On Mon, 22 Nov 2021 21:14:00 +0100 Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> wrote: [...] > > Thanks. While testing this properly, yet another bug showed up. The idx > in op->show remains 0 in all iterations, so I always see the same line > printed t->count times (or infinitely, ATM). Not sure if this only shows > on s390 due to endianness, but the reason is this: > > unsigned int idx = *(unsigned int *)v; > > IIUC, void *v is always the same as loff_t *ppos, and therefore idx also > should be *ppos. De-referencing the loff_t * with an unsigned int * only > gives the upper 32 bit half of the 64 bit value, which remains 0. > > This would be fixed e.g. with > > unsigned int idx = (unsigned int) *(loff_t *) v; > > With this fixed, my original patch actually also works for t->count > 0, > because then op->show would return w/o printing anything when idx reaches > t->count. For t->count > 0, it would even work w/o any extra checks in > op->start because of that, only "No data" would be printed infinitely. Oh, no, that would actually also fix the "No data" part, as op->show will then also return w/o printing in the next iteration, so that op->next would correctly end it all. This could also explain why it might all have worked fine on x86 (haven't verified), and really only showed on big-endian s390. Hmm, now I'm not so sure anymore if we really want the additional checks and return NULL in op->start, just to make it "double safe". > > It probably still makes sense to make this explicit in op->start, by > checking separately for !*ppos && !t->count, and returning NULL for > *ppos >= t->count, as you suggested. > > I think I will also make idx an unsigned long again, like it was before > commit 64dd68497be7, and similar to t->count. Not sure if it needs to > be, and with proper casting unsigned int is also possible, but why > change it?
