On Mon, 22 Nov 2021 21:14:00 +0100
Gerald Schaefer [off-list ref] 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?