| Message ID | c2ac0349b818d3f3787b939d5eb83e3c80fc205d.1467772840.git.viresh.kumar@linaro.org |
|---|---|
| State | Accepted |
| Headers | show |
Hi Viresh, A bit of background: at some point in time, the i2c adapter number (as represented internally by the kernel) could be different from the i2c device node number (as seen by user-space.) I put an end to this madness years ago, but it seems some legacy code from that time survived in i2c-dev. On Tue, 5 Jul 2016 19:57:06 -0700, Viresh Kumar wrote: > There is no code protecting i2c_dev to be freed after it is returned > from i2c_dev_get_by_minor() and using it to access the value which we > already have (minor) isn't safe really. I agree that i2c_dev_get_by_minor() looks racy by nature. It is possible that i2c_dev_get_by_minor() can be removed altogether. There are 2 other calling locations beyond the one you want to remove. If one can be removed then I suspect others can be removed as well (maybe with some more work though.) If i2c_dev_get_by_minor() needs to stay for whatever reason, then my next worry is that struct i2c_dev carries an unaccounted reference to an i2c_adapter. This looks seriously broken. i2c_dev->adap should only be set on open, and cleared on close. > Avoid using it and get the adapter directly from 'minor'. > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > --- > drivers/i2c/i2c-dev.c | 7 +------ > 1 file changed, 1 insertion(+), 6 deletions(-) > > diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c > index 6ecfd76270f2..66f323fd3982 100644 > --- a/drivers/i2c/i2c-dev.c > +++ b/drivers/i2c/i2c-dev.c > @@ -485,13 +485,8 @@ static int i2cdev_open(struct inode *inode, struct file *file) > unsigned int minor = iminor(inode); > struct i2c_client *client; > struct i2c_adapter *adap; > - struct i2c_dev *i2c_dev; > - > - i2c_dev = i2c_dev_get_by_minor(minor); > - if (!i2c_dev) > - return -ENODEV; > > - adap = i2c_get_adapter(i2c_dev->adap->nr); > + adap = i2c_get_adapter(minor); > if (!adap) > return -ENODEV; > This is the most simple fix to your immediate problem. However it doesn't address the big design issue.
Hi Jean, Thanks for the explanation. On 06-07-16, 19:04, Jean Delvare wrote: > Hi Viresh, > > A bit of background: at some point in time, the i2c adapter number (as > represented internally by the kernel) could be different from the i2c > device node number (as seen by user-space.) I put an end to this > madness years ago, but it seems some legacy code from that time > survived in i2c-dev. > > On Tue, 5 Jul 2016 19:57:06 -0700, Viresh Kumar wrote: > > There is no code protecting i2c_dev to be freed after it is returned > > from i2c_dev_get_by_minor() and using it to access the value which we > > already have (minor) isn't safe really. > > I agree that i2c_dev_get_by_minor() looks racy by nature. It is > possible that i2c_dev_get_by_minor() can be removed altogether. There > are 2 other calling locations beyond the one you want to remove. If one > can be removed then I suspect others can be removed as well (maybe with > some more work though.) > > If i2c_dev_get_by_minor() needs to stay for whatever reason, then my > next worry is that struct i2c_dev carries an unaccounted reference to > an i2c_adapter. This looks seriously broken. i2c_dev->adap should only > be set on open, and cleared on close. > > > Avoid using it and get the adapter directly from 'minor'. > > > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > > --- > > drivers/i2c/i2c-dev.c | 7 +------ > > 1 file changed, 1 insertion(+), 6 deletions(-) > > > > diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c > > index 6ecfd76270f2..66f323fd3982 100644 > > --- a/drivers/i2c/i2c-dev.c > > +++ b/drivers/i2c/i2c-dev.c > > @@ -485,13 +485,8 @@ static int i2cdev_open(struct inode *inode, struct file *file) > > unsigned int minor = iminor(inode); > > struct i2c_client *client; > > struct i2c_adapter *adap; > > - struct i2c_dev *i2c_dev; > > - > > - i2c_dev = i2c_dev_get_by_minor(minor); > > - if (!i2c_dev) > > - return -ENODEV; > > > > - adap = i2c_get_adapter(i2c_dev->adap->nr); > > + adap = i2c_get_adapter(minor); > > if (!adap) > > return -ENODEV; > > > > This is the most simple fix to your immediate problem. However it > doesn't address the big design issue. Yeah, I was just looking to fix the file operation paths for my particular problem and didn't try to do a core wide fix as I had little knowledge of the I2C subsystem :(
On Tue, 5 Jul 2016 19:57:06 -0700, Viresh Kumar wrote: > There is no code protecting i2c_dev to be freed after it is returned > from i2c_dev_get_by_minor() and using it to access the value which we > already have (minor) isn't safe really. > > Avoid using it and get the adapter directly from 'minor'. > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > --- > drivers/i2c/i2c-dev.c | 7 +------ > 1 file changed, 1 insertion(+), 6 deletions(-) > > diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c > index 6ecfd76270f2..66f323fd3982 100644 > --- a/drivers/i2c/i2c-dev.c > +++ b/drivers/i2c/i2c-dev.c > @@ -485,13 +485,8 @@ static int i2cdev_open(struct inode *inode, struct file *file) > unsigned int minor = iminor(inode); > struct i2c_client *client; > struct i2c_adapter *adap; > - struct i2c_dev *i2c_dev; > - > - i2c_dev = i2c_dev_get_by_minor(minor); > - if (!i2c_dev) > - return -ENODEV; > > - adap = i2c_get_adapter(i2c_dev->adap->nr); > + adap = i2c_get_adapter(minor); > if (!adap) > return -ENODEV; > Anyway, this is a good cleanup/optimization on its own, on top of fixing a race condition, independently of what we decide for the rest of the problems being discussed in this thread. So: Reviewed-by: Jean Delvare <jdelvare@suse.de> Tested-by: Jean Delvare <jdelvare@suse.de> Thanks,
On 07-07-16, 15:16, Jean Delvare wrote: > Anyway, this is a good cleanup/optimization on its own, on top of > fixing a race condition, independently of what we decide for the rest > of the problems being discussed in this thread. So: > > Reviewed-by: Jean Delvare <jdelvare@suse.de> > Tested-by: Jean Delvare <jdelvare@suse.de> Thanks Jean.
On Tue, Jul 05, 2016 at 07:57:06PM -0700, Viresh Kumar wrote: > There is no code protecting i2c_dev to be freed after it is returned > from i2c_dev_get_by_minor() and using it to access the value which we > already have (minor) isn't safe really. > > Avoid using it and get the adapter directly from 'minor'. > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> Applied to for-next, thanks! Jean, thanks for providing the background information! That was exactly the missing piece for me.
diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c index 6ecfd76270f2..66f323fd3982 100644 --- a/drivers/i2c/i2c-dev.c +++ b/drivers/i2c/i2c-dev.c @@ -485,13 +485,8 @@ static int i2cdev_open(struct inode *inode, struct file *file) unsigned int minor = iminor(inode); struct i2c_client *client; struct i2c_adapter *adap; - struct i2c_dev *i2c_dev; - - i2c_dev = i2c_dev_get_by_minor(minor); - if (!i2c_dev) - return -ENODEV; - adap = i2c_get_adapter(i2c_dev->adap->nr); + adap = i2c_get_adapter(minor); if (!adap) return -ENODEV;
There is no code protecting i2c_dev to be freed after it is returned from i2c_dev_get_by_minor() and using it to access the value which we already have (minor) isn't safe really. Avoid using it and get the adapter directly from 'minor'. Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> --- drivers/i2c/i2c-dev.c | 7 +------ 1 file changed, 1 insertion(+), 6 deletions(-)