On 02/09/2016 09:53 PM, Bjorn Helgaas wrote: > Hi Hannes, > > On Wed, Jan 13, 2016 at 12:25:33PM +0100, Hannes Reinecke wrote: >> It is not always possible to determine the actual size of the VPD >> data, so allow access to them if the size is set to '0'. >> >> Signed-off-by: Hannes Reinecke <[email protected]> >> --- >> drivers/pci/pci-sysfs.c | 20 ++++++++++++-------- >> 1 file changed, 12 insertions(+), 8 deletions(-) >> >> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c >> index eead54c..de327c3 100644 >> --- a/drivers/pci/pci-sysfs.c >> +++ b/drivers/pci/pci-sysfs.c >> @@ -772,10 +772,12 @@ static ssize_t read_vpd_attr(struct file *filp, struct >> kobject *kobj, >> struct pci_dev *dev = >> to_pci_dev(container_of(kobj, struct device, kobj)); >> >> - if (off > bin_attr->size) >> - count = 0; >> - else if (count > bin_attr->size - off) >> - count = bin_attr->size - off; >> + if (bin_attr->size > 0) { >> + if (off > bin_attr->size) >> + count = 0; >> + else if (count > bin_attr->size - off) >> + count = bin_attr->size - off; >> + } > > I'm trying to figure out why we do any of this checking here. It > seems like we could do all of it in pci_read_vpd(). > > Where is bin_attr->size set? I assume this is the "attr" allocated in > pci_create_capabilities_sysfs(). I see that before your series, we > set "attr->size = dev->vpd->len" there, but after patch 3/4 of your > series, we set it to 0 there, and I don't see a place that sets it to > anything else. > That is entirely correct. One of the joys of sysfs binary attributes.
In general, sysfs binary attributes are allowed to have a size of '0', but still can provide data when read. The size of '0' is just an indicator for "I don't know how much data I can provide, figure out yourself". (Cf fs/sysfs/file.c:sysfs_kf_bin_read() for details here) In our case we're not using that function, but rather our own pci_read_vpd(), which always exposes a size. So this patch just takes the same reasoning from sysfs_kf_bin_read(), and sets the size to '0', as initially we simply do _not_ know how much data will be provided. Cheers, Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking [email protected] +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)
