[PATCH v2 12/16] device-mapper: inform caller about already-existing device

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

Not only is this helpful for debugging, it also saves the caller an
ioctl in the case where a device should be used if it exists or created
otherwise.  To ensure existing userspace is not broken, this feature is
only enabled in strict mode.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/md/dm-ioctl.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 
17ece816d490b6c40d019da131ade44c9a201dab..44425093d3b908abf80e05e1fc99a26b17e18a42
 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -256,11 +256,13 @@ static void free_cell(struct hash_cell *hc)
        }
 }
 
+static void __dev_status(struct mapped_device *md, struct dm_ioctl *param);
+
 /*
  * The kdev_t and uuid of a device can never change once it is
  * initially inserted.
  */
-static int dm_hash_insert(const char *name, const char *uuid, struct 
mapped_device *md)
+static int dm_hash_insert(const char *name, const char *uuid, struct 
mapped_device *md, struct dm_ioctl *param)
 {
        struct hash_cell *cell, *hc;
 
@@ -277,6 +279,8 @@ static int dm_hash_insert(const char *name, const char 
*uuid, struct mapped_devi
        down_write(&_hash_lock);
        hc = __get_name_cell(name);
        if (hc) {
+               if (param)
+                       __dev_status(hc->md, param);
                dm_put(hc->md);
                goto bad;
        }
@@ -287,6 +291,8 @@ static int dm_hash_insert(const char *name, const char 
*uuid, struct mapped_devi
                hc = __get_uuid_cell(uuid);
                if (hc) {
                        __unlink_name(cell);
+                       if (param)
+                               __dev_status(hc->md, param);
                        dm_put(hc->md);
                        goto bad;
                }
@@ -901,12 +907,14 @@ static int dev_create(struct file *filp, struct dm_ioctl 
*param, size_t param_si
                m = MINOR(huge_decode_dev(param->dev));
 
        r = dm_create(m, &md, param->flags & DM_DISABLE_UEVENTS_FLAG);
-       if (r)
+       if (r) {
+               DMERR("Could not create device-mapper device");
                return r;
+       }
 
        param->flags &= ~DM_INACTIVE_PRESENT_FLAG;
 
-       r = dm_hash_insert(param->name, *param->uuid ? param->uuid : NULL, md);
+       r = dm_hash_insert(param->name, *param->uuid ? param->uuid : NULL, md, 
param);
        if (r) {
                dm_put(md);
                dm_destroy(md);
@@ -2269,7 +2277,6 @@ static int ctl_ioctl(struct file *file, uint command, 
struct dm_ioctl __user *us
                goto out;
        /* This XOR keeps only the flags validate_params has changed. */
        old_flags ^= param->flags;
-
        param->data_size = sloppy_checks(param) ? offsetof(struct dm_ioctl, 
data) : sizeof(struct dm_ioctl);
        r = fn(file, param, input_param_size);
 
@@ -2284,9 +2291,14 @@ static int ctl_ioctl(struct file *file, uint command, 
struct dm_ioctl __user *us
        param->flags |= old_flags;
 
        /*
-        * Copy the results back to userland.
+        * Copy the results back to userland if either:
+        *
+        * - The ioctl succeeded.
+        * - The ioctl is DM_DEV_CREATE, the return value is -EBUSY,
+        *   and strict parameter checking is enabled.
         */
-       if (!r && copy_to_user(user, param, param->data_size))
+       if ((!r || (!sloppy_checks(param) && cmd == DM_DEV_CREATE_CMD && r == 
-EBUSY)) &&
+           copy_to_user(user, param, param->data_size))
                r = -EFAULT;
 
 out:
@@ -2465,7 +2477,7 @@ int __init dm_early_create(struct dm_ioctl *dmi,
                return r;
 
        /* hash insert */
-       r = dm_hash_insert(dmi->name, *dmi->uuid ? dmi->uuid : NULL, md);
+       r = dm_hash_insert(dmi->name, *dmi->uuid ? dmi->uuid : NULL, md, NULL);
        if (r)
                goto err_destroy_dm;
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab