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

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

Signed-off-by: Borislav Petkov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michal Marek <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Borislav Petkov 2015-05-26 10:28:20 +02:00 • committed by Ingo Molnar
commit 113b5e3720
1 changed files with 44 additions and 0 deletions
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Documentation/x86/kernel-stacks
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@ -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: <[email protected]>:
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.