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Re: [dmidecode] [Patch 2/3] firmware: dmi_scan: add SBMIOS entry and DMI


From: Jean Delvare
Subject: Re: [dmidecode] [Patch 2/3] firmware: dmi_scan: add SBMIOS entry and DMI tables
Date: Thu, 16 Apr 2015 11:52:52 +0200

Hi Ivan,

On Thu,  2 Apr 2015 15:57:02 +0300, Ivan Khoronzhuk wrote:
> Some utils, like dmidecode and smbios, need to access SMBIOS entry
> table area in order to get information like SMBIOS version, size, etc.
> Currently it's done via /dev/mem. But for situation when /dev/mem
> usage is disabled, the utils have to use dmi sysfs instead, which
> doesn't represent SMBIOS entry and adds code/delay redundancy when direct
> access for table is needed.
> 
> So this patch creates dmi/tables and adds SMBIOS entry point to allow
> utils in question to work correctly without /dev/mem. Also patch adds
> raw dmi table to simplify dmi table processing in user space, as
> proposed by Jean Delvare.
> 
> Signed-off-by: Ivan Khoronzhuk <address@hidden>
> ---
>  .../ABI/testing/sysfs-firmware-dmi-tables          | 22 ++++++
>  drivers/firmware/dmi-sysfs.c                       | 11 ++-
>  drivers/firmware/dmi_scan.c                        | 80 
> ++++++++++++++++++++++
>  include/linux/dmi.h                                |  1 +
>  4 files changed, 107 insertions(+), 7 deletions(-)
>  create mode 100644 Documentation/ABI/testing/sysfs-firmware-dmi-tables

First of all: thanks for doing this, it looks mostly good now, and I've
tested it successfully on my own system.

> diff --git a/Documentation/ABI/testing/sysfs-firmware-dmi-tables 
> b/Documentation/ABI/testing/sysfs-firmware-dmi-tables
> new file mode 100644
> index 0000000..f46158c
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-firmware-dmi-tables
> @@ -0,0 +1,22 @@
> +What:                /sys/firmware/dmi/tables/
> +Date:                April 2015
> +Contact:     Ivan Khoronzhuk <address@hidden>
> +Description:
> +             The firmware provides DMI structures as a packed list of
> +             data referenced by a SMBIOS table entry point. The SMBIOS
> +             entry point contains general information, like SMBIOS
> +             version, DMI table size, etc. The structure, content and
> +             size of SMBIOS entry point is dependent on SMBIOS version.
> +             The format of SMBIOS entry point, equal as DMI structures
> +             can be read in SMBIOS specification.

"equal as" sounds strange, I think a simple "and" would be better.

> +
> +             The dmi/tables provides raw SMBIOS entry point and DMI tables
> +             through sysfs as an alternative to utilities reading them
> +             from /dev/mem. The raw SMBIOS entry point and DMI table are
> +             presented as raw attributes and are accessible via:

"binary attributes" rather than "raw attributes"? The "raw" nature is
already mentioned earlier in the sentence.

> +
> +             /sys/firmware/dmi/tables/smbios_entry_point
> +             /sys/firmware/dmi/tables/DMI
> +
> +             The complete DMI information can be taken using these two

"obtained" or "decoded" would sound better than "taken" IMHO.

> +             tables.
> diff --git a/drivers/firmware/dmi-sysfs.c b/drivers/firmware/dmi-sysfs.c
> index e0f1cb3..8e1a411 100644
> --- a/drivers/firmware/dmi-sysfs.c
> +++ b/drivers/firmware/dmi-sysfs.c
> @@ -566,7 +566,6 @@ static struct kobj_type dmi_sysfs_entry_ktype = {
>       .default_attrs = dmi_sysfs_entry_attrs,
>  };
>  
> -static struct kobject *dmi_kobj;
>  static struct kset *dmi_kset;
>  
>  /* Global count of all instances seen.  Only for setup */
> @@ -651,10 +650,11 @@ static int __init dmi_sysfs_init(void)
>       int error = -ENOMEM;

This initialization can be moved to the single error path left that
needs it. (I can do it myself in a separate patch if you don't want to
do it here.)

