Re: [Qemu-devel] RFC: guest-side retrieval of fw_cfg file

[Laszlo Ersek]

On 07/26/15 01:21, Gabriel L. Somlo wrote:
> On Tue, Jul 21, 2015 at 12:07:08AM +0200, Laszlo Ersek wrote:
>> On 07/20/15 23:19, Gabriel L. Somlo wrote:
>>
>> ... I just love "find", sorry. :) Anyway, I don't envision myself as a
>> user of this feature any time soon, so please feel free to ignore me.
> 
> How about this: I create /sys/firmware/fw_cfg/by_select with *all*
> entries, named after their selector key, in a flat folder.

I think this is nice. The fw_cfg directory blob itself is available at a
fixed key (FW_CFG_FILE_DIR, 0x0019, and it is even documented :)), so
for a fool-proof, non-interactive lookup, a C language program can
search the 0x0019 blob, grab the relevant key value, and then open
/sys/firmware/fw_cfg/by_select/<THAT_KEY>.

For interactive use or scripting, "by_name" can be used with "find" etc.
I think this is really nice.

Thanks
Laszlo

> Then, I also create /sys/firmware/fw_cfg/by_name/... where I try to
> build the folder hierarchy given by whatever the file names end up
> being, *on a best effort basis*, i.e. I may fail to create *some* of
> them, if they have brain-damaged names :)
> 
> The kernel will issue big scary warnings if a symlink already exists
> where I'm trying to add a new subdirectory of the same name, or the
> other way around.
> 
> The whole thing is below, tested and working on x86 and arm (so one
> example of IOPort and one example of MMIO).
> 
> I'm out all of next week, but will submit this to the kernel (and cc
> Greg-K-H as advised by Matt) once I get back.
> 
> In the mean time, any final words of wisdom, advice, testing, etc.
> much much appreciated !
> 
> Cheers,
> --Gabriel
> 
> 
> /*
>  * drivers/firmware/fw_cfg.c
>  *
>  * Expose entries from QEMU's firmware configuration (fw_cfg) device in
>  * sysfs (read-only, under "/sys/firmware/fw_cfg/...").
>  *
>  * Copyright 2015 Gabriel L. Somlo <address@hidden>
>  *
>  * This program is free software; you can redistribute it and/or modify
>  * it under the terms of the GNU General Public License v2 as published
>  * by the Free Software Foundation.
>  *
>  * This program is distributed in the hope that it will be useful, but
>  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
>  * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
>  * for more details.
>  *
>  * You should have received a copy of the GNU General Public License along
>  * with this program; if not, write to the Free Software Foundation, Inc.,
>  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
>  */
> 
> #include <linux/module.h>
> #include <linux/capability.h>
> #include <linux/slab.h>
> #include <linux/io.h>
> #include <linux/ioport.h>
> #include <linux/ctype.h>
> 
> /* selector values for "well-known" fw_cfg entries */
> #define FW_CFG_SIGNATURE  0x00
> #define FW_CFG_FILE_DIR   0x19
> 
> /* size in bytes of fw_cfg signature */
> #define FW_CFG_SIG_SIZE 4
> 
> /* fw_cfg "file name" is up to 56 characters (including terminating nul) */
> #define FW_CFG_MAX_FILE_PATH 56
> 
> /* fw_cfg file directory entry type */
> struct fw_cfg_file {
>       uint32_t size;
>       uint16_t select;
>       uint16_t reserved;
>       char name[FW_CFG_MAX_FILE_PATH];
> };
> 
> /* fw_cfg device i/o access options type */
> struct fw_cfg_access {
>       phys_addr_t start;
>       uint8_t size;
>       uint8_t ctrl_offset;
>       uint8_t data_offset;
>       bool is_mmio;
>       const char *name;
> };
> 
> /* fw_cfg device i/o access available options for known architectures */
> static struct fw_cfg_access fw_cfg_modes[] = {
>       { 0x510,       2, 0, 1, false, "fw_cfg on i386, sun4u" },
>       { 0x9020000,  10, 8, 0,  true, "fw_cfg on arm" },
>       { 0xd00000510, 3, 0, 2,  true, "fw_cfg on sun4m" },
>       { 0xf0000510,  3, 0, 2,  true, "fw_cfg on ppc/mac" },
>       { }
> };
> 
> /* fw_cfg device i/o currently selected option set */
