Messages in this thread | | | Date | Wed, 1 Apr 2020 16:22:26 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET |
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On Wed, Apr 01, 2020 at 09:14:02AM -0500, Josh Poimboeuf wrote: > On Wed, Apr 01, 2020 at 12:27:03AM +0200, Peter Zijlstra wrote: > > On Tue, Mar 31, 2020 at 04:20:40PM -0500, Josh Poimboeuf wrote: > > > On Tue, Mar 31, 2020 at 04:17:58PM -0500, Josh Poimboeuf wrote: > > > > > I'm not against adding a second/separate hint for this. In fact, I > > > > > almost considered teaching objtool how to interpret the whole IRET frame > > > > > so that we can do it without hints. It's just that that's too much code > > > > > for this one case. > > > > > > > > > > HINT_IRET_SELF ? > > > > > > > > Despite my earlier complaint about stack size knowledge, we could just > > > > forget the hint and make "iretq in C code" equivalent to "reduce stack > > > > size by arch_exception_stack_size()" and keep going. There's > > > > file->c_file which tells you it's a C file. > > > > > > Or maybe "iretq in an STT_FUNC" is better since this pattern could > > > presumably happen in a callable asm function. > > > > Like so then? > > I'd suggest a patch split like: > > 1) objtool: automagic IRET-in-func > 2) objtool: add RET_OFFSET > 3) ftrace: re-organize asm (and use RET_OFFSET hint) > 4) objtool: remove now-unused SAVE/RESTORE
Sure.
> > --- a/arch/x86/include/asm/orc_types.h > > +++ b/arch/x86/include/asm/orc_types.h > > @@ -58,8 +58,13 @@ > > #define ORC_TYPE_CALL 0 > > #define ORC_TYPE_REGS 1 > > #define ORC_TYPE_REGS_IRET 2 > > -#define UNWIND_HINT_TYPE_SAVE 3 > > -#define UNWIND_HINT_TYPE_RESTORE 4 > > + > > +/* > > + * RET_OFFSET: Used on instructions that terminate a function; mostly RETURN > > + * and sibling calls. On these, sp_offset denotes the expected offset from > > + * initial_func_cfi. > > + */ > > +#define UNWIND_HINT_TYPE_RET_OFFSET 3 > > I think this comment belongs at the UNWIND_HINT_RET_OFFSET macro > definition.
Humph, ok, but there's two of those :/
> > --- a/arch/x86/kernel/ftrace.c > > +++ b/arch/x86/kernel/ftrace.c > > @@ -282,7 +282,8 @@ static inline void tramp_free(void *tram > > > > /* Defined as markers to the end of the ftrace default trampolines */ > > extern void ftrace_regs_caller_end(void); > > -extern void ftrace_epilogue(void); > > +extern void ftrace_regs_caller_ret(void); > > +extern void ftrace_caller_end(void); > > extern void ftrace_caller_op_ptr(void); > > extern void ftrace_regs_caller_op_ptr(void); > > > > @@ -334,7 +335,7 @@ create_trampoline(struct ftrace_ops *ops > > call_offset = (unsigned long)ftrace_regs_call; > > } else { > > start_offset = (unsigned long)ftrace_caller; > > - end_offset = (unsigned long)ftrace_epilogue; > > + end_offset = (unsigned long)ftrace_caller_end; > > op_offset = (unsigned long)ftrace_caller_op_ptr; > > call_offset = (unsigned long)ftrace_call; > > } > > @@ -366,6 +367,13 @@ create_trampoline(struct ftrace_ops *ops > > if (WARN_ON(ret < 0)) > > goto fail; > > > > + if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) { > > + ip = ftrace_regs_caller_ret; > > + ret = probe_kernel_read(ip, (void *)retq, RET_SIZE); > > + if (WARN_ON(ret < 0)) > > + goto fail; > > + } > > + > > Hm? This function creates a trampoline but it looks like this change is > overwriting the original ftrace_64 code itself?
Ahh. So if you look at what the trampoline copies, you'll note we'll copy until -- but *NOT* including -- the jmp ftrace_epilogue. Instead we'll write a RET at the end.
However, due to splitting the return path, such that each instruction has a unique stack offset, we now have a second jmp ftrace_epilogue in the middle of the function. That too needs to be overwritten by a RET.
> > --- a/tools/objtool/Makefile > > +++ b/tools/objtool/Makefile > > @@ -31,7 +31,7 @@ INCLUDES := -I$(srctree)/tools/include \ > > -I$(srctree)/tools/arch/$(HOSTARCH)/include/uapi \ > > -I$(srctree)/tools/arch/$(SRCARCH)/include > > WARNINGS := $(EXTRA_WARNINGS) -Wno-switch-default -Wno-switch-enum -Wno-packed > > -CFLAGS := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -g $(INCLUDES) $(LIBELF_FLAGS) > > +CFLAGS := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -ggdb3 $(INCLUDES) $(LIBELF_FLAGS) > > LDFLAGS += $(LIBELF_LIBS) $(LIBSUBCMD) $(KBUILD_HOSTLDFLAGS) > > Why? Smells like a separate patch at least.
Oh, whoops :-) I keep doing this every time I need to run gdb on it. I'll make it go away.
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