Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1504015

Re: [PATCH 01/12] extarray: define helpers for arrays defined in linker scripts

Show all headers | View raw

On Wed, Oct 19, 2016 at 11:33:41AM +0200, Richard Biener wrote:
> On Wed, 19 Oct 2016, Peter Zijlstra wrote:

> > This is also an entirely different class of optimizations than the whole
> > pointer arithmetic is only valid inside an object thing.
> 
> Yes, it is not related to that.  I've opened 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78035 to track an
> inconsistency in that new optimization.
> 
> > The kernel very much relies on unbounded pointer arithmetic, including
> > overflow. Sure, C language says its UB, but we know our memory layout,
> > and it would be very helpful if we could define it.
> 
> It's well-defined and correctly handled if you do the arithmetic
> in uintptr_t.  No need for knobs.

So why not extend that to the pointers themselves and be done with it?

In any case, so you're saying our:

#define RELOC_HIDE(ptr, off)						\
({									\
	unsigned long __ptr;						\
	__asm__ ("" : "=r"(__ptr) : "0"(ptr));				\
	(typeof(ptr)) (__ptr + (off));					\
})

could be written like:

#define RELOC_HIDE(ptr, off)			\
({						\
	uintptr_t __ptr = (ptr);		\
	(typeof(ptr)) (__ptr + (off));		\
})

Without laundering it through inline asm?

Is there any advantage to doing so?

But this still means we need to be aware of this and use these macros to
launder our pointers.

Which gets us back to the issue that started this whole thread. We have
code that now gets miscompiled, silently.

That is a bad situation. So we need to either avoid the miscompilation,
or make it verbose.

> > Can't we get a knob extending -fno-strict-aliasing to define pointer
> > arithmetic outside of objects and overflow? I mean, we already use that,
> > we also use -fno-strict-overflow and a whole bunch of others.
> > 
> > At the very least, it would be nice to get a -W flag for when this alias
> > analysis stuff kills something so we can at least know when GCC goes and
> > defeats us.
> 
> What kind of warning do you envision?
> 
> "warning: optimized address comparison to always true/false"
> 
> ?  That would trigger all over the place.

That is indeed what I was thinking of. And I have no idea how often that
would trigger on the kernel.

I'm thinking that if this WARN isn't subject to false
positives we could live with that. Its the false positives that render
other warnings useless (too much noise on perfectly fine code etc..).

/me ponders..

So there might be a problem if this triggers in generic code due to
conditions at its use site. There we would not want to, nor could, fix
the generic code because in generic the thing would not be optimized. So
maybe we'd need an annotation still.

Hurm.

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH 00/12] external array access helpers Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
  [PATCH 07/12] tracing: declare __{start,stop}_ftrace_enum_maps as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
  [PATCH 01/12] extarray: define helpers for arrays defined in linker scripts Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
    Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-10-17 09:10 +0200
    Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-17 10:40 +0200
      Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Jiri Slaby <jslaby@suse.cz> - 2016-10-17 11:10 +0200
        Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-17 11:10 +0200
          Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-17 13:30 +0200
            Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-17 13:50 +0200
              Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-18 10:10 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-10-18 23:20 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Richard Biener <rguenther@suse.de> - 2016-10-19 16:30 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-19 16:40 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Richard Biener <rguenther@suse.de> - 2016-10-19 18:00 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-19 18:10 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Richard Biener <rguenther@suse.de> - 2016-10-19 17:10 +0200
                Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Peter Zijlstra <peterz@infradead.org> - 2016-10-19 18:20 +0200
              Re: [PATCH 01/12] extarray: define helpers for arrays defined in  linker scripts Jiri Slaby <jslaby@suse.cz> - 2016-10-19 16:50 +0200
  [PATCH 12/12] dynamic debug: declare table as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 17:30 +0200
  [PATCH 10/12] serial_core: declare __earlycon_table{,_end} as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 18:00 +0200
  [PATCH 08/12] tracing: declare __trace_bprintk_fmt/__tracepoint_str as external arrays Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 18:00 +0200
  [PATCH 09/12] tracing: declare __{start,stop}_syscalls_metadata as external array Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 18:20 +0200
  Re: [PATCH 00/12] external array access helpers Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-10-16 18:20 +0200
    Re: [PATCH 00/12] external array access helpers Vegard Nossum <vegard.nossum@oracle.com> - 2016-10-16 19:30 +0200
      Re: [PATCH 00/12] external array access helpers Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-10-17 09:10 +0200
    Re: [PATCH 00/12] external array access helpers Jiri Slaby <jslaby@suse.cz> - 2016-10-17 08:30 +0200

csiph-web