Re: [PATCH v9 01/12] vfio: Mediated device Core driver

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 20 Oct 2016 00:46:48 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 10/19/2016 4:46 AM, Alex Williamson wrote:
> > On Tue, 18 Oct 2016 02:52:01 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >   
> ...
> >> +static struct mdev_device *__find_mdev_device(struct parent_device *parent,
> >> +					      uuid_le uuid)
> >> +{
> >> +	struct device *dev;
> >> +
> >> +	dev = device_find_child(parent->dev, &uuid, _find_mdev_device);
> >> +	if (!dev)
> >> +		return NULL;
> >> +
> >> +	put_device(dev);
> >> +
> >> +	return to_mdev_device(dev);
> >> +}  
> > 
> > This function is only used by mdev_device_create() for the purpose of
> > checking whether a given uuid for a parent already exists, so the
> > returned device is not actually used.  However, at the point where
> > we're using to_mdev_device() here, we don't actually hold a reference to
> > the device, so that function call and any possible use of the returned
> > pointer by the callee is invalid.  I would either turn this into a
> > "get" function where the callee has a device reference and needs to do
> > a "put" on it or change this to a "exists" test where true/false is
> > returned and the function cannot be later mis-used to do a device
> > lookup where the reference isn't actually valid.
> >   
> 
> I'll change it to return 0 if not found and -EEXIST if found.
> 
> 
> >> +int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> >> +{
> >> +	int ret;
> >> +	struct mdev_device *mdev;
> >> +	struct parent_device *parent;
> >> +	struct mdev_type *type = to_mdev_type(kobj);
> >> +
> >> +	parent = mdev_get_parent(type->parent);
> >> +	if (!parent)
> >> +		return -EINVAL;
> >> +
> >> +	/* Check for duplicate */
> >> +	mdev = __find_mdev_device(parent, uuid);
> >> +	if (mdev) {
> >> +		ret = -EEXIST;
> >> +		goto create_err;
> >> +	}  
> > 
> > We check here whether the {parent,uuid} already exists, but what
> > prevents us racing with another create call with the same uuid?  ie.
> > neither exists at this point.  Will device_register() fail if the
> > device name already exists?  If so, should we just rely on the error
> > there and skip this duplicate check?  If not, we need a mutex to avoid
> > the race.
> >  
> 
> Yes, device_register() fails if device exists already with below
> warning. Is it ok to dump such warning? I think, this should be fine,
> right? then we can remove duplicate check.
> 
> If we want to avoid such warning, we should have duplication check.

We should avoid such warnings, bugs will get filed otherwise.  Thanks
for checking.  Thanks,

