Phil Mayers wrote: > For those not following the Fedora bug, it (or rather, it's dependency) > has been closed by Ulrich Drepper. He seems to be saying that the > FreeRadius code is incorrect and specifically that an invalid typecast > is triggering the compiler to generate bad code:
Interesting. So GCC doesn't complain, and it generates bad assembly for C code that it thinks is perfectly valid. > https://bugzilla.redhat.com/show_bug.cgi?id=448743#c6 > > Summary (as far as I can make out): Because the code looks like this: ... > ...the compiler can't detect that the "i" value is "used", and optimises > the code touching it away. i.e. GCC doesn't properly analyze the code that it optimizes, so it generates the wrong optimizations. GCC has a history of doing this... > I'm not expert enough in the C99 standard to judge whether the comments > at the above URL are correct or not; I suspect it's a matter of some > controversy, and will back slowly away now... ;o) I think your comments about the underlying cause are correct. Ulrich's comments about Posix are interesting: >>> The invalid casts are forbidden by ISO C. POSIX >>> does not and cannot guarantee anything about this type of use of >>> sockaddr_storage. To quote the Opengroup page: http://www.opengroup.org/onlinepubs/009695399/basedefs/sys/socket.h.html ... The <sys/socket.h> header shall define the sockaddr_storage structure. This structure shall be: * Large enough to accommodate all supported protocol-specific address structures * Aligned at an appropriate boundary so that pointers to it can be cast as pointers to protocol-specific address structures and used to access the fields of those structures without alignment problems ... i.e. the code in FreeRADIUS is correct. It uses sockaddr_stoage as a generic container for protocol-specific address structures. It casts a pointer to sockaddr_storage to a pointer to protocol-specific address structures. The recommendation to use a union to hold both sockaddr_storage && sockaddr_in is... interesting. If you have to do that, WTF is the use of sockaddr_storage? Looking on the net, I also found a fair amount of code using sockaddr_storage in the recommended Posix way. It appears that FreeRADIUS is OK, and just got hit by a GCC bug. i.e. for ANY code like this: foo = (type_t *) bar; switch (x) { case 1: foo->a = 1; break; ... } function(&bar); GCC will "optimize away" the line doing "foo->a = 1;". Nice. Alan DeKok. - List info/subscribe/unsubscribe? See http://www.freeradius.org/list/users.html
