fs/proc: Crash observed in next_tgid (fs/proc/base.c)

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Hi Kees Cook/Luis,

We are observing one kernel crash in next_tgid function through getdents64 path. Call stack is as shown below:

-000|has_group_leader_pid(inline)
-000|next_tgid(
| [X20] ns = 0xFFFFFF87CABB1AC0,
| [locdesc] iter = (
| [locdesc] tgid = 424,
| [locdesc] task = ?))
| [X21] p = 0xFFFFFFD0FFFFF948
| [X21] task = 0xFFFFFFD0FFFFF948
-001|proc_pid_readdir(
| [X20] file = 0xFFFFFFD1AC60FC40,
| [X19] ctx = 0xFFFFFF8027363E40)
| [X21] ns = 0xFFFFFF87CABB1AC0
-002|proc_root_readdir(
| [X20] file = 0xFFFFFFD1AC60FC40,
| [X19] ctx = 0xFFFFFF8027363E40)
-003|iterate_dir(
| [X19] file = 0xFFFFFFD1AC60FC40,
| [X22] ctx = 0xFFFFFF8027363E40)
| [X23] inode = 0xFFFFFFD1F20246D0
-004|SYSC_getdents64(inline)
-004|sys_getdents64(
| ?,
| ?,
| [X19] count = 4200)
| [X19] count = 4200
| [X20] f = ([X20] file = 0xAC60FC43AC60FC40, [X20] flags = 1207898624)
| [X0] error = -1720
-005|el0_svc_naked(asm)
-->|exception
-006|NUX:0x78C5AD7D38(asm)
---|end of frame


From this call stack,task: 0xFFFFFFD0FFFFF948, seems to be invalid. As(from ramdumps) it doesn't have any valid fields. And while trying to access the fields of this task struct in has_group_leader_pid, abort is happening.

From the dumps, its not clear why the task struct is coming to be some invalid (Possibly task has already exited). This issue is observed during normal monkey testing for long hours.

Could you please provide some pointers which could help in debugging this issue further.


Thanks,

Jitendra

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