On Tue, 12 Dec 2017 08:05:01 -0600 Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Tue, Dec 12, 2017 at 12:39:12PM +0100, Torsten Duwe wrote: > > Hi all, > > > > The "Power Architecture 64-Bit ELF V2 ABI" says in section 2.3.2.3: > > > > [...] There are several rules that must be adhered to in order to ensure > > reliable and consistent call chain backtracing: > > > > * Before a function calls any other function, it shall establish its > > own stack frame, whose size shall be a multiple of 16 bytes. > > What about leaf functions? If a leaf function doesn't establish a stack > frame, and it has inline asm which contains a blr to another function, > this ABI is broken. > > Also, even for non-leaf functions, is it possible for GCC to insert the > inline asm before it sets up the stack frame? (This is an occasional > problem on x86.) Inline asm must not have control transfer out of the statement unless it is asm goto. > > Also, what about hand-coded asm? Should follow the same rules if it uses the stack. > > > To me this sounds like the equivalent of HAVE_RELIABLE_STACKTRACE. > > This patch may be unneccessarily limited to ppc64le, but OTOH the only > > user of this flag so far is livepatching, which is only implemented on > > PPCs with 64-LE, a.k.a. ELF ABI v2. > > In addition to fixing the above issues, the unwinder also needs to > detect interrupts (i.e., preemption) and page faults on the stack of a > blocked task. If a function were preempted before it created a stack > frame, or if a leaf function blocked on a page fault, the stack trace > will skip the function's caller, so such a trace will need to be > reported to livepatch as unreliable. I don't think there is much problem there for powerpc. Stack frame creation and function call with return pointer are each atomic. > > Furthermore, the "reliable" unwinder needs to have a way to report an > error if it doesn't reach the end. This probably just means ensuring > that it reaches the user mode registers on the stack. > > And as Miroslav mentioned, once that's all done, implement > save_stack_trace_tsk_reliable(). > > I don't think the above is documented anywhere, it would be good to put > it in the livepatch doc. > Thanks, Nick -- 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
