Messages in this thread | | | Date | Wed, 26 Feb 2003 13:23:19 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: Invalid compilation without -fno-strict-aliasing |
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On Wed, 26 Feb 2003, Jean Tourrilhes wrote:
> 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 > -
This does absoultely nothing with egcs-2.91.66. I also modified
#define FORCE_TO_MEM(x) asm volatile(""::"r"(&(x))) |________ this to "memory"
and it still does nothing. The result of gcc -O2 -S -o xxx xxx.c just shows:
#AP #NOAP
With no code in-between.
I also changed it to: #define FORCE_TO_MEM(x) __asm__ __volatile__(""::"r"(&(x))) to no avail.
What's up?
Cheers, Dick Johnson Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips). Why is the government concerned about the lunatic fringe? Think about it.
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