Re: [PATCH] bpf: Tweak BPF jump table optimizations for objtool compatibility

[Alexei Starovoitov]

On Thu, May 07, 2020 at 09:07:33AM -0500, Josh Poimboeuf wrote:
> On Wed, May 06, 2020 at 05:03:57PM -0700, Alexei Starovoitov wrote:
> > > > > > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > > > > > index d7ee4c6bad48..05104c3cc033 100644
> > > > > > --- a/include/linux/compiler-gcc.h
> > > > > > +++ b/include/linux/compiler-gcc.h
> > > > > > @@ -171,4 +171,4 @@
> > > > > >  #define __diag_GCC_8(s)
> > > > > >  #endif
> > > > > >
> > > > > > -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> > > > > > +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer")))
> > > > > > --
> > > > > > 2.23.0
> > > > > >
> > > > > > I've tested it with gcc 8,9,10 and clang 11 with FP=y and with ORC=y.
> > > > > > All works.
> > > > > > I think it's safer to go with frame pointers even for ORC=y considering
> > > > > > all the pain this issue had caused. Even if objtool gets confused again
> > > > > > in the future __bpf_prog_run() will have frame pointers and kernel stack
> > > > > > unwinding can fall back from ORC to FP for that frame.
> > > > > > wdyt?
> > > > >
> > > > > It seems dangerous to me.  The GCC manual recommends against it.
> > > > 
> > > > The manual can says that it's broken. That won't stop the world from using it.
> > > > Just google projects that are using it. For example: qt, lz4, unreal engine, etc
> > > > Telling compiler to disable gcse via flag is a guaranteed way to avoid
> > > > that optimization that breaks objtool whereas messing with C code is nothing
> > > > but guess work. gcc can still do gcse.
> > > 
> > > But the manual's right, it is broken.  How do you know other important
> > > flags won't also be stripped?
> > 
> > What flags are you worried about?
> > I've checked that important things like -mno-red-zone, -fsanitize are preserved.
> 
> It's not any specific flags I'm worried about, it's all of them.  There
> are a lot of possibilities, with all the different configs, and arches.
> Flags are usually added for a good reason, so one randomly missing flag
> could have unforeseen results.
> 
> And I don't have any visibility into how GCC decides which flags to
> drop, and when.  But the docs aren't comforting.

That doc change landed 5 years ago:
https://patchwork.ozlabs.org/project/gcc/patch/20151213081911.GA320@x4/
Sure it's 'broken' by whatever definition of broken.
Yet gcc has
$ git grep '__attribute__((optimize' gcc/testsuite/|wc -l
34 tests to make sure it stays working.
And gcc is using it to bootstrap itself. See LIBGCC2_UNWIND_ATTRIBUTE.
The doc is expressing desire and trying to discourage its use,
but that attribute is not going anywhere.

> Even if things seem to work now, that could (silently) change at any
> point in time.  This time objtool warned about the missing frame
> pointer, but that's not necessarily going to happen for other flags.
> 
> If we go this route, I would much rather do -fno-gcse on a file-wide
> basis.

The fix for broken commit 3193c0836f20 has to be backported all the way
to 5.3 release. I'd like to minimize conflicts.
For that very reason I'm not even renaming #define __no_fgcse.