[syzbot] [fs?] [trace?] WARNING in eventfs_create_events_dir

[syzbot] [fs?] [trace?] WARNING in eventfs_create_events_dir

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    f4cdf7ca9a1f Merge tag 'media/v7.3-1' of git://git.kernel...
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=10d46549580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e4b57019f58edf16
dashboard link: https://syzkaller.appspot.com/bug?extid=3ef80b4ed02226d04a06
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1577ba25580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/865d16295f11/disk-f4cdf7ca.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/71f476ad78fc/vmlinux-f4cdf7ca.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e19d5dbdc9b7/bzImage-f4cdf7ca.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3ef80b4ed02226d04a06@syzkaller.appspotmail.com

Could not create tracefs 'set_event_pid' entry
Could not create tracefs 'set_event_notrace_pid' entry
------------[ cut here ]------------
!list_empty(&ei->children)
WARNING: fs/tracefs/event_inode.c:128 at free_ei fs/tracefs/event_inode.c:128 [inline], CPU#1: syz.0.20/6104
WARNING: fs/tracefs/event_inode.c:128 at cleanup_ei fs/tracefs/event_inode.c:150 [inline], CPU#1: syz.0.20/6104
WARNING: fs/tracefs/event_inode.c:128 at cleanup_ei fs/tracefs/event_inode.c:145 [inline], CPU#1: syz.0.20/6104
WARNING: fs/tracefs/event_inode.c:128 at eventfs_create_events_dir+0xa3d/0xbe0 fs/tracefs/event_inode.c:834, CPU#1: syz.0.20/6104
Modules linked in:
CPU: 1 UID: 0 PID: 6104 Comm: syz.0.20 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
RIP: 0010:free_ei fs/tracefs/event_inode.c:128 [inline]
RIP: 0010:cleanup_ei fs/tracefs/event_inode.c:150 [inline]
RIP: 0010:cleanup_ei fs/tracefs/event_inode.c:145 [inline]
RIP: 0010:eventfs_create_events_dir+0xa3d/0xbe0 fs/tracefs/event_inode.c:834
Code: 00 e8 57 f7 f9 00 e9 ab fb ff ff e8 2d 6d e6 fd 48 8b 7c 24 10 be 03 00 00 00 e8 3e f7 f9 00 e9 c2 fd ff ff e8 14 6d e6 fd 90 <0f> 0b 90 e9 59 fe ff ff e8 f6 0a 56 fe e9 4b f7 ff ff 48 89 44 24
RSP: 0018:ffffc90002617b88 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff888077746000 RCX: 0000000000000000
RDX: ffff88802f489f40 RSI: ffffffff8424403c RDI: ffffffff8e679928
RBP: ffff88802737f210 R08: 00000000ffffffff R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff88802737f250
R13: 0000000000000003 R14: ffff88802737f24c R15: ffff88802737f200
FS:  00007f6c427fe6c0(0000) GS:ffff888123ccc000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2cf63fff CR3: 0000000026acc000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 create_event_toplevel_files+0x12d/0x1a0 kernel/trace/trace_events.c:4593
 event_trace_add_tracer+0x72/0x260 kernel/trace/trace_events.c:4624
 trace_array_create_dir+0x8e/0x210 kernel/trace/trace.c:8548
 trace_array_create_systems+0x8b3/0xc30 kernel/trace/trace.c:8639
 trace_array_create kernel/trace/trace.c:8666 [inline]
 instance_mkdir+0xca/0x140 kernel/trace/trace.c:8681
 tracefs_syscall_mkdir+0x10e/0x180 fs/tracefs/inode.c:121
 vfs_mkdir+0x364/0x850 fs/namei.c:5410
 filename_mkdirat+0x48b/0x5e0 fs/namei.c:5443
 __do_sys_mkdirat fs/namei.c:5464 [inline]
 __se_sys_mkdirat fs/namei.c:5461 [inline]
 __x64_sys_mkdirat+0x89/0xc0 fs/namei.c:5461
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6c4319e0d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f6c427fe028 EFLAGS: 00000246 ORIG_RAX: 0000000000000102
RAX: ffffffffffffffda RBX: 00007f6c43425fa0 RCX: 00007f6c4319e0d9
RDX: 00000000000001ff RSI: 0000200000000140 RDI: ffffffffffffff9c
RBP: 00007f6c43235024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6c43426038 R14: 00007f6c43425fa0 R15: 00007fff53277798
 </TASK>


---
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Forwarded: [PATCH] eventfs: Initialize ei->children and ei->list in init_ei()

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com.

***

Subject: [PATCH] eventfs: Initialize ei->children and ei->list in init_ei()
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master


eventfs_create_events_dir() allocates the eventfs_inode via
alloc_root_ei(), but only calls INIT_LIST_HEAD() on ei->children
and ei->list after the tracefs_get_inode() check. If that check
fails, the code jumps to the fail label and calls cleanup_ei(),
which calls free_ei():

	WARN_ON_ONCE(!list_empty(&ei->children));

Since ei was allocated with kzalloc(), ei->children.next is NULL
at this point, not a self-referencing pointer. list_empty() checks
head->next == head, so it returns false on an uninitialized list
head, triggering a false-positive WARN_ON_ONCE() even though the
list was never used.

eventfs_create_dir() has the same latent issue: alloc_ei() is
called before INIT_LIST_HEAD(), leaving a window where an early
failure path could hit cleanup_ei() on an uninitialized list head.

Move the INIT_LIST_HEAD() calls into init_ei(), which is called
by both alloc_ei() and alloc_root_ei() immediately after
allocation. This guarantees every eventfs_inode has a valid,
self-linked, empty children/list the moment it is allocated,
regardless of which failure path runs afterward.

Reported-by: syzbot+3ef80b4ed02226d04a06@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3ef80b4ed02226d04a06
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 fs/tracefs/event_inode.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 604ba3e841d2..6e3513b13cfa 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -438,6 +438,8 @@ static inline struct eventfs_inode *init_ei(struct eventfs_inode *ei, const char
 	if (!ei->name)
 		return NULL;
 	kref_init(&ei->kref);
+	INIT_LIST_HEAD(&ei->children);
+	INIT_LIST_HEAD(&ei->list);
 	return ei;
 }
 
@@ -729,8 +731,6 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 	ei->entries = entries;
 	ei->nr_entries = size;
 	ei->data = data;
-	INIT_LIST_HEAD(&ei->children);
-	INIT_LIST_HEAD(&ei->list);
 
 	scoped_guard(mutex, &eventfs_mutex) {
 		if (!parent->is_freed)
@@ -802,9 +802,6 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	ei->attr.uid = uid;
 	ei->attr.gid = gid;
 
-	INIT_LIST_HEAD(&ei->children);
-	INIT_LIST_HEAD(&ei->list);
-
 	ti = get_tracefs(inode);
 	ti->flags |= TRACEFS_EVENT_INODE;
 	ti->private = ei;
-- 
2.34.1