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

Josh Poimboeuf
2017-03-23T13:24:58Z

On Thu, Mar 23, 2017 at 08:38:20AM +0100, Ingo Molnar wrote:
>
> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > On Wed, Mar 22, 2017 at 08:46:16AM +0100, Ingo Molnar wrote:
> > >
> > > * Jiri Slaby <jslaby@suse.cz> wrote:
> > >
> > > > On 03/22/2017, 08:25 AM, Ingo Molnar wrote:
> > > > >
> > > > > * Pavel Machek <pavel@ucw.cz> wrote:
> > > > >
> > > > >> Hi!
> > > > >>
> > > > >>> -ENTRY(saved_rbp) .quad 0
> > > > >>> -ENTRY(saved_rsi) .quad 0
> > > > >>> -ENTRY(saved_rdi) .quad 0
> > > > >>> -ENTRY(saved_rbx) .quad 0
> > > > >>> +SYM_DATA_START(saved_rbp) .quad 0
> > > > >>> +SYM_DATA_START(saved_rsi) .quad 0
> > > > >>> +SYM_DATA_START(saved_rdi) .quad 0
> > > > >>> +SYM_DATA_START(saved_rbx) .quad 0
> > > > >>
> > > > >> Does it make sense to call it SYM_DATA_*START* when there's no
> > > > >> corresponding end?
> > > > >
> > > > > That looks like a bug - I think we should strive for them to always be in pairs.
> > > > >
> > > > > Jiri, Josh, could objtool help here perhaps, to detect 'non-terminated'
> > > > > SYM_*_START() uses? This could be done by emitting debug data into a special
> > > > > section and then analyzing that section for unpaired entries. The section can be
> > > > > discarded in the final link, it won't show up in the kernel image.
> > > >
> > > > It should be easier than that. No introduction of other info needed --
> > > > every global symbol without a ".type" or ".size" (i.e. SYM_*_END) should
> > > > be a bug now.
> > >
> > > I'm all for that!
> >
> > It would be easy to add this checking to objtool since it already reads
> > the symbol table. The hard part is figuring out the logistics. :-)
> >
> > - Should the warnings be on by default?
>
> Yes, if objtool is running. Keep it simple.
>
> > - Part of the "objtool check" command or something else?
>
> Yes - I think it's still within the 'object file check' functionality.
>
> > - Separate config option or just include it with
> > CONFIG_STACK_VALIDATION?
>
> Yeah, but I'd rename CONFIG_STACK_VALIDATION to CONFIG_OBJ_VALIDATION or such. As
> I predicted early on, objtool will go beyond stack checking! ;-)
>
> > - Should all asm files be checked, including those currently skipped by
> > objtool with OBJECT_FILES_NON_STANDARD?
>
> The symbol syntax check should definitely be for all files, yes.

That all sounds reasonable. I'll work something up.

> Could we perhaps emit 'non-standard stack frames' information into the .o itself
> (via a flag or a special section?), so that objtool can decide on its own whether
> to complain about any weirdnesses there?

For the OBJECT_FILES_NON_STANDARD case, where the whole file is
"special", we can just provide a flag to "objtool check" to tell it to
skip stack checking for that file, but still do the symbol checks.

> > > Can we detect double ends as well - i.e. do a build check of the full syntax of
> > > these symbol definition primitives?
> >
> > Detecting double ends would be a little trickier. The second SYM_*_END
> > supersedes the first, so that information isn't in the ELF symbol table.
>
> Indeed.
>
> > We could use a special section to annotate all the macro uses and have
> > objtool do the checking, similar to what you suggested earlier.
>
> That might be useful for other purposes as well - such as the non-standard stack
> frame annotations?

To start with we can try going without all the special sections (other
than the SYM_END double end check). If we end up finding another case
which isn't covered then we can always add the special sections later.

--
Josh