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author | Linus Torvalds <torvalds@linux-foundation.org> | 2015-06-23 00:09:32 (GMT) |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2015-06-23 00:09:32 (GMT) |
commit | 5ef6ca4f24b59af7f7c2c19502a3923a4bc10e0a (patch) | |
tree | 24f0a7f99ca3f091f9a0c3127d4d37054340fa2c | |
parent | b3ba283d831fed464a1f9c18e7ee82b020ab1a1e (diff) | |
parent | 113b5e3720e79ad938374163c1b8e295521dc9cf (diff) | |
download | linux-5ef6ca4f24b59af7f7c2c19502a3923a4bc10e0a.tar.xz |
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
-rw-r--r-- | Documentation/x86/kernel-stacks (renamed from Documentation/x86/x86_64/kernel-stacks) | 54 |
1 files changed, 47 insertions, 7 deletions
diff --git a/Documentation/x86/x86_64/kernel-stacks b/Documentation/x86/kernel-stacks index e3c8a49..0f3a6c2 100644 --- a/Documentation/x86/x86_64/kernel-stacks +++ b/Documentation/x86/kernel-stacks @@ -1,3 +1,6 @@ +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. |