Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute

[Gerald Schaefer]

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?