linux-kernel - Re: [PATCH v4 3/3] exec: Add support for 64 byte 'tsk->comm

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Message-ID: <202505222041.B639D482FB@keescook>
Date: Thu, 22 May 2025 20:48:44 -0700
From: Kees Cook <kees@...nel.org>
To: Bhupesh <bhupesh@...lia.com>
Cc: akpm@...ux-foundation.org, kernel-dev@...lia.com,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
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Subject: Re: [PATCH v4 3/3] exec: Add support for 64 byte 'tsk->comm_ext'

On Wed, May 21, 2025 at 11:53:37AM +0530, Bhupesh wrote:
> Historically due to the 16-byte length of TASK_COMM_LEN, the
> users of 'tsk->comm' are restricted to use a fixed-size target
> buffer also of TASK_COMM_LEN for 'memcpy()' like use-cases.
> 
> To fix the same, Linus suggested in [1] that we can add the
> following union inside 'task_struct':
>        union {
>                char    comm[TASK_COMM_LEN];
>                char    comm_ext[TASK_COMM_EXT_LEN];
>        };

I remain unconvinced that this is at all safe. With the existing
memcpy() and so many places using %s and task->comm, this feels very
very risky to me.

Can we just make it separate, instead of a union? Then we don't have to
touch comm at all.

> and then modify '__set_task_comm()' to pass 'tsk->comm_ext'
> to the existing users.

We can use set_task_comm() to set both still...

-- 
Kees Cook