[PATCH 2/3] cxl: Introduce afu_desc sysfs attribute

[Frederic Barrat]

Hi Vaibhav,

There's one thing bugging me here, see below


Le 14/03/2017 à 05:06, Vaibhav Jain a écrit :
> This patch introduces a new afu sysfs attribute named afu_desc. This
> binary attribute provides access to raw contents of the afu descriptor
> to user-space. Direct access to afu descriptor is useful for libcxl
> that can use it to determine if the CXL card has been fenced or
> provide application access to afu attributes beyond one defined in
> CAIA.
>
> We introduce three new backend-ops:
>
> * afu_desc_size(): Return the size in bytes of the afu descriptor.
>
> * afu_desc_read(): Copy into a provided buffer contents of afu
>   descriptor starting at specific offset.
>
> * afu_desc_mmap(): Memory map the afu descriptor to the given
>   vm_area_struct.
>
> If afu_desc_size() > 0 the afu_desc attribute gets created for the AFU.
> The bin_attribute callbacks route the calls to corresponding cxl backend
> implementation.
>
> Signed-off-by: Vaibhav Jain <vaibhav at linux.vnet.ibm.com>
> ---
>  Documentation/ABI/testing/sysfs-class-cxl |  9 +++++++
>  drivers/misc/cxl/cxl.h                    |  9 +++++++
>  drivers/misc/cxl/sysfs.c                  | 45 +++++++++++++++++++++++++++++++
>  3 files changed, 63 insertions(+)
>
> diff --git a/Documentation/ABI/testing/sysfs-class-cxl b/Documentation/ABI/testing/sysfs-class-cxl
> index 640f65e..9ac84c4 100644
> --- a/Documentation/ABI/testing/sysfs-class-cxl
> +++ b/Documentation/ABI/testing/sysfs-class-cxl
> @@ -6,6 +6,15 @@ Example: The real path of the attribute /sys/class/cxl/afu0.0s/irqs_max is
>
>  Slave contexts (eg. /sys/class/cxl/afu0.0s):
>
> +What:           /sys/class/cxl/<afu>/afu_desc
> +Date:           March 2016
> +Contact:        linuxppc-dev at lists.ozlabs.org
> +Description:    read only
> +                AFU Descriptor contents. The contents of this file are
> +		binary contents of the AFU descriptor. LIBCXL library can
> +		use this file to read afu descriptor and in some special cases
> +		determine if the cxl card has been fenced.
> +
>  What:           /sys/class/cxl/<afu>/afu_err_buf
>  Date:           September 2014
>  Contact:        linuxppc-dev at lists.ozlabs.org
> diff --git a/drivers/misc/cxl/cxl.h b/drivers/misc/cxl/cxl.h
> index ef683b7..1c43d06 100644
> --- a/drivers/misc/cxl/cxl.h
> +++ b/drivers/misc/cxl/cxl.h
> @@ -426,6 +426,9 @@ struct cxl_afu {
>  	u64 eb_len, eb_offset;
>  	struct bin_attribute attr_eb;
>
> +	/* Afu descriptor */
> +	struct bin_attribute attr_afud;
> +
>  	/* pointer to the vphb */
>  	struct pci_controller *phb;
>
> @@ -995,6 +998,12 @@ struct cxl_backend_ops {
>  	int (*afu_cr_write16)(struct cxl_afu *afu, int cr_idx, u64 offset, u16 val);
>  	int (*afu_cr_write32)(struct cxl_afu *afu, int cr_idx, u64 offset, u32 val);
>  	ssize_t (*read_adapter_vpd)(struct cxl *adapter, void *buf, size_t count);
> +	/* Access to AFU descriptor */
> +	ssize_t (*afu_desc_size)(struct cxl_afu *afu);
> +	ssize_t (*afu_desc_read)(struct cxl_afu *afu, char *buf, loff_t off,
> +				 size_t count);
> +	int (*afu_desc_mmap)(struct cxl_afu *afu, struct file *filp,
> +			     struct vm_area_struct *vma);
>  };
>  extern const struct cxl_backend_ops cxl_native_ops;
>  extern const struct cxl_backend_ops cxl_guest_ops;
> diff --git a/drivers/misc/cxl/sysfs.c b/drivers/misc/cxl/sysfs.c
> index a8b6d6a..fff3468 100644
> --- a/drivers/misc/cxl/sysfs.c
> +++ b/drivers/misc/cxl/sysfs.c
> @@ -426,6 +426,26 @@ static ssize_t afu_eb_read(struct file *filp, struct kobject *kobj,
>  	return cxl_ops->afu_read_err_buffer(afu, buf, off, count);
>  }
>
> +static ssize_t afu_desc_read(struct file *filp, struct kobject *kobj,
> +			     struct bin_attribute *bin_attr, char *buf,
> +			     loff_t off, size_t count)
> +{
> +	struct cxl_afu *afu = to_cxl_afu(kobj_to_dev(kobj));
> +
> +	return cxl_ops->afu_desc_read ?
> +		cxl_ops->afu_desc_read(afu, buf, off, count) : -EIO;
> +}
> +
> +static int afu_desc_mmap(struct file *filp, struct kobject *kobj,
> +			 struct bin_attribute *attr, struct vm_area_struct *vma)
> +{
> +	struct cxl_afu *afu = to_cxl_afu(kobj_to_dev(kobj));
> +
> +	return cxl_ops->afu_desc_mmap ?
> +		cxl_ops->afu_desc_mmap(afu, filp, vma) : -EINVAL;
> +}
> +
> +
>  static struct device_attribute afu_attrs[] = {
>  	__ATTR_RO(mmio_size),
>  	__ATTR_RO(irqs_min),
> @@ -625,6 +645,9 @@ void cxl_sysfs_afu_remove(struct cxl_afu *afu)
>  	struct afu_config_record *cr, *tmp;
>  	int i;
>
> +	if (afu->attr_afud.size > 0)
> +		device_remove_bin_file(&afu->dev, &afu->attr_afud);
> +
>  	/* remove the err buffer bin attribute */
>  	if (afu->eb_len)
>  		device_remove_bin_file(&afu->dev, &afu->attr_eb);
> @@ -686,6 +709,28 @@ int cxl_sysfs_afu_add(struct cxl_afu *afu)
>  		list_add(&cr->list, &afu->crs);
>  	}
>
> +	/* Create the sysfs binattr for afu-descriptor */
> +	afu->attr_afud.size = cxl_ops->afu_desc_size ?
> +		cxl_ops->afu_desc_size(afu) : 0;
> +
> +	if (afu->attr_afud.size > 0) {
> +		sysfs_attr_init(&afu->attr_afud.attr);
> +		afu->attr_afud.attr.name = "afu_desc";
> +		afu->attr_afud.attr.mode = 0444;


So I had a long pause here, wondering if we are showing too much 
information to ANY user. Most of the content of the AFU descriptor is 
already world-readable through other sysfs attributes, the only ones 
troubling me are offset 0x50 and 0x58, ie. the page resolution interrupt 
handling. We currently don't support it (yet), but we need to think 
about it. A user process, by monitoring offset x50 could get a glimpse 
of the layout of the address space of other processes. Not the actual 
content, but at least what addresses are valid. I think that's already 
too much.

Why not just exporting an area only covering 64 bits (and guaranteed not 
to be all 1's) and call it mmio_check? It would seem safer to me.

   Fred