>       int val;
>  
> -     /* Set up our directory */
> -     dmi_kobj = kobject_create_and_add("dmi", firmware_kobj);
> -     if (!dmi_kobj)
> +     if (!dmi_kobj) {
> +             pr_err("dmi-sysfs: dmi entry is absent.\n");
> +             error = -ENOSYS;
>               goto err;
> +     }
>  
>       dmi_kset = kset_create_and_add("entries", NULL, dmi_kobj);
>       if (!dmi_kset)
> @@ -675,7 +675,6 @@ static int __init dmi_sysfs_init(void)
>  err:
>       cleanup_entry_list();
>       kset_unregister(dmi_kset);
> -     kobject_put(dmi_kobj);
>       return error;
>  }
>  
> @@ -685,8 +684,6 @@ static void __exit dmi_sysfs_exit(void)
>       pr_debug("dmi-sysfs: unloading.\n");
>       cleanup_entry_list();
>       kset_unregister(dmi_kset);
> -     kobject_del(dmi_kobj);
> -     kobject_put(dmi_kobj);
>  }
>  
>  module_init(dmi_sysfs_init);
> diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
> index d3aae09..bb19f8b 100644
> --- a/drivers/firmware/dmi_scan.c
> +++ b/drivers/firmware/dmi_scan.c
> @@ -10,6 +10,9 @@
>  #include <asm/dmi.h>
>  #include <asm/unaligned.h>
>  
> +struct kobject *dmi_kobj;
> +EXPORT_SYMBOL_GPL(dmi_kobj);
> +
>  /*
>   * DMI stands for "Desktop Management Interface".  It is part
>   * of and an antecedent to, SMBIOS, which stands for System
> @@ -20,6 +23,9 @@ static const char dmi_empty_string[] = "        ";
>  static u32 dmi_ver __initdata;
>  static u32 dmi_len;
>  static u16 dmi_num;
> +static u8 smbios_entry_point[32];
> +static int smbios_entry_point_size;
> +
>  /*
>   * Catch too early calls to dmi_check_system():
>   */
> @@ -118,6 +124,7 @@ static void dmi_decode_table(u8 *buf,
>  }
>  
>  static phys_addr_t dmi_base;
> +static u8 *dmi_table;

This variable is only ever used in a single function (dmi_init). Does
it actually need to be a global variable? I suspect not.

>  
>  static int __init dmi_walk_early(void (*decode)(const struct dmi_header *,
>               void *))
> @@ -476,6 +483,8 @@ static int __init dmi_present(const u8 *buf)
>       if (memcmp(buf, "_SM_", 4) == 0 &&
>           buf[5] < 32 && dmi_checksum(buf, buf[5])) {
>               smbios_ver = get_unaligned_be16(buf + 6);
> +             smbios_entry_point_size = buf[5];
> +             memcpy(smbios_entry_point, buf, smbios_entry_point_size);
>  
>               /* Some BIOS report weird SMBIOS version, fix that up */
>               switch (smbios_ver) {
> @@ -508,6 +517,9 @@ static int __init dmi_present(const u8 *buf)
>                                       dmi_ver >> 8, dmi_ver & 0xFF,
>                                       (dmi_ver < 0x0300) ? "" : ".x");
>                       } else {
> +                             smbios_entry_point_size = 15;
> +                             memcpy(smbios_entry_point, buf,
> +                                    smbios_entry_point_size);
>                               dmi_ver = (buf[14] & 0xF0) << 4 |
>                                          (buf[14] & 0x0F);
>                               pr_info("Legacy DMI %d.%d present.\n",
> @@ -535,6 +547,8 @@ static int __init dmi_smbios3_present(const u8 *buf)
>               dmi_ver &= 0xFFFFFF;
>               dmi_len = get_unaligned_le32(buf + 12);
>               dmi_base = get_unaligned_le64(buf + 16);
> +             smbios_entry_point_size = buf[6];
> +             memcpy(smbios_entry_point, buf, smbios_entry_point_size);
>  
>               /*
>                * The 64-bit SMBIOS 3.0 entry point no longer has a field
> @@ -638,6 +652,72 @@ void __init dmi_scan_machine(void)
>       dmi_initialized = 1;
>  }
>  
> +static ssize_t raw_table_read(struct file *file, struct kobject *kobj,
> +                           struct bin_attribute *attr, char *buf,
> +                           loff_t pos, size_t count)
> +{
> +     memcpy(buf, attr->private + pos, count);
> +     return count;
> +}
> +
> +static BIN_ATTR(smbios_entry_point, S_IRUSR, raw_table_read, NULL, 0);

This one could be world-readable as it contains no sensitive
information.

