Commit 5ef6ca4f authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge branch 'x86-debug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 debugging documentation updates from Ingo Molnar:
 "Documentation updates about x86 kernel stacks"

* 'x86-debug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/Documentation: Adapt Ingo's explanation on printing backtraces
  x86/Documentation: Remove STACKFAULT_STACK bulletpoint
  x86/Documentation: Move kernel-stacks doc one level up
parents b3ba283d 113b5e37
Kernel stacks on x86-64 bit
---------------------------
Most of the text from Keith Owens, hacked by AK
x86_64 page size (PAGE_SIZE) is 4K.
......@@ -56,13 +59,6 @@ If that assumption is ever broken then the stacks will become corrupt.
The currently assigned IST stacks are :-
* STACKFAULT_STACK. EXCEPTION_STKSZ (PAGE_SIZE).
Used for interrupt 12 - Stack Fault Exception (#SS).
This allows the CPU to recover from invalid stack segments. Rarely
happens.
* DOUBLEFAULT_STACK. EXCEPTION_STKSZ (PAGE_SIZE).
Used for interrupt 8 - Double Fault Exception (#DF).
......@@ -99,3 +95,47 @@ The currently assigned IST stacks are :-
assumptions about the previous state of the kernel stack.
For more details see the Intel IA32 or AMD AMD64 architecture manuals.
Printing backtraces on x86
--------------------------
The question about the '?' preceding function names in an x86 stacktrace
keeps popping up, here's an indepth explanation. It helps if the reader
stares at print_context_stack() and the whole machinery in and around
arch/x86/kernel/dumpstack.c.
Adapted from Ingo's mail, Message-ID: <20150521101614.GA10889@gmail.com>:
We always scan the full kernel stack for return addresses stored on
the kernel stack(s) [*], from stack top to stack bottom, and print out
anything that 'looks like' a kernel text address.
If it fits into the frame pointer chain, we print it without a question
mark, knowing that it's part of the real backtrace.
If the address does not fit into our expected frame pointer chain we
still print it, but we print a '?'. It can mean two things:
- either the address is not part of the call chain: it's just stale
values on the kernel stack, from earlier function calls. This is
the common case.
- or it is part of the call chain, but the frame pointer was not set
up properly within the function, so we don't recognize it.
This way we will always print out the real call chain (plus a few more
entries), regardless of whether the frame pointer was set up correctly
or not - but in most cases we'll get the call chain right as well. The
entries printed are strictly in stack order, so you can deduce more
information from that as well.
The most important property of this method is that we _never_ lose
information: we always strive to print _all_ addresses on the stack(s)
that look like kernel text addresses, so if debug information is wrong,
we still print out the real call chain as well - just with more question
marks than ideal.
[*] For things like IRQ and IST stacks, we also scan those stacks, in
the right order, and try to cross from one stack into another
reconstructing the call chain. This works most of the time.
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