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SubjectRe: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
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

> --- 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.

> --- 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?

> --- 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.

--
Josh

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