Re: [PATCH] mm: shrinker: fix NULL pointer dereference in debugfs

[Muchun Song <muchun.song@linux.dev>]

> On Jun 17, 2026, at 17:00, Qi Zheng <qi.zheng@linux.dev> wrote:
> 
> From: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> The shrinker_debugfs_add() creates both "count" and "scan" debugfs files
> unconditionally.
> 
> That assumes every shrinker implements both count_objects() and
> scan_objects(), which is not guaranteed. For example, the xen-backend
> shrinker sets count_objects() but leaves scan_objects() NULL, so writing
> to its scan file calls through a NULL function pointer and panics the
> kernel:
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> RIP: 0010:0x0
> Code: Unable to access opcode bytes at 0xffffffffffffffd6.
> Call Trace:
> <TASK>
> shrinker_debugfs_scan_write+0x12e/0x270
> full_proxy_write+0x5f/0x90
> vfs_write+0xde/0x420
> ? filp_flush+0x75/0x90
> ? filp_close+0x1d/0x30
> ? do_dup2+0xb8/0x120
> ksys_write+0x68/0xf0
> ? filp_flush+0x75/0x90
> do_syscall_64+0xb3/0x5b0
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> The count path has the same issue in principle if a shrinker omits
> count_objects().
> 
> To fix it, only create "count" and "scan" debugfs files when the
> corresponding callbacks are present.
> 
> Fixes: bbf535fd6f06 ("mm: shrinkers: add scan interface for shrinker debugfs")
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.