linux-kernel - Re: [PATCH V2] staging: gpib: Remove a dead condition in if statement

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Message-ID: <f3fdd6b8-53bb-4e9d-bb32-3816035d4d52@stanley.mountain>
Date: Wed, 16 Oct 2024 18:00:26 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: "Everest K.C." <everestkc@...restkc.com.np>
Cc: Greg KH <gregkh@...uxfoundation.org>, dpenkler@...il.com,
	skhan@...uxfoundation.org, linux-staging@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] staging: gpib: Remove a dead condition in if statement

On Wed, Oct 16, 2024 at 06:54:00AM -0600, Everest K.C. wrote:
> > > diff --git a/drivers/staging/gpib/eastwood/fluke_gpib.c b/drivers/staging/gpib/eastwood/fluke_gpib.c
> > > index f9f149db222d..51b4f9891a34 100644
> > > --- a/drivers/staging/gpib/eastwood/fluke_gpib.c
> > > +++ b/drivers/staging/gpib/eastwood/fluke_gpib.c
> > > @@ -644,7 +644,7 @@ static int fluke_dma_read(gpib_board_t *board, uint8_t *buffer,
> > >        */
> > >       usleep_range(10, 15);
> > >       residue = fluke_get_dma_residue(e_priv->dma_channel, dma_cookie);
> > > -     if (WARN_ON_ONCE(residue > length || residue < 0))
> > > +     if (WARN_ON_ONCE(residue > length))
> >
> > No, this is incorrect, now we never notice is the call to
> > fluke_get_dma_residue() has failed.  Please fix that bug instead (hint,
> > Covertity is giving you a pointer to where something might be wrong, but
> > this change is NOT how to fix it.)
> I need a little guidance here.
> My best guess to fix the bug would be to make fluke_get_dma_residue()
> return an int instead of unsigned int or size_t. But theoretically the
> maximum value of residue can be UINT_MAX, and casting it to int will
> result in a negative number, which in turn will cause  the error check
> condition to evaluate to true.
> The best solution I see would be to make fluke_get_dma_residue() return
> an int (-1 for error and 0 for success). Then pass the address of residue
> reference to fluke_get_dma_residue() to be updated.
> Am I on the right track ?

Functions shouldn't return -1 on error, they should return proper error codes.

> 
> Also,I searched for the functions with names that match get_dma_residue
> in the kernel source code and found that they return unsigned int. I also
> noticed that no error checks have been made to check if get_dma_residue
> was successful.

$ git grep get_dma_residue | grep static | grep -v gpib

arch/alpha/include/asm/dma.h:static __inline__ int get_dma_residue(unsigned int dmanr)
arch/arm/mach-footbridge/dma-isa.c:static int isa_get_dma_residue(unsigned int chan, dma_t *dma)
arch/m68k/include/asm/floppy.h:static int vdma_get_dma_residue(unsigned int dummy)
arch/mips/include/asm/dma.h:static __inline__ int get_dma_residue(unsigned int dmanr)
arch/mips/include/asm/mach-au1x00/au1000_dma.h:static inline int get_dma_residue(unsigned int dmanr)
arch/mips/include/asm/mach-generic/floppy.h:static inline int fd_get_dma_residue(void)
arch/mips/include/asm/mach-jazz/floppy.h:static inline int fd_get_dma_residue(void)
arch/parisc/include/asm/dma.h:static __inline__ int get_dma_residue(unsigned int dmanr)
arch/parisc/include/asm/floppy.h:static int vdma_get_dma_residue(unsigned int dummy)
arch/powerpc/include/asm/dma.h:static __inline__ int get_dma_residue(unsigned int dmanr)
arch/powerpc/include/asm/floppy.h:static int vdma_get_dma_residue(unsigned int dummy)
arch/sh/drivers/dma/dma-pvr2.c:static int pvr2_get_dma_residue(struct dma_channel *chan)
arch/sh/drivers/dma/dma-sh.c:static int sh_dmac_get_dma_residue(struct dma_channel *chan)
arch/sparc/include/asm/floppy_64.h:static unsigned int sun_get_dma_residue(void)
arch/sparc/include/asm/floppy_64.h:static unsigned int sun_pci_get_dma_residue(void)
arch/sparc/include/asm/parport_64.h:static inline unsigned int get_dma_residue(unsigned int dmanr)
arch/x86/include/asm/dma.h:static inline int get_dma_residue(unsigned int dmanr)
arch/x86/include/asm/floppy.h:static int vdma_get_dma_residue(unsigned int dummy)

Only the Sparc functions return unsigned int.  The rest return int.

The return value is going to be between 0 and priv->dma_buffer_size (0x7ff).
It's set in fluke_allocate_private().

Just make the return value an int.

regards,
dan carpenter