From: Borislav Petkov <bp@xxxxxxx> 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@xxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: Michal Marek <mmarek@xxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: X86 ML <x86@xxxxxxxxxx> Cc: live-patching@xxxxxxxxxxxxxxx Cc: lkml <linux-kernel@xxxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/20150521101614.GA10889@xxxxxxxxx --- Documentation/x86/kernel-stacks | 44 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/Documentation/x86/kernel-stacks b/Documentation/x86/kernel-stacks index c3c935b9d56e..0f3a6c201943 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@xxxxxxxxx>: + +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. -- 2.3.5 -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