> static struct fw_cfg_access *fw_cfg_mode;
> 
> /* fw_cfg device i/o register addresses */
> static void __iomem *fw_cfg_dev_base;
> static void __iomem *fw_cfg_dev_ctrl;
> static void __iomem *fw_cfg_dev_data;
> 
> /* atomic access to fw_cfg device (potentially slow i/o, so using mutex) */
> static DEFINE_MUTEX(fw_cfg_dev_lock);
> 
> /* pick apropriate endianness for selector key */
> static inline uint16_t fw_cfg_sel_endianness(uint16_t select)
> {
>       return fw_cfg_mode->is_mmio ? cpu_to_be16(select) : cpu_to_le16(select);
> }
> 
> /* type for fw_cfg "directory scan" visitor/callback function */
> typedef int (*fw_cfg_file_callback)(const struct fw_cfg_file *f);
> 
> /* run a given callback on each fw_cfg directory entry */
> static int fw_cfg_scan_dir(fw_cfg_file_callback callback)
> {
>       int ret = 0;
>       uint32_t count, i;
>       struct fw_cfg_file f;
> 
>       mutex_lock(&fw_cfg_dev_lock);
>       iowrite16(fw_cfg_sel_endianness(FW_CFG_FILE_DIR), fw_cfg_dev_ctrl);
>       ioread8_rep(fw_cfg_dev_data, &count, sizeof(count));
>       for (i = 0; i < be32_to_cpu(count); i++) {
>               ioread8_rep(fw_cfg_dev_data, &f, sizeof(f));
>               ret = callback(&f);
>               if (ret)
>                       break;
>       }
>       mutex_unlock(&fw_cfg_dev_lock);
>       return ret;
> }
> 
> /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
> static inline void fw_cfg_read_blob(uint16_t select,
>                                    void *buf, loff_t pos, size_t count)
> {
>       mutex_lock(&fw_cfg_dev_lock);
>       iowrite16(fw_cfg_sel_endianness(select), fw_cfg_dev_ctrl);
>       while (pos-- > 0)
>               ioread8(fw_cfg_dev_data);
>       ioread8_rep(fw_cfg_dev_data, buf, count);
>       mutex_unlock(&fw_cfg_dev_lock);
> }
> 
> /* clean up fw_cfg device i/o setup */
> static void fw_cfg_io_cleanup(void)
> {
>       if (fw_cfg_mode->is_mmio) {
>               iounmap(fw_cfg_dev_base);
>               release_mem_region(fw_cfg_mode->start, fw_cfg_mode->size);
>       } else {
>               ioport_unmap(fw_cfg_dev_base);
>               release_region(fw_cfg_mode->start, fw_cfg_mode->size);
>       }
> }
> 
> /* probe and map fw_cfg device */
> static int __init fw_cfg_io_probe(void)
> {
>       char sig[FW_CFG_SIG_SIZE];
> 
>       for (fw_cfg_mode = &fw_cfg_modes[0];
>            fw_cfg_mode->start; fw_cfg_mode++) {
> 
>               phys_addr_t start = fw_cfg_mode->start;
>               uint8_t size = fw_cfg_mode->size;
> 
>               /* reserve and map mmio or ioport region */
>               if (fw_cfg_mode->is_mmio) {
>                       if (!request_mem_region(start, size, fw_cfg_mode->name))
>                               continue;
>                       fw_cfg_dev_base = ioremap(start, size);
>                       if (!fw_cfg_dev_base) {
>                               release_mem_region(start, size);
>                               continue;
>                       }
>               } else {
>                       if (!request_region(start, size, fw_cfg_mode->name))
>                               continue;
>                       fw_cfg_dev_base = ioport_map(start, size);
>                       if (!fw_cfg_dev_base) {
>                               release_region(start, size);
>                               continue;
>                       }
>               }
> 
>               /* set control and data register addresses */
>               fw_cfg_dev_ctrl = fw_cfg_dev_base + fw_cfg_mode->ctrl_offset;
>               fw_cfg_dev_data = fw_cfg_dev_base + fw_cfg_mode->data_offset;
> 
>               /* verify fw_cfg device signature */
>               fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
>               if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) == 0)
>                       /* success, we're done */
>                       return 0;