Alex
 
> [  610.847958] ------------[ cut here ]------------
> [  610.855377] WARNING: CPU: 15 PID: 19839 at fs/sysfs/dir.c:31
> sysfs_warn_dup+0x64/0x80
> [  610.865798] sysfs: cannot create duplicate filename
> '/devices/pci0000:80/0000:80:02.0/0000:83:00.0/0000:84:08.0/0000:85:00.0/83b8f4f2-509f-382f-3c1e-e6bfe0fa1234'
> [  610.885101] Modules linked in:[  610.888039]  nvidia(POE)
> vfio_iommu_type1 vfio_mdev mdev vfio nfsv4 dns_resolver nfs fscache
> sb_edac edac_core x86_pkg_temp_thermal intel_powerclamp coretemp
> kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul crc32c_intel
> ghash_clmulni_intel aesni_intel glue_helper lrw gf128mul ablk_helper
> cryptd nfsd auth_rpcgss nfs_acl lockd mei_me grace iTCO_wdt
> iTCO_vendor_support mei ipmi_si pcspkr ioatdma i2c_i801 lpc_ich shpchp
> i2c_smbus mfd_core ipmi_msghandler acpi_pad uinput sunrpc xfs libcrc32c
> sd_mod mgag200 drm_kms_helper syscopyarea sysfillrect sysimgblt
> fb_sys_fops ttm drm igb ahci libahci ptp libata pps_core dca
> i2c_algo_bit i2c_core dm_mirror dm_region_hash dm_log dm_mod [last
> unloaded: mdev]
> [  610.963835] CPU: 15 PID: 19839 Comm: bash Tainted: P           OE
> 4.8.0-next-20161013+ #0
> [  610.973779] Hardware name: Supermicro
> SYS-2027GR-5204A-NC024/X9DRG-HF, BIOS 1.0c 02/28/2013
> [  610.983769]  ffffc90009323ae0 ffffffff813568bf ffffc90009323b30
> 0000000000000000
> [  610.992867]  ffffc90009323b20 ffffffff81085511 0000001f00001000
> ffff8808839ef000
> [  611.001954]  ffff88108b30f900 ffff88109ae368e8 ffff88109ae580b0
> ffff881099cc0818
> [  611.011055] Call Trace:
> [  611.015087]  [<ffffffff813568bf>] dump_stack+0x63/0x84
> [  611.021784]  [<ffffffff81085511>] __warn+0xd1/0xf0
> [  611.028115]  [<ffffffff8108558f>] warn_slowpath_fmt+0x5f/0x80
> [  611.035379]  [<ffffffff812a4e80>] ? kernfs_path_from_node+0x50/0x60
> [  611.043148]  [<ffffffff812a86c4>] sysfs_warn_dup+0x64/0x80
> [  611.050109]  [<ffffffff812a87ae>] sysfs_create_dir_ns+0x7e/0x90
> [  611.057481]  [<ffffffff81359891>] kobject_add_internal+0xc1/0x340
> [  611.065018]  [<ffffffff81359d45>] kobject_add+0x75/0xd0
> [  611.071635]  [<ffffffff81483829>] device_add+0x119/0x610
> [  611.078314]  [<ffffffff81483d3a>] device_register+0x1a/0x20
> [  611.085261]  [<ffffffffa03c748d>] mdev_device_create+0xdd/0x200 [mdev]
> [  611.093143]  [<ffffffffa03c7768>] create_store+0xa8/0xe0 [mdev]
> [  611.100385]  [<ffffffffa03c76ab>] mdev_type_attr_store+0x1b/0x30 [mdev]
> [  611.108309]  [<ffffffff812a7d8a>] sysfs_kf_write+0x3a/0x50
> [  611.115096]  [<ffffffff812a78bb>] kernfs_fop_write+0x10b/0x190
> [  611.122231]  [<ffffffff81224e97>] __vfs_write+0x37/0x140
> [  611.128817]  [<ffffffff811cea84>] ? handle_mm_fault+0x724/0xd80
> [  611.135976]  [<ffffffff81225da2>] vfs_write+0xb2/0x1b0
> [  611.142354]  [<ffffffff81003510>] ? syscall_trace_enter+0x1d0/0x2b0
> [  611.149836]  [<ffffffff812271f5>] SyS_write+0x55/0xc0
> [  611.156065]  [<ffffffff81003a47>] do_syscall_64+0x67/0x180
> [  611.162734]  [<ffffffff816d41eb>] entry_SYSCALL64_slow_path+0x25/0x25
> [  611.170345] ---[ end trace b05a73599da2ba3f ]---
> [  611.175940] ------------[ cut here ]------------
> 
> 
> 
> >> +static ssize_t create_store(struct kobject *kobj, struct device *dev,
> >> +			    const char *buf, size_t count)
> >> +{
> >> +	char *str;
> >> +	uuid_le uuid;
> >> +	int ret;
> >> +
> >> +	if (count < UUID_STRING_LEN)
> >> +		return -EINVAL;  
> > 
> > 
> > Can't we also test for something unreasonably large?
> >   
> 
> Ok. I'll add that check.
> 
> >   
> >> +
> >> +	str = kstrndup(buf, count, GFP_KERNEL);
> >> +	if (!str)
> >> +		return -ENOMEM;
> >> +
> >> +	ret = uuid_le_to_bin(str, &uuid);  
> > 
> > nit, we can kfree(str) here regardless of the return.
> >   
> >> +	if (!ret) {
> >> +
> >> +		ret = mdev_device_create(kobj, dev, uuid);
> >> +		if (ret)
> >> +			pr_err("mdev_create: Failed to create mdev device\n");  
> > 
> > What value does this pr_err add?  It doesn't tell us why it failed and
> > the user will already know if failed by the return value of their write.
> >   
> 
> Ok, will remove it.
> 
> >> +		else
> >> +			ret = count;
> >> +	}
> >> +
> >> +	kfree(str);
> >> +	return ret;
> >> +}  
> 
> ...
> 
> >> +static inline struct mdev_driver *to_mdev_driver(struct device_driver *drv)
> >> +{
> >> +	return drv ? container_of(drv, struct mdev_driver, driver) : NULL;
> >> +}
> >> +
> >> +static inline struct mdev_device *to_mdev_device(struct device *dev)
> >> +{
> >> +	return dev ? container_of(dev, struct mdev_device, dev) : NULL;  
> > 
> > 
> > Do we really need this NULL dev/drv behavior?  I don't see that any of
> > the callers can pass NULL into these.  The PCI equivalents don't
> > support this behavior and it doesn't seem they need to.  Thanks,
> >  
> 
> Ok, I'll update that.
> 
> Kirti