[PATCH v5.5] of/fdt: export fdt blob as /sys/firmware/fdt

[Mark Rutland]

On Tue, Nov 18, 2014 at 04:51:45PM +0000, Grant Likely wrote:
> On Fri, 14 Nov 2014 18:05:35 +0100
> , Ard Biesheuvel <ard.biesheuvel at linaro.org>
>  wrote:
> > Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
> > that was passed to the kernel by the bootloader. This allows userland
> > applications such as kexec to access the raw binary.
> > 
> > The fact that this node does not reside under /sys/firmware/device-tree
> > is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
> > communicate just the UEFI and ACPI entry points, but the FDT is never
> > unflattened and used to configure the system.
> > 
> > A CRC32 checksum is calculated over the entire FDT blob, and verified
> > at late_initcall time. The sysfs entry is instantiated only if the
> > checksum is valid, i.e., if the FDT blob has not been modified in the
> > mean time. Otherwise, a warning is printed.
> > 
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> 
> So, I have a doubt...
> 
> When this patch is merged, we have two separate ABIs for extracting the
> DT from the kernel. /proc/device-tree and /sys/firmware/fdt. Merging it
> into mainline means we're committed to supporting both interfaces.
> 
> The whole purpose of this patch is to support kexec. Specifically, kexec
> on ACPI platforms that don't currently unflatten the tree.*  Would it
> not be better to always unflatten the tree and have only one interface
> that kexec needs to use to obtain the tree?

The whole reasoning for not unflattening the DTB in the presence of ACPI
is that it's unlikely we can stop drivers using DTB information in
addition to ACPI, leaving us with a completely non-standard mess. We
really don't want people mixing the two, and it's going to be a struggle
if the information is readily available (even preventing probing based
on compatible string isn't enough because of some drivers which probe
from an initcall or look up magic paths).

So if we were to unflatten the tree in the ACPI case, at a minimum it
needs to be unflattened to a separate root that only exists for sysfs,
and is not at exposed to the mercy of the rest of the kernel.

> * It also helps with exposing the reserved map to userspace, but kexec
>   has done without that feature for years, and it is in the process of
>   being deprecated in favour of /reserved-memory anyway.

This is the first I'd heard of the reserve map being deprecated, and
we're going to have DTs with reserved map entries for a long time going
forwards.

There are also other kernels (e.g. Xen) using DT that I believe handle
reserve map entries but not (currently) reserved-memory nodes, and I'd
rather we didn't have the kernel rewrite things they understand into
things they didn't. For debugging on a production kernel I'd also like
to have as close to the original information provided as possible
exposed to userspace.

Can't we expose the header fields under something like
/sys/firmware/devicetree/dtb-header/, parallel to the usual
/sys/firmware/devicetree/base for nodes?

Mark.

