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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568a547d2sm8947987f8f.19.2025.06.15.13.57.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 15 Jun 2025 13:57:37 -0700 (PDT) Date: Sun, 15 Jun 2025 21:57:36 +0100 From: David Laight To: Jin D Cc: deller@gmx.de, linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fbdev: i740: Fix potential divide by zero Message-ID: <20250615215736.3d850bdd@pumpkin> In-Reply-To: References: <20250614051837.3544959-1-alexguo1023@gmail.com> <20250615093003.6e524fa9@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-fbdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Sun, 15 Jun 2025 16:43:58 -0400 Jin D wrote: > > bpp =3D var->bits_per_pixel; > > + if (!var->pixclock){ > > + dev_err(info->device, "pixclock must not be zero\n"); > > + return -EINVAL; > > + } > > switch (bpp) { > > case 1 ... 8: > > bpp =3D 8; =20 >=20 > The value used in the switch condition is var->bits_per_pixel. I can not > find a deterministic relationship between var->bits_per_pixel and > var->pixclock. Brain-fade ... >=20 > On Sun, Jun 15, 2025 at 4:30=E2=80=AFAM David Laight > wrote: >=20 > > On Sat, 14 Jun 2025 01:18:37 -0400 > > Alex Guo wrote: > > =20 > > > Variable var->pixclock can be set by user. In case it equals to > > > zero, divide by zero would occur in 4 switch branches in > > > i740fb_decode_var. > > > Similar crashes have happened in other fbdev drivers. We fix this > > > by checking whether 'pixclock' is zero. =20 > > > > Doesn't it already hit the 'default' clause of the switch statement? > > > > David > > =20 > > > > > > Similar commit: commit 16844e58704 ("video: fbdev: tridentfb: > > > Error out if 'pixclock' equals zero") > > > > > > Signed-off-by: Alex Guo > > > --- > > > drivers/video/fbdev/i740fb.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/drivers/video/fbdev/i740fb.c b/drivers/video/fbdev/i740f= b.c > > > index 9b74dae71472..861e9e397b4e 100644 > > > --- a/drivers/video/fbdev/i740fb.c > > > +++ b/drivers/video/fbdev/i740fb.c > > > @@ -419,6 +419,10 @@ static int i740fb_decode_var(const struct =20 > > fb_var_screeninfo *var, =20 > > > > > > > > > bpp =3D var->bits_per_pixel; > > > + if (!var->pixclock){ > > > + dev_err(info->device, "pixclock must not be zero\n"); > > > + return -EINVAL; > > > + } > > > switch (bpp) { > > > case 1 ... 8: > > > bpp =3D 8; =20 > > > > =20