> 
>               /* clean up before probing next access mode */
>               fw_cfg_io_cleanup();
>       }
> 
>       return -ENODEV;
> }
> 
> /* fw_cfg_sysfs_entry type */
> struct fw_cfg_sysfs_entry {
>       struct kobject kobj;
>       struct fw_cfg_file f;
>       struct list_head list;
> };
> 
> /* get fw_cfg_sysfs_entry from kobject member */
> static inline struct fw_cfg_sysfs_entry *to_entry(struct kobject *kobj)
> {
>       return container_of(kobj, struct fw_cfg_sysfs_entry, kobj);
> }
> 
> /* fw_cfg_sysfs_attribute type */
> struct fw_cfg_sysfs_attribute {
>       struct attribute attr;
>       ssize_t (*show)(struct fw_cfg_sysfs_entry *entry, char *buf);
> };
> 
> /* get fw_cfg_sysfs_attribute from attribute member */
> static inline struct fw_cfg_sysfs_attribute *to_attr(struct attribute *attr)
> {
>       return container_of(attr, struct fw_cfg_sysfs_attribute, attr);
> }
> 
> /* global cache of fw_cfg_sysfs_entry objects */
> static LIST_HEAD(fw_cfg_entry_cache);
> 
> /* kobjects removed lazily by kernel, mutual exclusion needed */
> static DEFINE_SPINLOCK(fw_cfg_cache_lock);
> 
> static inline void fw_cfg_sysfs_cache_enlist(struct fw_cfg_sysfs_entry *entry)
> {
>       spin_lock(&fw_cfg_cache_lock);
>       list_add_tail(&entry->list, &fw_cfg_entry_cache);
>       spin_unlock(&fw_cfg_cache_lock);
> }
> 
> static inline void fw_cfg_sysfs_cache_delist(struct fw_cfg_sysfs_entry *entry)
> {
>       spin_lock(&fw_cfg_cache_lock);
>       list_del(&entry->list);
>       spin_unlock(&fw_cfg_cache_lock);
> }
> 
> static void fw_cfg_sysfs_cache_cleanup(void)
> {
>       struct fw_cfg_sysfs_entry *entry, *next;
> 
>       list_for_each_entry_safe(entry, next, &fw_cfg_entry_cache, list) {
>               /* will end up invoking fw_cfg_sysfs_cache_delist()
>                * via each object's release() method (i.e. destructor) */
>               kobject_put(&entry->kobj);
>       }
> }
> 
> /* default_attrs: per-entry attributes and show methods */
> 
> #define FW_CFG_SYSFS_ATTR(_attr) \
> struct fw_cfg_sysfs_attribute fw_cfg_sysfs_attr_##_attr = { \
>       .attr = { .name = __stringify(_attr), .mode = 0400 }, \
>       .show = fw_cfg_sysfs_show_##_attr, \
> }
> 
> static ssize_t fw_cfg_sysfs_show_size(struct fw_cfg_sysfs_entry *e, char *buf)
> {
>       return sprintf(buf, "%d\n", e->f.size);
> }
> 
> static ssize_t fw_cfg_sysfs_show_select(struct fw_cfg_sysfs_entry *e, char 
> *buf)
> {
>       return sprintf(buf, "%d\n", e->f.select);
> }
> 
> static ssize_t fw_cfg_sysfs_show_name(struct fw_cfg_sysfs_entry *e, char *buf)
> {
>       return sprintf(buf, "%s\n", e->f.name);
> }
> 
> static FW_CFG_SYSFS_ATTR(size);
> static FW_CFG_SYSFS_ATTR(select);
> static FW_CFG_SYSFS_ATTR(name);
> 
> static struct attribute *fw_cfg_sysfs_entry_attrs[] = {
>       &fw_cfg_sysfs_attr_size.attr,
>       &fw_cfg_sysfs_attr_select.attr,
>       &fw_cfg_sysfs_attr_name.attr,
>       NULL,
> };
> 
> /* sysfs_ops: find fw_cfg_[entry, attribute] and call appropriate show method 
> */
> static ssize_t fw_cfg_sysfs_attr_show(struct kobject *kobj, struct attribute 
> *a,
>                                     char *buf)
> {
>       struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
>       struct fw_cfg_sysfs_attribute *attr = to_attr(a);
> 
>       if (!capable(CAP_SYS_ADMIN))
>               return -EACCES;
> 
>       return attr->show(entry, buf);
> }
> 
> static const struct sysfs_ops fw_cfg_sysfs_attr_ops = {
>       .show = fw_cfg_sysfs_attr_show,
> };
> 
> /* release: destructor, to be called via kobject_put() */
> static void fw_cfg_sysfs_release_entry(struct kobject *kobj)
> {
>       struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> 
>       fw_cfg_sysfs_cache_delist(entry);
>       kfree(entry);
> }
> 
> /* kobj_type: ties together all properties required to register an entry */
> static struct kobj_type fw_cfg_sysfs_entry_ktype = {
