Re: [PATCH] libnvdimm/labels: Fix divide error in nd_label_data_init()

[Ira Weiny <ira.weiny@intel.com>]

Robert Richter wrote:
> If a CXL memory device returns a broken zero LSA size in its memory
> device information (Identify Memory Device (Opcode 4000h), CXL
> spec. 3.1, 8.2.9.9.1.1), a divide error occurs in the libnvdimm
> driver:
> 
>  Oops: divide error: 0000 [#1] PREEMPT SMP NOPTI
>  RIP: 0010:nd_label_data_init+0x10e/0x800 [libnvdimm]
> 
> Code and flow:
> 
> 1) CXL Command 4000h returns LSA size = 0,
> 2) config_size is assigned to zero LSA size (CXL pmem driver):
> 
> drivers/cxl/pmem.c:             .config_size = mds->lsa_size,
> 
> 3) max_xfer is set to zero (nvdimm driver):
> 
> drivers/nvdimm/label.c: max_xfer = min_t(size_t, ndd->nsarea.max_xfer, config_size);
> drivers/nvdimm/label.c: if (read_size < max_xfer) {
> drivers/nvdimm/label.c-         /* trim waste */
> 
> 4) DIV_ROUND_UP() causes division by zero:
> 
> drivers/nvdimm/label.c:         max_xfer -= ((max_xfer - 1) - (config_size - 1) % max_xfer) /
> drivers/nvdimm/label.c:                     DIV_ROUND_UP(config_size, max_xfer);

I think this is the wrong DIV_ROUND_UP which is failing because read_size is
never less than max_xfer is it?

I believe the failing DIV_ROUND_UP is after if statement here:

 489         /* Make our initial read size a multiple of max_xfer size */
 490         read_size = min(DIV_ROUND_UP(read_size, max_xfer) * max_xfer,
 491                         config_size);

Apparently nvdimm_get_config_data() was intended to check for this implicitly
but it is too late.

Anyway all this side tracked me a bit.

I assume this is a broken device which is in the real world?  The fix looks
fine.  But could you re-spin with a clean up of the commit message and I'll
queue it up.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

[snip]