On 11/22/21 21:33, Gerald Schaefer wrote: > 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; Uh, good catch. I was actually looking suspiciously at how we cast signed to unsigned, but didn't occur to me that shortening together with endiannes is the problem. >> >> 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 all this experience I'm now inclined to rather follow more the example in Documentation/filesystems/seq_file.rst and don't pass around the pointer that we got as ppos in slab_debugfs_start(), and that seq_file.c points to m->index. In that example an own value is kmalloced: loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL); while we could just make this a field of loc_track? >> 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". I guess we don't. >> >> 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? >