>       .default_attrs = fw_cfg_sysfs_entry_attrs,
>       .sysfs_ops = &fw_cfg_sysfs_attr_ops,
>       .release = fw_cfg_sysfs_release_entry,
> };
> 
> /* raw-read method and attribute */
> static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
>                                    struct bin_attribute *bin_attr,
>                                    char *buf, loff_t pos, size_t count)
> {
>       struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> 
>       if (!capable(CAP_SYS_ADMIN))
>               return -EACCES;
> 
>       if (pos > entry->f.size)
>               return -EINVAL;
> 
>       if (count > entry->f.size - pos)
>               count = entry->f.size - pos;
> 
>       fw_cfg_read_blob(entry->f.select, buf, pos, count);
>       return count;
> }
> 
> static struct bin_attribute fw_cfg_sysfs_attr_raw = {
>       .attr = { .name = "raw", .mode = 0400 },
>       .read = fw_cfg_sysfs_read_raw,
> };
> 
> /*
>  * kset_find_obj() is not EXPORTed from lib/kobject.c, clone it here for now.
>  * FIXME: patch adding EXPORT in lib/kobject.c tested working, but then
>  *      building this module for older kernels wouldn't work anymore...
>  */
> struct kobject *my_kset_find_obj(struct kset *kset, const char *name)
> {
>         struct kobject *k;
>         struct kobject *ret = NULL;
> 
>         spin_lock(&kset->list_lock);
> 
>         list_for_each_entry(k, &kset->list, entry) {
>                 if (kobject_name(k) && !strcmp(kobject_name(k), name)) {
>                         ret = kobject_get(k);
>                         break;
>                 }
>         }
> 
>         spin_unlock(&kset->list_lock);
>         return ret;
> }
> 
> /*
>  * 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 void __init fw_cfg_build_symlink(struct kset *dir,
>                                       struct kobject *target,
>                                       const char *name)
> {
>       struct kset *subdir;
>       struct kobject *ko;
>       char *name_copy, *p, *tok;
> 
>       if (!dir || !target || !name || !*name) {
>               pr_err("fw_cfg: invalid argument to fw_cfg_build_symlink\n");
>               return;
>       }
> 
>       /* clone a copy of name for parsing */
>       name_copy = p = kstrdup(name, GFP_KERNEL);
>       if (!name_copy) {
>               pr_err("fw_cfg: kstrdup failed while "
>                      "creating symlink %s\n", name);
>               return;
>       }
> 
>       /* 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) {
>                       if (sysfs_create_link(&dir->kobj, target, tok)) {
>                               pr_err("fw_cfg: can't create symlink %s "
>                                      "of %s\n", tok, name);
>                       }
>                       break;
>               }
> 
>               /* does the current dir contain an item named after tok ? */
>               ko = my_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) {
>                               pr_err("fw_cfg: non-folder token %s already "
>                                      "exists while creating %s\n", tok, name);
>                               break;
>                       }
> 
>                       /* descend into already existing subdirectory */
>                       dir = to_kset(ko);
>               } else {
>                       /* create new subdirectory kset */
>                       subdir = kzalloc(sizeof(struct kset), GFP_KERNEL);
>                       if (!subdir) {
>                               pr_err("fw_cfg: alloc error creating subdir %s "
>                                      "of %s\n", tok, name);
>                               break;
>                       }
>                       subdir->kobj.kset = dir;
>                       subdir->kobj.ktype = dir->kobj.ktype;
>                       if (kobject_set_name(&subdir->kobj, "%s", tok)) {
>                               kfree(subdir);
