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

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

On Mon, Mar 20, 2017 at 04:32:09PM +0100, Jiri Slaby wrote:
> On 03/20/2017, 02:32 PM, Josh Poimboeuf wrote:
> > On Mon, Mar 20, 2017 at 01:32:14PM +0100, Jiri Slaby wrote:
> >> This is a start of series to cleanup macros used for starting functions,
> >> data, globals etc. across x86. When we have all this sorted out, this
> >> will help to inject DWARF unwinding info by objtool later.
> >>
> >> The goal is forcing SYM_FUNC_START to emit .cfi_startproc and
> >> SYM_FUNC_END to emit .cfi_endproc. Automatically at best.
> > 
> > Do we still want to emit .cfi_startproc/endproc from the macro?  From
> > our last discussion, that seemed to be up in the air.
> > 
> >   https://lkml.kernel.org/r/20170217211804.j6l2d7t5mfzqzmbt@treble
> 
> "Automatically at best" above means "completely from objtool". I am
> still uncertain whether it will work 100% or we would have to help by
> generating some pieces from the added macros. In particular, the ALIASes
> are evil which cause harm here:
> 
> fun_alias:
> fun:
> <code>
> .size fun, .-fun
> .type fun STT_FUNC
> .size fun_alias, .-fun_alias
> .type fun_alias STT_FUNC
> 
> Both cannot create (overlapping) .cfi_startproc/endproc, only the inner
> shall.
> 
> But it seems so far, that we might be able to deal with all of that from
> objtool... (I have not been thinking about this particular thing deep
> enough yet.) Some sort of "from the last label that is marked as
> STT_FUNC till its .size" might work.

Ok.

> > What did you think about making CFI read-only for .c object files and
> > write-only for .S object files?
> 
> There are those functions like sync_core() or native_save_fl() with
> inline asm. And they seem to need a) read-write support, or b) manual
> annotation. I would like to avoid b) for sure.

Ah, so I guess those inline asm functions cause problems because they
muck with the stack pointer with pushes and pops?

I don't think manual annotation of inline asm would be so bad.  IIUC, it
would only mean replacing the pushes and pops with a macro which does
the CFI-annotated version, like PUSH_CFI and POP_CFI.  And the benefit
would be that objtool doesn't have to try to rewrite a bunch of .c
object files.

Objtool read-write worries me because it gives more responsibility to
objtool.  It could be tricky to insert CFI instructions within the ones
already created by gcc.  Also, while unlikely, a bug in objtool could
theoretically corrupt an object file and brick the kernel.  Also I
wonder how all those extra file writes would affect build performance.

If at all possible, I would rather objtool stay out of the way of the
compiler and let gcc do its job of generating CFI.

-- 
Josh

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