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SubjectRe: Invalid compilation without -fno-strict-aliasing
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On Tue, Feb 25, 2003 at 11:33:09PM -0500, Albert Cahalan wrote:
> Jean Tourrilhes writes:
>
> > It looks like a compiler bug to me...
> > Some users have complained that when the following
> > code is compiled without the -fno-strict-aliasing,
> > the order of the write and memcpy is inverted (which
> > mean a bogus len is mem-copied into the stream).
> > Code (from linux/include/net/iw_handler.h) :
> > --------------------------------------------
> > static inline char *
> > iwe_stream_add_event(char * stream, /* Stream of events */
> > char * ends, /* End of stream */
> > struct iw_event *iwe, /* Payload */
> > int event_len) /* Real size of payload */
> > {
> > /* Check if it's possible */
> > if((stream + event_len) < ends) {
> > iwe->len = event_len;
> > memcpy(stream, (char *) iwe, event_len);
> > stream += event_len;
> > } return stream;
> > }
> > --------------------------------------------
> > IMHO, the compiler should have enough context to
> > know that the reordering is dangerous. Any suggestion
> > to make this simple code more bullet proof is welcomed.
> >
> > Have fun...
>
> Since (char*) is special, I agree that it's a bug.
> In any case, a warning sure would be nice!
>
> Now for the fun. Pass iwe->len into this
> macro before the memcpy, and all should be well.
>
> #define FORCE_TO_MEM(x) asm volatile(""::"r"(&(x)))
>
> Like this:
>
> iwe->len = event_len;
> FORCE_TO_MEM(iwe->len);
> memcpy(stream, (char *) iwe, event_len);

I'll try that, that sounds absolutely clever (but I only
understand half of it).
Thanks a lot !

Jean
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