>                               pr_err("fw_cfg: set_name failed on subdir %s "
>                                      "of %s\n", tok, name);
>                               break;
>                       }
>                       if (kset_register(subdir)) {
>                               kfree(subdir);
>                               pr_err("fw_cfg: register failed on subdir %s "
>                                      "of %s\n", tok, name);
>                               break;
>                       }
> 
>                       /* descend into newly created subdirectory */
>                       dir = subdir;
>               }
>       }
> 
>       /* we're done with cloned copy of name */
>       kfree(name_copy);
> }
> 
> /* 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_select, 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;
> 
> /* callback function to register an individual fw_cfg file */
> static int __init fw_cfg_register_file(const struct fw_cfg_file *f)
> {
>       int err;
>       struct fw_cfg_sysfs_entry *entry;
> 
>       /* allocate new entry */
>       entry = kzalloc(sizeof(*entry), GFP_KERNEL);
>       if (!entry)
>               return -ENOMEM;
> 
>       /* set file entry information */
>       entry->f.size = be32_to_cpu(f->size);
>       entry->f.select = be16_to_cpu(f->select);
>       strcpy(entry->f.name, f->name);
> 
>       /* register entry under "/sys/firmware/fw_cfg/by_select/" */
>       err = kobject_init_and_add(&entry->kobj, &fw_cfg_sysfs_entry_ktype,
>                                  fw_cfg_sel_ko, "%d", entry->f.select);
>       if (err)
>               goto err_register;
> 
>       /* add raw binary content access */
>       err = sysfs_create_bin_file(&entry->kobj, &fw_cfg_sysfs_attr_raw);
>       if (err)
>               goto err_add_raw;
> 
>       /* try adding "/sys/firmware/fw_cfg/by_name/" symlink (best-effort) */
>       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;
> 
> err_add_raw:
>       kobject_del(&entry->kobj);
> err_register:
>       kfree(entry);
>       return err;
> }
> 
> /* unregister top-level or by_select folder */
> static inline void fw_cfg_kobj_cleanup(struct kobject *kobj)
> {
>       kobject_del(kobj);
>       kobject_put(kobj);
> }
> 
> static int __init fw_cfg_sysfs_init(void)
> {
>       int err;
> 
>       /* probe for the fw_cfg "hardware" */
>       err = fw_cfg_io_probe();
>       if (err)
>               return err;
> 
>       /* create sysfs toplevel, by_select, and by_name folders */
>       err = -ENOMEM;
>       fw_cfg_top_ko = kobject_create_and_add("fw_cfg", firmware_kobj);
>       if (!fw_cfg_top_ko)
>               goto err_sysfs_top;
>       fw_cfg_sel_ko = kobject_create_and_add("by_select", fw_cfg_top_ko);
>       if (!fw_cfg_sel_ko)
>               goto err_sysfs_sel;
>       fw_cfg_fname_kset = kset_create_and_add("by_name", NULL, fw_cfg_top_ko);
>       if (!fw_cfg_fname_kset)
>               goto err_sysfs_name;
> 
>       /* process fw_cfg file directory entry, registering each file */
>       err = fw_cfg_scan_dir(fw_cfg_register_file);
>       if (err)
>               goto err_scan;
> 
>       /* success */
>       pr_debug("fw_cfg: loaded.\n");
>       return 0;
> 
> err_scan:
>       fw_cfg_sysfs_cache_cleanup();
>       fw_cfg_kset_unregister_recursive(fw_cfg_fname_kset);
> err_sysfs_name:
>       fw_cfg_kobj_cleanup(fw_cfg_sel_ko);
> err_sysfs_sel:
>       fw_cfg_kobj_cleanup(fw_cfg_top_ko);
> err_sysfs_top:
>       fw_cfg_io_cleanup();
>       return err;
> }
> 
> static void __exit fw_cfg_sysfs_exit(void)
> {
>       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_kobj_cleanup(fw_cfg_top_ko);
>       fw_cfg_io_cleanup();
> }
> 
> module_init(fw_cfg_sysfs_init);
> module_exit(fw_cfg_sysfs_exit);
> MODULE_AUTHOR("Gabriel L. Somlo <address@hidden>");
> MODULE_DESCRIPTION("QEMU fw_cfg sysfs support");
> MODULE_LICENSE("GPL");
>