | |
52 | Events with different IST codes (i.e. with different stacks) can be | |
53 | nested. For example, a debug interrupt can safely be interrupted by an | |
54 | NMI. arch/x86_64/kernel/entry.S::paranoidentry adjusts the stack | |
55 | pointers on entry to and exit from all IST events, in theory allowing | |
56 | IST events with the same code to be nested. However in most cases, the | |
57 | stack size allocated to an IST assumes no nesting for the same code. | |
58 | If that assumption is ever broken then the stacks will become corrupt. | |
59 | ||
60 | The currently assigned IST stacks are :- | |
61 | ||
352f7bae AK |
62 | * DOUBLEFAULT_STACK. EXCEPTION_STKSZ (PAGE_SIZE). |
63 | ||
64 | Used for interrupt 8 - Double Fault Exception (#DF). | |
65 | ||
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66 | Invoked when handling one exception causes another exception. Happens |
67 | when the kernel is very confused (e.g. kernel stack pointer corrupt). | |
68 | Using a separate stack allows the kernel to recover from it well enough | |
69 | in many cases to still output an oops. | |
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70 | |
71 | * NMI_STACK. EXCEPTION_STKSZ (PAGE_SIZE). | |
72 | ||
73 | Used for non-maskable interrupts (NMI). | |
74 | ||
75 | NMI can be delivered at any time, including when the kernel is in the | |
76 | middle of switching stacks. Using IST for NMI events avoids making | |
77 | assumptions about the previous state of the kernel stack. | |
78 | ||
79 | * DEBUG_STACK. DEBUG_STKSZ | |
80 | ||
81 | Used for hardware debug interrupts (interrupt 1) and for software | |
82 | debug interrupts (INT3). | |
83 | ||
84 | When debugging a kernel, debug interrupts (both hardware and | |
85 | software) can occur at any time. Using IST for these interrupts | |
86 | avoids making assumptions about the previous state of the kernel | |
87 | stack. | |
88 | ||
89 | * MCE_STACK. EXCEPTION_STKSZ (PAGE_SIZE). | |
90 | ||
91 | Used for interrupt 18 - Machine Check Exception (#MC). | |
92 | ||
93 | MCE can be delivered at any time, including when the kernel is in the | |
94 | middle of switching stacks. Using IST for MCE events avoids making | |
95 | assumptions about the previous state of the kernel stack. | |
96 | ||
97 | For more details see the Intel IA32 or AMD AMD64 architecture manuals. | |
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98 | |
99 | ||
100 | Printing backtraces on x86 | |
101 | -------------------------- | |
102 | ||
103 | The question about the '?' preceding function names in an x86 stacktrace | |
104 | keeps popping up, here's an indepth explanation. It helps if the reader | |
105 | stares at print_context_stack() and the whole machinery in and around | |
106 | arch/x86/kernel/dumpstack.c. | |
107 | ||
108 | Adapted from Ingo's mail, Message-ID: <20150521101614.GA10889@gmail.com>: | |
109 | ||
110 | We always scan the full kernel stack for return addresses stored on | |
111 | the kernel stack(s) [*], from stack top to stack bottom, and print out | |
112 | anything that 'looks like' a kernel text address. | |
113 | ||
114 | If it fits into the frame pointer chain, we print it without a question | |
115 | mark, knowing that it's part of the real backtrace. | |
116 | ||
117 | If the address does not fit into our expected frame pointer chain we | |
118 | still print it, but we print a '?'. It can mean two things: | |
119 | ||
120 | - either the address is not part of the call chain: it's just stale | |
121 | values on the kernel stack, from earlier function calls. This is | |
122 | the common case. | |
123 | ||
124 | - or it is part of the call chain, but the frame pointer was not set | |
125 | up properly within the function, so we don't recognize it. | |
126 | ||
127 | This way we will always print out the real call chain (plus a few more | |
128 | entries), regardless of whether the frame pointer was set up correctly | |
129 | or not - but in most cases we'll get the call chain right as well. The | |
130 | entries printed are strictly in stack order, so you can deduce more | |
131 | information from that as well. | |
132 | ||
133 | The most important property of this method is that we _never_ lose | |
134 | information: we always strive to print _all_ addresses on the stack(s) | |
135 | that look like kernel text addresses, so if debug information is wrong, | |
136 | we still print out the real call chain as well - just with more question | |
137 | marks than ideal. | |
138 | ||
139 | [*] For things like IRQ and IST stacks, we also scan those stacks, in | |
140 | the right order, and try to cross from one stack into another | |
141 | reconstructing the call chain. This works most of the time. |