Hi Josh, On Wed, 8 May 2019 13:48:48 -0500 Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Wed, May 08, 2019 at 05:39:07PM +0200, Peter Zijlstra wrote: > > On Wed, May 08, 2019 at 07:42:48AM -0500, Josh Poimboeuf wrote: > > > On Wed, May 08, 2019 at 02:04:16PM +0200, Peter Zijlstra wrote: > > > > > > Do the x86_64 variants also want some ORC annotation? > > > > > > Maybe so. Though it looks like regs->ip isn't saved. The saved > > > registers might need to be tweaked. I'll need to look into it. > > > > What all these sites do (and maybe we should look at unifying them > > somehow) is turn a CALL frame (aka RET-IP) into an exception frame (aka > > pt_regs). > > > > So regs->ip will be the return address (which is fixed up to be the CALL > > address in the handler). > > But from what I can tell, trampoline_handler() hard-codes regs->ip to > point to kretprobe_trampoline(), and the original return address is > placed in regs->sp. > > Masami, is there a reason why regs->ip doesn't have the original return > address and regs->sp doesn't have the original SP? I think that would > help the unwinder understand things. Yes, for regs->ip, there is a histrical reason. Since previously, we had an int3 at trampoline, so the user (kretprobe) handler expects that regs->ip is trampoline address and ri->ret_addr is original return address. It is better to check the other archs, but I think it is possible to change the regs->ip to original return address, since no one cares such "fixed address". :) For the regs->sp, there are 2 reasons. For x86-64, it's just for over-optimizing (reduce stack usage). I think we can make a gap for putting return address, something like "kretprobe_trampoline:\n" #ifdef CONFIG_X86_64 " pushq %rsp\n" /* Make a gap for return address */ " pushq 0(%rsp)\n" /* Copy original stack pointer */ " pushfq\n" SAVE_REGS_STRING " movq %rsp, %rdi\n" " call trampoline_handler\n" /* Push the true return address to the bottom */ " movq %rax, 20*8(%rsp)\n" RESTORE_REGS_STRING " popfq\n" " addq $8, %rsp\n" /* Skip original stack pointer */ For i386 (x86-32), there is no other way to keep ®s->sp as the original stack pointer. It has to be changed with this series, maybe as same as x86-64. Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
