Re: [PATCH v2] powerpc/papr_scm: Fix warning triggered by perf_stats_show()

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

On Sat, Sep 12, 2020 at 01:44:51PM +0530, Vaibhav Jain wrote:
> A warning is reported by the kernel in case perf_stats_show() returns
> an error code. The warning is of the form below:
> 
>  papr_scm ibm,persistent-memory:ibm,pmemory@44100001:
>  	  Failed to query performance stats, Err:-10
>  dev_attr_show: perf_stats_show+0x0/0x1c0 [papr_scm] returned bad count
>  fill_read_buffer: dev_attr_show+0x0/0xb0 returned bad count
> 
> On investigation it looks like that the compiler is silently truncating the
> return value of drc_pmem_query_stats() from 'long' to 'int', since the
> variable used to store the return code 'rc' is an 'int'. This
> truncated value is then returned back as a 'ssize_t' back from
> perf_stats_show() to 'dev_attr_show()' which thinks of it as a large
> unsigned number and triggers this warning..
> 
> To fix this we update the type of variable 'rc' from 'int' to
> 'ssize_t' that prevents the compiler from truncating the return value
> of drc_pmem_query_stats() and returning correct signed value back from
> perf_stats_show().
> 
> Fixes: 2d02bf835e573 ('powerpc/papr_scm: Fetch nvdimm performance
>        stats from PHYP')
> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>

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

> ---
> Changelog:
> 
> v2: Added an explicit cast to the expression calling 'seq_buf_used()'
>     and triggering this issue. [ Ira ]
> ---
>  arch/powerpc/platforms/pseries/papr_scm.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
> index a88a707a608aa..5493bc847bd08 100644
> --- a/arch/powerpc/platforms/pseries/papr_scm.c
> +++ b/arch/powerpc/platforms/pseries/papr_scm.c
> @@ -785,7 +785,8 @@ static int papr_scm_ndctl(struct nvdimm_bus_descriptor *nd_desc,
>  static ssize_t perf_stats_show(struct device *dev,
>  			       struct device_attribute *attr, char *buf)
>  {
> -	int index, rc;
> +	int index;
> +	ssize_t rc;
>  	struct seq_buf s;
>  	struct papr_scm_perf_stat *stat;
>  	struct papr_scm_perf_stats *stats;
> @@ -820,7 +821,7 @@ static ssize_t perf_stats_show(struct device *dev,
>  
>  free_stats:
>  	kfree(stats);
> -	return rc ? rc : seq_buf_used(&s);
> +	return rc ? rc : (ssize_t)seq_buf_used(&s);
>  }
>  DEVICE_ATTR_ADMIN_RO(perf_stats);
>  
> -- 
> 2.26.2
> 
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