firmware: create directory hierarchy for sysfs fw_cfg entries

Each fw_cfg entry of type "file" has an associated 56-char,
nul-terminated ASCII string which represents its name. While
the fw_cfg device doesn't itself impose any specific naming
convention, QEMU developers have traditionally used path name
semantics (i.e. "etc/acpi/rsdp") to descriptively name the
various fw_cfg "blobs" passed into the guest.

This patch attempts, on a best effort basis, to create a
directory hierarchy representing the content of fw_cfg file
names, under /sys/firmware/qemu_fw_cfg/by_name.

Upon successful creation of all directories representing the
"dirname" portion of a fw_cfg file, a symlink will be created
to represent the "basename", pointing at the appropriate
/sys/firmware/qemu_fw_cfg/by_key entry. If a file name is not
suitable for this procedure (e.g., if its basename or dirname
components collide with an already existing dirname component
or basename, respectively) the corresponding fw_cfg blob is
skipped and will remain available in sysfs only by its selector
key value.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 148 insertions, 3 deletions

diff --git a/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
index e9e58d4ea60a..011dda4f8e8a 100644
--- a/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
+++ b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
@@ -56,3 +56,45 @@ Description:
                           entry via the control register, and reading a number
                           of bytes equal to the blob size from the data
                           register.
+
+                --- Listing fw_cfg blobs by file name ---
+
+                While the fw_cfg device does not impose any specific naming
+                convention on the blobs registered in the file directory,
+                QEMU developers have traditionally used path name semantics
+                to give each blob a descriptive name. For example:
+
+                        "bootorder"
+                        "genroms/kvmvapic.bin"
+                        "etc/e820"
+                        "etc/boot-fail-wait"
+                        "etc/system-states"
+                        "etc/table-loader"
+                        "etc/acpi/rsdp"
+                        "etc/acpi/tables"
+                        "etc/smbios/smbios-tables"
+                        "etc/smbios/smbios-anchor"
+                        ...
+
+                In addition to the listing by unique selector key described
+                above, the fw_cfg sysfs driver also attempts to build a tree
+                of directories matching the path name components of fw_cfg
+                blob names, ending in symlinks to the by_key entry for each
+                "basename", as illustrated below (assume current directory is
+                /sys/firmware):
+
+                    qemu_fw_cfg/by_name/bootorder -> ../by_key/38
+                    qemu_fw_cfg/by_name/etc/e820 -> ../../by_key/35
+                    qemu_fw_cfg/by_name/etc/acpi/rsdp -> ../../../by_key/41
+                    ...
+
+                Construction of the directory tree and symlinks is done on a
+                "best-effort" basis, as there is no guarantee that components
+                of fw_cfg blob names are always "well behaved". I.e., there is
+                the possibility that a symlink (basename) will conflict with
+                a dirname component of another fw_cfg blob, in which case the
+                creation of the offending /sys/firmware/qemu_fw_cfg/by_name
+                entry will be skipped.
+
+                The authoritative list of entries will continue to be found
+                under the /sys/firmware/qemu_fw_cfg/by_key directory.
diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index 83e8a5c8f887..19f6851be87f 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -334,9 +334,103 @@ static struct bin_attribute fw_cfg_sysfs_attr_raw = {
         .read = fw_cfg_sysfs_read_raw,
 };
 