> +struct bin_attribute bin_attr_dmi_table =
> +                     __BIN_ATTR(DMI, S_IRUSR, raw_table_read, NULL, 0);

I do not understand why you don't use BIN_ATTR here too? I tried naming
the attribute bin_attr_DMI and it seems to work just fine, checkpatch
doesn't even complain!

> +
> +static int __init dmi_init(void)
> +{
> +     int ret = -ENOMEM;
> +     struct kobject *tables_kobj = NULL;
> +
> +     if (!dmi_available) {
> +             ret = -ENOSYS;
> +             goto err;
> +     }

This is weird. Can this actually happen?

We currently have two ways to enter this module: dmi_scan_machine(),
which is called by the architecture code, and dmi_init(), which is
called at subsys_initcall time. As far as I can see,
core/arch_initcalls are guaranteed to be always called before
subsys_initcalls. If we can rely on that, the test above is not needed.
If for any reason we can't, that means that dmi_init() should not be a
subsys_initcall, but should instead be called explicitly at the end of
dmi_scan_machine().

> +
> +     /* Set up dmi directory at /sys/firmware/dmi */
> +     dmi_kobj = kobject_create_and_add("dmi", firmware_kobj);
> +     if (!dmi_kobj)
> +             goto err;
> +
> +     tables_kobj = kobject_create_and_add("tables", dmi_kobj);
> +     if (!tables_kobj)
> +             goto err;
> +
> +     bin_attr_smbios_entry_point.size = smbios_entry_point_size;
> +     bin_attr_smbios_entry_point.private = smbios_entry_point;
> +     ret = sysfs_create_bin_file(tables_kobj, &bin_attr_smbios_entry_point);
> +     if (ret)
> +             goto err;
> +
> +     dmi_table = dmi_remap(dmi_base, dmi_len);
> +     if (!dmi_table)
> +             goto err;

At this point "ret" has value 0 so you will take the error path but
return with success. No good. I suggest that you move the call to
dmi_remap before the first call to sysfs_create_bin_file above, so that
"ret" still has value -ENOMEM.

> +
> +     bin_attr_dmi_table.size = dmi_len;
> +     bin_attr_dmi_table.private = dmi_table;
> +     ret = sysfs_create_bin_file(tables_kobj, &bin_attr_dmi_table);
> +     if (ret) {
> +             dmi_unmap(dmi_table);

Doing this here goes against the logic of having a single error path.
Instead you should have an additional error label before err: and jump
there.

> +             goto err;
> +     }
> +
> +     return 0;
> +err:
> +     pr_err("dmi: Firmware registration failed.\n");
> +
> +     if (tables_kobj)
> +             sysfs_remove_bin_file(tables_kobj,
> +                                   &bin_attr_smbios_entry_point);
> +     kobject_del(tables_kobj);
> +     kobject_put(tables_kobj);

These last two calls could be moved inside the conditional above. Even
better would be a separate error label so that you know exactly what
needs to be undone.

> +     kobject_del(dmi_kobj);
> +     kobject_put(dmi_kobj);
> +     dmi_kobj = NULL;

I'm wondering, wouldn't it make sense to keep dmi_kobj alive (with an
appropriate comment), so that dmi-sysfs has a chance to load? As it is
now, a bug or some unexpected behavior in this new code could cause a
regression for dmi-sysfs users. Just because I don't like dmi_sysfs
doesn't mean we can break it ;-)

> +
> +     return ret;
> +}
> +subsys_initcall(dmi_init);
> +
>  /**
>   * dmi_set_dump_stack_arch_desc - set arch description for dump_stack()
>   *
> diff --git a/include/linux/dmi.h b/include/linux/dmi.h
> index f820f0a..9f55f46 100644
> --- a/include/linux/dmi.h
> +++ b/include/linux/dmi.h
> @@ -93,6 +93,7 @@ struct dmi_dev_onboard {
>       int devfn;
>  };
>  
> +extern struct kobject *dmi_kobj;

struct kobject is defined in <linux/kobject.h> so I think you should
include that file to avoid random build failures in the future.

>  extern int dmi_check_system(const struct dmi_system_id *list);
>  const struct dmi_system_id *dmi_first_match(const struct dmi_system_id 
> *list);
>  extern const char * dmi_get_system_info(int field);


-- 
Jean Delvare
SUSE L3 Support



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