> 
> g.
> 
> (side comment: I just realized that if I do merge this patch, it needs
> to include documentation of the ABI in /Documentation/ABI/*/sysfs-8)
> 
> > ---
> > 
> > v5.5: based on Grant's changes
> > """
> > Actually, I made a couple of changes when I merged it. I removed the
> > older debugfs interface since it overlaps, and I added tests for
> > initial_boot_params to make sure it doesn't try to run on an invalid
> > FDT
> > """
> > but I removed the second fdt_check_header() again in the late init code, as it
> > would result in a corrupted FDT to be silently ignored. Note that
> > initial_boot_params can only be set if fdt_check_header() succeeded the first
> > time, so a failure occurring the second time should produce a warning, and
> > the CRC check will catch it anyway. The CRC check itself is fixed to use the
> > API as it is supposed to. I also moved the CRC variable definition inside the
> > #ifdef OF_EARLY_FLATTREE region.
> > 
> > v4: use pr_warn() instead of WARN()
> > v3: keep checksum instead of copying the entire blob, and WARN on mismatch
> > 
> >  drivers/of/Kconfig |  1 +
> >  drivers/of/fdt.c   | 43 +++++++++++++++++++++++++++----------------
> >  2 files changed, 28 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> > index 1a13f5b722c5..0348c208343c 100644
> > --- a/drivers/of/Kconfig
> > +++ b/drivers/of/Kconfig
> > @@ -23,6 +23,7 @@ config OF_FLATTREE
> >  	bool
> >  	select DTC
> >  	select LIBFDT
> > +	select CRC32
> >  
> >  config OF_EARLY_FLATTREE
> >  	bool
> > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > index d1ffca8b34ea..2bbda0775f57 100644
> > --- a/drivers/of/fdt.c
> > +++ b/drivers/of/fdt.c
> > @@ -9,6 +9,7 @@
> >   * version 2 as published by the Free Software Foundation.
> >   */
> >  
> > +#include <linux/crc32.h>
> >  #include <linux/kernel.h>
> >  #include <linux/initrd.h>
> >  #include <linux/memblock.h>
> > @@ -22,6 +23,7 @@
> >  #include <linux/libfdt.h>
> >  #include <linux/debugfs.h>
> >  #include <linux/serial_core.h>
> > +#include <linux/sysfs.h>
> >  
> >  #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
> >  #include <asm/page.h>
> > @@ -425,6 +427,8 @@ void *initial_boot_params;
> >  
> >  #ifdef CONFIG_OF_EARLY_FLATTREE
> >  
> > +static u32 of_fdt_crc32;
> > +
> >  /**
> >   * res_mem_reserve_reg() - reserve all memory described in 'reg' property
> >   */
> > @@ -996,6 +1000,8 @@ bool __init early_init_dt_verify(void *params)
> >  
> >  	/* Setup flat device-tree pointer */
> >  	initial_boot_params = params;
> > +	of_fdt_crc32 = crc32_be(~0, initial_boot_params,
> > +				fdt_totalsize(initial_boot_params));
> >  
> >  	/* check device tree validity */
> >  	if (fdt_check_header(params)) {
> > @@ -1080,27 +1086,32 @@ void __init unflatten_and_copy_device_tree(void)
> >  	unflatten_device_tree();
> >  }
> >  
> > -#if defined(CONFIG_DEBUG_FS) && defined(DEBUG)
> > -static struct debugfs_blob_wrapper flat_dt_blob;
> > -
> > -static int __init of_flat_dt_debugfs_export_fdt(void)
> > +#ifdef CONFIG_SYSFS
> > +static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
> > +			       struct bin_attribute *bin_attr,
> > +			       char *buf, loff_t off, size_t count)
> >  {
> > -	struct dentry *d = debugfs_create_dir("device-tree", NULL);
> > -
> > -	if (!d)
> > -		return -ENOENT;
> > +	memcpy(buf, initial_boot_params + off, count);
> > +	return count;
> > +}
> >  
> > -	flat_dt_blob.data = initial_boot_params;
> > -	flat_dt_blob.size = fdt_totalsize(initial_boot_params);
> > +static int __init of_fdt_raw_init(void)
> > +{
> > +	static struct bin_attribute of_fdt_raw_attr =
> > +		__BIN_ATTR(fdt, S_IRUSR, of_fdt_raw_read, NULL, 0);
> >  
> > -	d = debugfs_create_blob("flat-device-tree", S_IFREG | S_IRUSR,
> > -				d, &flat_dt_blob);
> > -	if (!d)
> > -		return -ENOENT;
> > +	if (!initial_boot_params)
> > +		return 0;
> >  
> > -	return 0;
> > +	if (of_fdt_crc32 != crc32_be(~0, initial_boot_params,
> > +				     fdt_totalsize(initial_boot_params))) {
> > +		pr_warn("fdt: not creating '/sys/firmware/fdt': CRC check failed\n");
> > +		return 0;
> > +	}
> > +	of_fdt_raw_attr.size = fdt_totalsize(initial_boot_params);
> > +	return sysfs_create_bin_file(firmware_kobj, &of_fdt_raw_attr);
> >  }
> > -module_init(of_flat_dt_debugfs_export_fdt);
> > +late_initcall(of_fdt_raw_init);
> >  #endif
> >  
> >  #endif /* CONFIG_OF_EARLY_FLATTREE */
> > -- 
> > 1.8.3.2
> > 
> 
>