Messages in this thread | | | Date | Wed, 2 Jan 2002 08:54:41 -0700 | From | Tom Rini <> | Subject | Re: [PATCH] C undefined behavior fix |
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On Wed, Jan 02, 2002 at 08:11:39AM -0500, Jakub Jelinek wrote: > > On Wed, Jan 02, 2002 at 12:41:28PM +0200, Momchil Velikov wrote: > > >>>>> "Florian" == Florian Weimer <fw@deneb.enyo.de> writes: > > > > Florian> Momchil Velikov <velco@fadata.bg> writes: > > >> - strcpy(namep, RELOC("linux,phandle")); > > >> + memcpy (namep, RELOC("linux,phandle"), sizeof("linux,phandle")); > > > > Florian> Doesn't this still trigger undefined behavior, as far as the C > > Florian> standard is concerned? It's probably a better idea to fix the linker, > > Florian> so that it performs proper relocation.
This has been suggested too. And if someone implemements this in the 2.5.x timeframe it might even go in (as long as it works w/ gcc-2.95.x, which really should be doable).
> > Well, strictly speaking it _is_ undefined, however adding/subtracting > > __PAGE_OFFSET is far too common operation and one can resonably expect > > to get away with it in the _vast_ majority of cases. IMHO, it is > > better to fix the particular case, which triggers the undefined > > behaviour, as these cases are bound to be _very_ rare. > > IMHO the best thing is to change the RELOC macro, so that gcc cannot optimize > this. > E.g.: > -#define PTRRELOC(x) ((typeof(x))((unsigned long)(x) + offset)) > +#define PTRRELOC(x) ({ unsigned long __x = (unsigned long)(x); \ > asm ("" : "=r" (__x) : "0" (__x)); \ > (typeof(x))(__x + offset); }) > This way gcc cannot assume anything about it.
This is what Franz Sirl suggested awhile back, which should work, but doesn't look too nice. If gcc-3.0.x is going to be expected to produce a working kernel on all arches, I suppose this is the best fix for now.
-- Tom Rini (TR1265) http://gate.crashing.org/~trini/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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