[PATCH] 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.

Fixes: 6adc4a22f20b ("fault-inject: add ratelimit option")
Cc: [email protected]
Signed-off-by: Michael Liang <[email protected]>
---
 drivers/infiniband/hw/hfi1/fault.c |  1 -
 include/linux/fault-inject.h       | 10 ++++++++--
 lib/fault-inject.c                 |  7 +++++--
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/infiniband/hw/hfi1/fault.c b/drivers/infiniband/hw/hfi1/fault.c
index 4ab72ef03ba1..941a0b96590b 100644
--- a/drivers/infiniband/hw/hfi1/fault.c
+++ b/drivers/infiniband/hw/hfi1/fault.c
@@ -216,7 +216,6 @@ int hfi1_fault_init_debugfs(struct hfi1_ibdev *ibd)
 	ibd->fault->attr.interval = 1;
 	ibd->fault->attr.require_end = ULONG_MAX;
 	ibd->fault->attr.stacktrace_depth = 32;
-	ibd->fault->attr.dname = NULL;
 	ibd->fault->attr.verbose = 0;
 	ibd->fault->enable = false;
 	ibd->fault->opcode = false;
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
@@ -18,6 +18,13 @@ enum fault_flags {
 #include <linux/configfs.h>
 #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
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);
-- 
2.34.1