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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>
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@ -95,3 +95,47 @@ The currently assigned IST stacks are :-
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assumptions about the previous state of the kernel stack.
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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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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) [*], 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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[*] 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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