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    Subject[tip:x86/debug] x86/Documentation: Adapt Ingo' s explanation on printing backtraces
    Commit-ID:  113b5e3720e79ad938374163c1b8e295521dc9cf
    Gitweb: http://git.kernel.org/tip/113b5e3720e79ad938374163c1b8e295521dc9cf
    Author: Borislav Petkov <bp@suse.de>
    AuthorDate: Tue, 26 May 2015 10:28:20 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Wed, 27 May 2015 14:39:47 +0200

    x86/Documentation: Adapt Ingo's explanation on printing backtraces

    Hold it down for future reference, as the question about the
    question mark in stack traces keeps popping up.

    Signed-off-by: Borislav Petkov <bp@suse.de>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Michal Marek <mmarek@suse.cz>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: live-patching@vger.kernel.org
    Link: http://lkml.kernel.org/r/1432628901-18044-18-git-send-email-bp@alien8.de
    Link: http://lkml.kernel.org/r/20150521101614.GA10889@gmail.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    Documentation/x86/kernel-stacks | 44 +++++++++++++++++++++++++++++++++++++++++
    1 file changed, 44 insertions(+)

    diff --git a/Documentation/x86/kernel-stacks b/Documentation/x86/kernel-stacks
    index c3c935b..0f3a6c2 100644
    --- a/Documentation/x86/kernel-stacks
    +++ b/Documentation/x86/kernel-stacks
    @@ -95,3 +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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