Re: [PATCH 1/1] exec: do not sleep in TASK_TRACED under ->cred_guard_mutex

From: David Howells
Date: Fri Sep 04 2009 - 04:41:35 EST

Oleg Nesterov <oleg@xxxxxxxxxx> wrote:

> But I strongly believe we should blame another patch
> "CRED: Make execve() take advantage of copy-on-write credentials"
> a6f76f23d297f70e2a6b3ec607f7aeeea9e37e8d
> The tracee must not sleep in TASK_TRACED holding this mutex (it was named
> cred_exec_mutex). Even if we remove ->cred_guard_mutex from mm_for_maps()
> and proc_pid_attr_write(), another task doing PTRACE_ATTACH should not
> hang until it is killed or the tracee resumes.

Ummm... I don't see how this is relevant.

Yes, a task must not sleep in TASK_TRACED if it is holding this mutex, but how
does that apply to do_exec(), mm_for_maps() or proc_pid_attr_write()? A
process can't be in any of those three if it is in the TASK_TRACED state.

A process can only be in the TASK_TRACED state in one of two ways: its parent
moved it there from the TASK_STOPPED state, or it put itself in that state -
neither of which should apply here.

Apart from that, I've no objection to dropping the guard semaphore earlier.

I do think though, the problem is elsewhere. Are we failing to unlock the
semaphore somewhere? Or double locking it, I wonder? Has Tom tried lockdep?

Btw, should mm_for_maps() use mutex_lock_interruptible()? There doesn't seem
any point making it non-interruptible (except for kill signals) - unless that
would muck up seqfile handling.

> +int prepare_bprm_creds(struct linux_binprm *bprm)
> +{


> +void free_bprm(struct linux_binprm *bprm)
> +{

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