Messages in this thread | | | Date | Thu, 2 Apr 2020 17:40:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/7] objtool: Add support for return trampoline call |
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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.
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