Messages in this thread | | | Date | Mon, 26 Nov 2018 16:43:56 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 3/4] x86/static_call: Add out-of-line static call implementation |
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On Mon, Nov 26, 2018 at 07:54:59AM -0600, Josh Poimboeuf wrote:
> +void static_call_bp_handler(void); > +void *bp_handler_dest; > + > +asm(".pushsection .text, \"ax\" \n" > + ".globl static_call_bp_handler \n" > + ".type static_call_bp_handler, @function \n" > + "static_call_bp_handler: \n" > + "ANNOTATE_RETPOLINE_SAFE \n" > + "jmp *bp_handler_dest \n" > + ".popsection \n"); > + > +void arch_static_call_transform(void *site, void *tramp, void *func) > +{ > + s32 dest_relative; > + unsigned long insn; > + unsigned char insn_opcode; > + unsigned char opcodes[CALL_INSN_SIZE]; > + > + insn = (unsigned long)tramp; > + > + mutex_lock(&text_mutex); > + > + insn_opcode = *(unsigned char *)insn; > + if (insn_opcode != 0xe8 && insn_opcode != 0xe9) { > + WARN_ONCE(1, "unexpected static call insn opcode 0x%x at %pS", > + insn_opcode, (void *)insn); > + goto done; > + } > + > + dest_relative = (long)(func) - (long)(insn + CALL_INSN_SIZE); > + > + opcodes[0] = insn_opcode; > + memcpy(&opcodes[1], &dest_relative, CALL_INSN_SIZE - 1); > + > + /* Set up the variable for the breakpoint handler: */ > + bp_handler_dest = func; > + > + /* Patch the call site: */ > + text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE, > + static_call_bp_handler);
I'm confused by the whole static_call_bp_handler thing; why not jump straight to @func ?
Also, what guarantees this other thread will have gotten from static_call_bp_handler and executed the actual indirect JMP instruction by the time we re-write @bp_handler_dest again?
> +done: > + mutex_unlock(&text_mutex); > +}
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