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, 8 insertions, 2 deletions

diff --git a/include/linux/fault-inject.h b/include/linux/fault-inject.h
index 58fd14c82270..5c74748a53f3 100644
--- a/include/linux/fault-inject.h
+++ b/include/linux/fault-inject.h
@@ -19,6 +19,13 @@ enum fault_flags {
 #include <linux/ratelimit.h>
 
 /*
+ * Length of the debugfs directory name embedded in struct fault_attr.
+ * Chosen to accommodate every in-tree caller of fault_create_debugfs_attr()
+ * (the longest is "fail_dma_array_full", 19 chars) with generous headroom.
+ */
+#define FAULT_ATTR_DNAME_LEN	64
+
+/*
  * For explanation of the elements of this struct, see
  * Documentation/fault-injection/fault-injection.rst
  */
@@ -37,7 +44,7 @@ struct fault_attr {
 
 	unsigned long count;
 	struct ratelimit_state ratelimit_state;
-	struct dentry *dname;
+	char dname[FAULT_ATTR_DNAME_LEN];
 };
 
 #define FAULT_ATTR_INITIALIZER {					\
@@ -47,7 +54,6 @@ struct fault_attr {
 		.stacktrace_depth = 32,					\
 		.ratelimit_state = RATELIMIT_STATE_INIT_DISABLED,	\
 		.verbose = 2,						\
-		.dname = NULL,						\
 	}
 
 #define DECLARE_FAULT_ATTR(name) struct fault_attr name = FAULT_ATTR_INITIALIZER