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

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

On Thu, Mar 23, 2017 at 08:38:20AM +0100, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> > On Wed, Mar 22, 2017 at 08:46:16AM +0100, Ingo Molnar wrote:
> > > 
> > > * Jiri Slaby <jslaby@xxxxxxx> wrote:
> > > 
> > > > On 03/22/2017, 08:25 AM, Ingo Molnar wrote:
> > > > > 
> > > > > * Pavel Machek <pavel@xxxxxx> 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

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