fault-inject: fix dentry leak

fault_create_debugfs_attr() has always taken an extra dentry reference on
the created directory (attr->dname = dget(dir)) so that fail_dump() could
print the name via %pd from any context.  Nothing anywhere in the tree
ever calls dput() on attr->dname.

For callers with a matching teardown, that unmatched reference causes one
dentry plus its attached inode to leak per fault_create_debugfs_attr /
debugfs_remove_recursive cycle.  simple_recursive_removal() drops
debugfs's own +1 ref on the child dentry, but the dget()'d ref keeps its
refcount at 1: the dentry ends up unhashed but pinned, and its inode is
never freed.

Boot-once callers (mm/failslab, block/blk-core, etc.) leak exactly once at
init and never destroy the tree, so the impact there is bounded.  But
per-lifecycle callers (drivers/nvme, drivers/infiniband/hw/hfi1,
drivers/mmc, drivers/iommu/iommufd, drivers/media, drivers/misc,
drivers/gpu/drm/msm, drivers/crypto, net/sunrpc) leak on every
create/destroy cycle.

We observed this in production: an NVMe/RDMA host repeatedly reconnecting
to a target that rejected the CRTO Property Get went through ~50 nvme
controller create/destroy cycles per second, and dentry and inode_cache
grew by ~13k pinned objects per 240 s -- unrecoverable through
drop_caches.  Byte math matched a per-cycle 1-dentry / 1-inode leak from
the "fault_inject" directory dentry.

Fix this by not holding any external reference in fault_attr.  Embed the
directory name as a fixed-size char array (FAULT_ATTR_DNAME_LEN, 64 bytes)
inside struct fault_attr, copied by strscpy() at
fault_create_debugfs_attr() time.  fail_dump() prints it via %s.

Advantages of an embedded array over kstrdup() + kfree() paired with a new
destroy API:

  - Zero API footprint.  No new export and no caller changes required:
    callers already own their fault_attr's memory and free it when
    they are done, and now that suffices.
  - No allocation on the create path.
  - fault_create_debugfs_attr() cannot fail from the name-copy step.
  - No lifetime coupling between attr->dname and debugfs; the string
    is valid for exactly as long as the containing struct.

The 64-byte length accommodates every in-tree caller with generous
headroom (the longest current name is "fail_dma_array_full", 19 chars).

The user-visible fail_dump() format changes from "name %pd" to "name %s",
but the printed content is identical -- %pd on the created directory
renders the same string that was passed in as @name.

drivers/infiniband/hw/hfi1/fault.c drops a now-invalid "attr.dname = NULL"
statement; the surrounding kzalloc() already zero-initialises the array.

Link: https://lore.kernel.org/20260821181527.3271414-1-mliang@purestorage.com
Fixes: 6adc4a22f20b ("fault-inject: add ratelimit option")
Signed-off-by: Michael Liang <mliang@purestorage.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Akinbou Mita <akinobu.mita@gmail.com>
Cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/lib/fault-inject.c b/lib/fault-inject.c
index 999053fa133e..02916ef2761c 100644
--- a/lib/fault-inject.c
+++ b/lib/fault-inject.c
@@ -5,6 +5,7 @@
 #include <linux/debugfs.h>
 #include <linux/sched.h>
 #include <linux/stat.h>
+#include <linux/string.h>
 #include <linux/types.h>
 #include <linux/fs.h>
 #include <linux/export.h>
@@ -64,7 +65,7 @@ static void fail_dump(struct fault_attr *attr)
 {
 	if (attr->verbose > 0 && __ratelimit(&attr->ratelimit_state)) {
 		printk(KERN_NOTICE "FAULT_INJECTION: forcing a failure.\n"
-		       "name %pd, interval %lu, probability %lu, "
+		       "name %s, interval %lu, probability %lu, "
 		       "space %d, times %d\n", attr->dname,
 		       attr->interval, attr->probability,
 		       atomic_read(&attr->space),
@@ -261,7 +262,9 @@ struct dentry *fault_create_debugfs_attr(const char *name,
 	debugfs_create_xul("reject-end", mode, dir, &attr->reject_end);
 #endif /* CONFIG_FAULT_INJECTION_STACKTRACE_FILTER */
 
-	attr->dname = dget(dir);
+	if (strscpy(attr->dname, name, sizeof(attr->dname)) == -E2BIG)
+		pr_warn("FAULT_INJECTION: name '%s' truncated to '%s'\n",
+			name, attr->dname);
 	return dir;
 }
 EXPORT_SYMBOL_GPL(fault_create_debugfs_attr);