-/* kobjects representing top-level and by_key folders */
+/*
+ * Create a kset subdirectory matching each '/' delimited dirname token
+ * in 'name', starting with sysfs kset/folder 'dir'; At the end, create
+ * a symlink directed at the given 'target'.
+ * NOTE: We do this on a best-effort basis, since 'name' is not guaranteed
+ * to be a well-behaved path name. Whenever a symlink vs. kset directory
+ * name collision occurs, the kernel will issue big scary warnings while
+ * refusing to add the offending link or directory. We follow up with our
+ * own, slightly less scary error messages explaining the situation :)
+ */
+static int fw_cfg_build_symlink(struct kset *dir,
+                                struct kobject *target, const char *name)
+{
+        int ret;
+        struct kset *subdir;
+        struct kobject *ko;
+        char *name_copy, *p, *tok;
+
+        if (!dir || !target || !name || !*name)
+                return -EINVAL;
+
+        /* clone a copy of name for parsing */
+        name_copy = p = kstrdup(name, GFP_KERNEL);
+        if (!name_copy)
+                return -ENOMEM;
+
+        /* create folders for each dirname token, then symlink for basename */
+        while ((tok = strsep(&p, "/")) && *tok) {
+
+                /* last (basename) token? If so, add symlink here */
+                if (!p || !*p) {
+                        ret = sysfs_create_link(&dir->kobj, target, tok);
+                        break;
+                }
+
+                /* does the current dir contain an item named after tok ? */
+                ko = kset_find_obj(dir, tok);
+                if (ko) {
+                        /* drop reference added by kset_find_obj */
+                        kobject_put(ko);
+
+                        /* ko MUST be a kset - we're about to use it as one ! */
+                        if (ko->ktype != dir->kobj.ktype) {
+                                ret = -EINVAL;
+                                break;
+                        }
+
+                        /* descend into already existing subdirectory */
+                        dir = to_kset(ko);
+                } else {
+                        /* create new subdirectory kset */
+                        subdir = kzalloc(sizeof(struct kset), GFP_KERNEL);
+                        if (!subdir) {
+                                ret = -ENOMEM;
+                                break;
+                        }
+                        subdir->kobj.kset = dir;
+                        subdir->kobj.ktype = dir->kobj.ktype;
+                        ret = kobject_set_name(&subdir->kobj, "%s", tok);
+                        if (ret) {
+                                kfree(subdir);
+                                break;
+                        }
+                        ret = kset_register(subdir);
+                        if (ret) {
+                                kfree(subdir);
+                                break;
+                        }
+
+                        /* descend into newly created subdirectory */
+                        dir = subdir;
+                }
+        }
+
+        /* we're done with cloned copy of name */
+        kfree(name_copy);
+        return ret;
+}
+
+/* recursively unregister fw_cfg/by_name/ kset directory tree */
+static void fw_cfg_kset_unregister_recursive(struct kset *kset)
+{
+        struct kobject *k, *next;
+
+        list_for_each_entry_safe(k, next, &kset->list, entry)
+                /* all set members are ksets too, but check just in case... */
+                if (k->ktype == kset->kobj.ktype)
+                        fw_cfg_kset_unregister_recursive(to_kset(k));
+
+        /* symlinks are cleanly and automatically removed with the directory */
+        kset_unregister(kset);
+}
+
+/* kobjects & kset representing top-level, by_key, and by_name folders */
 static struct kobject *fw_cfg_top_ko;
 static struct kobject *fw_cfg_sel_ko;
+static struct kset *fw_cfg_fname_kset;
 
 /* register an individual fw_cfg file */
 static int fw_cfg_register_file(const struct fw_cfg_file *f)
@@ -363,6 +457,9 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
         if (err)
                 goto err_add_raw;
 
+        /* try adding "/sys/firmware/qemu_fw_cfg/by_name/" symlink */
+        fw_cfg_build_symlink(fw_cfg_fname_kset, &entry->kobj, entry->f.name);
+
         /* success, add entry to global cache */
         fw_cfg_sysfs_cache_enlist(entry);
         return 0;
@@ -417,18 +514,21 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
 
         /* NOTE: If we supported multiple fw_cfg devices, we'd first create
          * a subdirectory named after e.g. pdev->id, then hang per-device
-         * by_key subdirectories underneath it. However, only
+         * by_key (and by_name) subdirectories underneath it. However, only
          * one fw_cfg device exist system-wide, so if one was already found
          * earlier, we might as well stop here.
          */
         if (fw_cfg_sel_ko)
                 return -EBUSY;
 
-        /* create by_key subdirectory of /sys/firmware/qemu_fw_cfg/ */
+        /* create by_key and by_name subdirs of /sys/firmware/qemu_fw_cfg/ */
         err = -ENOMEM;
         fw_cfg_sel_ko = kobject_create_and_add("by_key", fw_cfg_top_ko);
         if (!fw_cfg_sel_ko)
                 goto err_sel;
+        fw_cfg_fname_kset = kset_create_and_add("by_name", NULL, fw_cfg_top_ko);
+        if (!fw_cfg_fname_kset)
+                goto err_name;
 
         /* initialize fw_cfg device i/o from platform data */
         err = fw_cfg_do_platform_probe(pdev);
@@ -457,6 +557,8 @@ err_dir:
 err_rev:
         fw_cfg_io_cleanup();
 err_probe:
+        fw_cfg_kset_unregister_recursive(fw_cfg_fname_kset);
+err_name:
         fw_cfg_kobj_cleanup(fw_cfg_sel_ko);
 err_sel:
         return err;
@@ -466,6 +568,7 @@ static int fw_cfg_sysfs_remove(struct platform_device *pdev)
 {
         pr_debug("fw_cfg: unloading.\n");
         fw_cfg_sysfs_cache_cleanup();
+        fw_cfg_kset_unregister_recursive(fw_cfg_fname_kset);
         fw_cfg_kobj_cleanup(fw_cfg_sel_ko);
         fw_cfg_io_cleanup();
         return 0;