153 lines
6.5 KiB
ReStructuredText
153 lines
6.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=============
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Kernel Stacks
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=============
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Kernel stacks on x86-64 bit
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===========================
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Most of the text from Keith Owens, hacked by AK
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x86_64 page size (PAGE_SIZE) is 4K.
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Like all other architectures, x86_64 has a kernel stack for every
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active thread. These thread stacks are THREAD_SIZE (2*PAGE_SIZE) big.
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These stacks contain useful data as long as a thread is alive or a
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zombie. While the thread is in user space the kernel stack is empty
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except for the thread_info structure at the bottom.
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In addition to the per thread stacks, there are specialized stacks
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associated with each CPU. These stacks are only used while the kernel
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is in control on that CPU; when a CPU returns to user space the
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specialized stacks contain no useful data. The main CPU stacks are:
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* Interrupt stack. IRQ_STACK_SIZE
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Used for external hardware interrupts. If this is the first external
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hardware interrupt (i.e. not a nested hardware interrupt) then the
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kernel switches from the current task to the interrupt stack. Like
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the split thread and interrupt stacks on i386, this gives more room
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for kernel interrupt processing without having to increase the size
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of every per thread stack.
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The interrupt stack is also used when processing a softirq.
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Switching to the kernel interrupt stack is done by software based on a
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per CPU interrupt nest counter. This is needed because x86-64 "IST"
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hardware stacks cannot nest without races.
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x86_64 also has a feature which is not available on i386, the ability
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to automatically switch to a new stack for designated events such as
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double fault or NMI, which makes it easier to handle these unusual
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events on x86_64. This feature is called the Interrupt Stack Table
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(IST). There can be up to 7 IST entries per CPU. The IST code is an
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index into the Task State Segment (TSS). The IST entries in the TSS
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point to dedicated stacks; each stack can be a different size.
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An IST is selected by a non-zero value in the IST field of an
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interrupt-gate descriptor. When an interrupt occurs and the hardware
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loads such a descriptor, the hardware automatically sets the new stack
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pointer based on the IST value, then invokes the interrupt handler. If
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the interrupt came from user mode, then the interrupt handler prologue
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will switch back to the per-thread stack. If software wants to allow
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nested IST interrupts then the handler must adjust the IST values on
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entry to and exit from the interrupt handler. (This is occasionally
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done, e.g. for debug exceptions.)
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Events with different IST codes (i.e. with different stacks) can be
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nested. For example, a debug interrupt can safely be interrupted by an
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NMI. arch/x86_64/kernel/entry.S::paranoidentry adjusts the stack
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pointers on entry to and exit from all IST events, in theory allowing
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IST events with the same code to be nested. However in most cases, the
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stack size allocated to an IST assumes no nesting for the same code.
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If that assumption is ever broken then the stacks will become corrupt.
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The currently assigned IST stacks are:
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* ESTACK_DF. EXCEPTION_STKSZ (PAGE_SIZE).
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Used for interrupt 8 - Double Fault Exception (#DF).
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Invoked when handling one exception causes another exception. Happens
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when the kernel is very confused (e.g. kernel stack pointer corrupt).
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Using a separate stack allows the kernel to recover from it well enough
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in many cases to still output an oops.
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* ESTACK_NMI. EXCEPTION_STKSZ (PAGE_SIZE).
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Used for non-maskable interrupts (NMI).
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NMI can be delivered at any time, including when the kernel is in the
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middle of switching stacks. Using IST for NMI events avoids making
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assumptions about the previous state of the kernel stack.
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* ESTACK_DB. EXCEPTION_STKSZ (PAGE_SIZE).
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Used for hardware debug interrupts (interrupt 1) and for software
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debug interrupts (INT3).
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When debugging a kernel, debug interrupts (both hardware and
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software) can occur at any time. Using IST for these interrupts
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avoids making assumptions about the previous state of the kernel
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stack.
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To handle nested #DB correctly there exist two instances of DB stacks. On
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#DB entry the IST stackpointer for #DB is switched to the second instance
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so a nested #DB starts from a clean stack. The nested #DB switches
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the IST stackpointer to a guard hole to catch triple nesting.
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* ESTACK_MCE. EXCEPTION_STKSZ (PAGE_SIZE).
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Used for interrupt 18 - Machine Check Exception (#MC).
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MCE can be delivered at any time, including when the kernel is in the
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middle of switching stacks. Using IST for MCE events avoids making
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assumptions about the previous state of the kernel stack.
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For more details see the Intel IA32 or AMD AMD64 architecture manuals.
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Printing backtraces on x86
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==========================
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The question about the '?' preceding function names in an x86 stacktrace
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keeps popping up, here's an indepth explanation. It helps if the reader
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stares at print_context_stack() and the whole machinery in and around
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arch/x86/kernel/dumpstack.c.
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Adapted from Ingo's mail, Message-ID: <20150521101614.GA10889@gmail.com>:
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We always scan the full kernel stack for return addresses stored on
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the kernel stack(s) [1]_, from stack top to stack bottom, and print out
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anything that 'looks like' a kernel text address.
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If it fits into the frame pointer chain, we print it without a question
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mark, knowing that it's part of the real backtrace.
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If the address does not fit into our expected frame pointer chain we
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still print it, but we print a '?'. It can mean two things:
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- either the address is not part of the call chain: it's just stale
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values on the kernel stack, from earlier function calls. This is
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the common case.
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- or it is part of the call chain, but the frame pointer was not set
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up properly within the function, so we don't recognize it.
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This way we will always print out the real call chain (plus a few more
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entries), regardless of whether the frame pointer was set up correctly
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or not - but in most cases we'll get the call chain right as well. The
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entries printed are strictly in stack order, so you can deduce more
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information from that as well.
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The most important property of this method is that we _never_ lose
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information: we always strive to print _all_ addresses on the stack(s)
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that look like kernel text addresses, so if debug information is wrong,
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we still print out the real call chain as well - just with more question
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marks than ideal.
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.. [1] For things like IRQ and IST stacks, we also scan those stacks, in
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the right order, and try to cross from one stack into another
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reconstructing the call chain. This works most of the time.
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