Re: [PATCH v2 02/10] x86: assembly, FUNC_START for fn, DATA_START for data

Josh Poimboeuf
2017-03-22T15:33:43Z

On Wed, Mar 22, 2017 at 04:01:08PM +0100, Jiri Slaby wrote:
> On 03/22/2017, 03:11 PM, Josh Poimboeuf wrote:
> > Or, here's a much easier way to do it, without involving objtool:
> >
> > --- a/include/linux/linkage.h
> > +++ b/include/linux/linkage.h
> > @@ -138,9 +138,17 @@
> > name:
> > #endif
> >
> > +#ifndef CHECK_DUP_SYM_END
> > +#define CHECK_DUP_SYM_END(name) \
> > + .pushsection .discard.sym_func_end ASM_NL \
> > + SYM_END_##name: .byte 0 ASM_NL \
> > + .popsection
> > +#endif
> > +
> > /* SYM_END -- use only if you have to */
> > #ifndef SYM_END
> > #define SYM_END(name, sym_type) \
> > + CHECK_DUP_SYM_END(name) ASM_NL \
> > .type name sym_type ASM_NL \
> > .size name, .-name
> > #endif
>
> I tried this approach and it didn't work for me inside .macros. Oh,
> well, the name cannot be first, so now, we can have a check for both
> correct pairing _and_ duplicate ends in one:
>
> #define SYM_CHECK_START(name) \
> .pushsection .rodata.bubak ASM_NL \
> .long has_no_SYM_END_##name - . ASM_NL \
> .popsection
>
> #define SYM_CHECK_END(name) \
> has_no_SYM_END_##name:
>
> /* SYM_START -- use only if you have to */
> #ifndef SYM_START
> #define SYM_START(name, align, visibility, entry) \
> SYM_CHECK_START(name) ASM_NL \
> visibility(name) ASM_NL \
> align ASM_NL \
> name: ASM_NL \
> entry
> #endif
>
> /* SYM_END -- use only if you have to */
> #ifndef SYM_END
> #define SYM_END(name, sym_type, exit) \
> exit ASM_NL \
> SYM_CHECK_END(name) ASM_NL \
> .type name sym_type ASM_NL \
> .size name, .-name
> #endif
>
>
> So for the ftrace mistake I did:
>
> AS arch/x86/kernel/mcount_64.o
> /home/latest/linux/arch/x86/kernel/mcount_64.S: Assembler messages:
> /home/latest/linux/arch/x86/kernel/mcount_64.S:192: Error: symbol
> `has_no_SYM_END_ftrace_caller' is already defined
>
>
> or if I remove SYM_END_FUNC completely:
> LD vmlinux.o
> MODPOST vmlinux.o
> arch/x86/built-in.o:(.rodata.bubak+0x130): undefined reference to
> `has_no_SYM_END_ftrace_stub'
>
>
> Sad is that this occurs only during linking, so I cannot put it in the
> .discard section -- ideas?

Ah, interesting idea but I can't think of a way to do the missing end
check before link time.

But it would be easy for objtool to check for a missing end because the
symbol would have a zero size.

--
Josh