Messages in this thread | | | Subject | Re: [PATCH 4/7] objtool: Add support for return trampoline call | From | Julien Thierry <> | Date | Fri, 3 Apr 2020 09:11:55 +0100 |
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On 4/2/20 4:40 PM, Peter Zijlstra wrote: > On Thu, Apr 02, 2020 at 04:31:05PM +0100, Julien Thierry wrote: >> My understanding is that once you find an intra-function call, either you >> hit a return, ending the branch, so the return should undo the modification >> the intra-function call did (whether is it a retpoline return or not). >> Otherwise, the intra-function call branch will need to reach an end in some >> way (e.g. hitting a CONTEXT_SWITCH instruction, calling a >> dead_end_function). >> >> Am I missing something? > > The thing is basically doing: > > mov $n, cx > 1: call 2f > 2: dec cx > jnz 1b > add 8*n, sp > > So it does N calls to self, then subtracts N words from the stack. > > The reason being that the CPU has a return-stack-buffer for predicting > returns, and call/ret being naturally paired, that works. The above > is a software flush of the RSB. >
Ah, lovely... Maybe that's where SAVE/RESTORE unwind hints could be nice ;) .
Otherwise, I don't really have a good suggestion for this...
-- Julien Thierry
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