Re: [PATCH v2] nvmem: rmem: Fix return value of rmem_read()

[Srinivas Kandagatla]

On 07/02/2024 06:35, Joy Chakraborty wrote:
> On Wed, Feb 7, 2024 at 4:06 AM Srinivas Kandagatla
> <srinivas.kandagatla@xxxxxxxxxx> wrote:
> > On 06/02/2024 04:24, Joy Chakraborty wrote:
> > > reg_read() callback registered with nvmem core expects an integer error
> > > as a return value but rmem_read() returns the number of bytes read, as a
> > > result error checks in nvmem core fail even when they shouldn't.
> > >
> > > Return 0 on success where number of bytes read match the number of bytes
> > > requested and a negative error -EINVAL on all other cases.
> > >
> > > Fixes: 5a3fa75a4d9c ("nvmem: Add driver to expose reserved memory as nvmem")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Joy Chakraborty <joychakr@xxxxxxxxxx>
> > > ---
> > >    drivers/nvmem/rmem.c | 7 ++++++-
> > >    1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/nvmem/rmem.c b/drivers/nvmem/rmem.c
> > > index 752d0bf4445e..a74dfa279ff4 100644
> > > --- a/drivers/nvmem/rmem.c
> > > +++ b/drivers/nvmem/rmem.c
> > > @@ -46,7 +46,12 @@ static int rmem_read(void *context, unsigned int offset,
> > >
> > >        memunmap(addr);
> > >
> > > -     return count;
> > > +     if (count != bytes) {
> >
> > How can this fail unless the values set in priv->mem->size is incorrect
>
> That should be correct since it would be fetched from the reserved
> memory definition in the device tree.
>
> > Only case I see this failing with short reads is when offset cross the
> > boundary of priv->mem->size.
> >
> >
> > can you provide more details on the failure usecase, may be with actual
> > values of offsets, bytes and priv->mem->size?
>
> This could very well happen if a fixed-layout defined for the reserved
> memory has a cell which defines an offset and size greater than the
> actual size of the reserved mem.

No that should just be blocked from core layer, atleast which is what is 
checked bin_attr_nvmem_read(), if checks are missing in other places 
then that needs fixing.


> For E.g. if the device tree node is as follows
> reserved-memory {
>      #address-cells = <1>;
>      #size-cells = <1>;
>      ranges;
>      nvmem@1000 {
>          compatible = "nvmem-rmem";
>          reg = <0x1000 0x400>;
>          no-map;
>          nvmem-layout {
>              compatible = "fixed-layout";
>              #address-cells = <1>;
>              #size-cells = <1>;
>              calibration@13ff {
>                  reg = <0x13ff 0x2>;

this is out of range, core should just err out.

--srini

>              };
>          };
>      };
> };
> If we try to read the cell "calibration" which crosses the boundary of
> the reserved memory then it will lead to a short read.
> Though, one might argue that the protection against such cell
> definition should be there during fixed-layout parsing in core itself
> but that is not there now and would not be a fix.
>
> What I am trying to fix here is not exactly short reads but how the
> return value of rmem_read() is treated by the nvmem core, where it
> treats a non-zero return from read as an error currently. Hence
> returning the number of bytes read leads to false failures if we try
> to read a cell.
>
>
> > > +             dev_err(priv->dev, "Failed read memory (%d)\n", count);
> > > +             return -EINVAL;
> > > +     }
> > > +
> >
> > > +     return 0;
> >
> > thanks,
> > srini
> >
> > >    }
> > >
> > >    static int rmem_probe(struct platform_device *pdev)