On Wed, Jan 21, 2004 at 02:47:17PM +0100, Maciej W. Rozycki wrote: > On Tue, 20 Jan 2004, Daniel Jacobowitz wrote: > > > No, I'm pretty sure Pavel's right. > > > > -#ifdef __MIPSEB__ > > case 1: these.sig[0] = these32.sig[0] | (((long)these32.sig[1]) << 32); > > -#endif > > -#ifdef __MIPSEL__ > > - case 1: these.sig[0] = these32.sig[1] | (((long)these32.sig[0]) << 32); > > -#endif > > > > Consider a 64-bit sigset. 32-bit userland, 64-bit kernel. Here's a > > userland sigset with signal 33 set, only, on a little endian target. > > Word 1, least significant bit, right? > > Right, but... > > > byte address in memory > > 1 2 3 4 5 6 7 8 > > val 0 0 0 0 0 0 0 1 > > ... this is incorrect -- it would be right for big-endian; word #1, bit #1 > for little-endian is: > > byte address in memory > 1 2 3 4 5 6 7 8 > val 0 0 0 0 1 0 0 0 > > > > Obviously, as a 64-bit integer the sigset looks different. There it's > > supposed to be 1 << (33 - 1). > > val 0 0 0 1 0 0 0 0 > > Again, for little-endian it should actually be: > > val 0 0 0 0 1 0 0 0 > > i.e. the whole operation is actually a no-op, except that the 64-bit > vector is assured to be properly aligned for doubleword accesses. Re-reading what I wrote, the above was actually supposed to be a big-endian example. D'oh! If you pretend I wrote "big endian" up at the top, then it makes sense. > As a side note -- that's the reason certain C code portability problems > related to the width of the machine word only get actually discovered when > problematic software is run on a big-endian processor. I've been hit by > this property once -- I was porting a 16-bit program and it appeared to > run just fine on both a 32-bit (i386) and a 64-bit (Alpha) little-endian > CPU, but when run on a 32-bit big-endian one (SPARC) I discovered a few > more bits to be cleaned up. > > > So the correct algorithm to convert a userspace sigset to a kernel > > sigset is to shift the second word left 32 bits, and leave the first > > word right aligned, and or them together. Which is what using the > > __MIPSEB__ case does. > > But this conclusion is of course right. > > Maciej > > -- > + Maciej W. Rozycki, Technical University of Gdansk, Poland + > +--------------------------------------------------------------+ > + e-mail: macro@ds2.pg.gda.pl, PGP key available + > -- Daniel Jacobowitz MontaVista Software Debian GNU/Linux Developer
