Re: [PATCH] scsi: gdth: Only call dma_free_coherent when buf is not NULL in ioc_general

Nathan Chancellor
2019-03-22T15:26:53Z

On Fri, Mar 22, 2019 at 03:30:08PM +0100, Arnd Bergmann wrote:
> On Fri, Mar 8, 2019 at 10:14 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> wrote:
> >
> > On Thu, Mar 7, 2019 at 3:19 PM Nathan Chancellor
> > <natechancellor@gmail.com> wrote:
> > >
> > > When building with -Wsometimes-uninitialized, Clang warns:
> > >
> > > drivers/scsi/gdth.c:3662:6: warning: variable 'paddr' is used
> > > uninitialized whenever 'if' condition is false
> > > [-Wsometimes-uninitialized]
> > >
> > > Don't attempt to call dma_free_coherent when buf is NULL (meaning that
> > > we never called dma_alloc_coherent and initialized paddr), which avoids
> > > this warning.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/402
> > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > > ---
> > > drivers/scsi/gdth.c | 5 +++--
> > > 1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/scsi/gdth.c b/drivers/scsi/gdth.c
> > > index e7f1dd4f3b66..0ca9b4393770 100644
> > > --- a/drivers/scsi/gdth.c
> > > +++ b/drivers/scsi/gdth.c
> > > @@ -3697,8 +3697,9 @@ static int ioc_general(void __user *arg, char *cmnd)
> > >
> > > rval = 0;
> > > out_free_buf:
> > > - dma_free_coherent(&ha->pdev->dev, gen.data_len + gen.sense_len, buf,
> > > - paddr);
> > > + if (buf)
> > > + dma_free_coherent(&ha->pdev->dev, gen.data_len + gen.sense_len,
> > > + buf, paddr);
> > > return rval;
> > > }
>
> I came up with a different fix for this, but I think yours is better
>
> Reviewed-by: Arnd Bergmann <arnd@arndb.de>
>
> For reference, this was my version:

This is also what I had initially but I felt this was more future proof
and matches how the rest of the tree handles calls to dma_free_coherent.

Thanks for the review!
Nathan

>
> diff --git a/drivers/scsi/gdth.c b/drivers/scsi/gdth.c
> index e7f1dd4f3b66..c01f243902e1 100644
> --- a/drivers/scsi/gdth.c
> +++ b/drivers/scsi/gdth.c
> @@ -3697,8 +3697,9 @@ static int ioc_general(void __user *arg, char *cmnd)
>
> rval = 0;
> out_free_buf:
> - dma_free_coherent(&ha->pdev->dev, gen.data_len + gen.sense_len, buf,
> - paddr);
> + if (gen.data_len + gen.sense_len > 0)
> + dma_free_coherent(&ha->pdev->dev, gen.data_len +
> gen.sense_len, buf,
> + paddr);
> return rval;
> }
>
>
> > Alternatively, paddr is a dma_addr_t defined in include/linux/types.h:
> >
> > #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> > typedef u64 dma_addr_t;
> > #else
> > typedef u32 dma_addr_t;
> > #endif
> >
> > Just initializing it to zero might be simpler than complicating the
> > control flow of this function further. Thoughts?
>
> No, blindly shutting up warnings is almost never the
> right solution, even when they are false positives. The code
> might change in the future and the bogus initialization would
> then prevent the compiler from warning about a new bug.
>
> Arnd