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

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

On Sat, Sep 12, 2020 at 01:36:46AM +0530, Vaibhav Jain wrote:
> Thanks for reviewing this patch Ira,
> 
> 
> Ira Weiny <ira.weiny@intel.com> writes:
> 
> > On Thu, Sep 10, 2020 at 02:52:12PM +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>
> >> ---
> >>  arch/powerpc/platforms/pseries/papr_scm.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
> >> index a88a707a608aa..9f00b61676ab9 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;
> >
> > I'm not sure this is really fixing everything here.
> 
> The issue is with the statement in perf_stats_show():
> 
> 'return rc ? rc : seq_buf_used(&s);'
> 
> The function seq_buf_used() returns an 'unsigned int' and with 'rc'
> typed as 'int', forces a promotion of the expression to 'unsigned int'
> which causes a loss of signedness of 'rc' and compiler silently
> assigns this unsigned value to the function return typed as 'signed
> long'.
> 
> Making 'rc', a 'signed long' forces a promotion of the expresion to
> 'signed long' which preserves the signedness of 'rc' and will also be
> compatible with the function return type.

Ok, ok, I read this all wrong.

FWIW I would also cast seq_buf_used() to ssize_t to show you know what you are
doing there.

> 
> >
> > drc_pmem_query_stats() can return negative errno's.  Why are those not checked
> > somewhere in perf_stats_show()?
> >
> For the specific invocation 'drc_pmem_query_stats(p, stats, 0)' we only
> expect return value 'rc <=0' with '0' indicating a successful fetch of
> nvdimm performance stats from hypervisor. Hence there are no explicit
> checks for negative error codes in the functions as all return values
> !=0 indicate an error.
> 
> 
> > It seems like all this fix is handling is a > 0 return value: 'ret[0]' from
> > line 289 in papr_scm.c...  Or something?
> No, in case the arg 'num_stats' is '0' and 'buff_stats != NULL' the
> variable 'size' is assigned a non-zero value hence that specific branch
> you mentioned  is never taken. Instead in case of success
> drc_pmem_query_stats() return '0' and in case of an error a negative
> error code is returned.
> 
> >
> > Worse yet drc_pmem_query_stats() is returning ssize_t which is a signed value.
> > Therefore, it should not be returning -errno.  I'm surprised the static
> > checkers did not catch that.
> Didnt quite get the assertion here. The function is marked to return
> 'ssize_t' because we can return both +ve for success and -ve values to
> indicate errors.

Sorry I was reading this as size_t and meant to say unsigned...  I was looking
at this too quickly.

> 
> >
> > I believe I caught similar errors with a patch series before which did not pay
> > attention to variable types.
> >
> > Please audit this code for these types of errors and ensure you are really
> > doing the correct thing when using the sysfs interface.  I'm pretty sure bad
> > things will eventually happen (if they are not already) if you return some
> > really big number to the sysfs core from *_show().
> I think this problem is different compared to what you had previously pointed
> to. The values returned from drc_pmem_query_stats() can be stored in an
> 'int' variable too, however it was the silent promotion of a signed type
> to unsigned type was what caused this specific issue.

Ok this makes more sense now.  Sorry about not looking more carefully.

But I still think matching up the return of seq_buf_used() is worth it.  I
don't particularly like depending on 'automatic' promotions which make
reviewing code harder like this.

And sorry if my email seemed harsh I did not mean it to be.  I just like when
types are more explicit because I feel like it can avoid issues like this.
(Specifically my confusion over the types...)

:-D

Thanks,
Ira
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