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Date: Fri, 24 Apr 2026 03:50:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Shengzhuo Wei" <me@cherr.cc>
Cc: "John Stultz" <johnstul@us.ibm.com>, "Yao Zi" <me@ziyao.cc>,
 <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>, Alexey
 Dobriyan <adobriyan@gmail.com>
Subject: Re: [PATCH] proc: fix comm_write return value when truncated or
 error
Message-Id: <20260424035028.a811dd6bc8248b640a9fd7c2@linux-foundation.org>
In-Reply-To: <20260424-fix_proc_write_return-v1-1-7a793c2aad32@cherr.cc>
References: <20260424-fix_proc_write_return-v1-1-7a793c2aad32@cherr.cc>
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On Fri, 24 Apr 2026 04:06:21 +0800 "Shengzhuo Wei" <me@cherr.cc> wrote:

> When count exceeds TASK_COMM_LEN-1, comm_write() copies at most
> TASK_COMM_LEN-1 bytes but returns the original count. This violates
> write(2) semantics, which require returning the number of bytes
> actually written.
> 
> The count parameter is size_t and should not be repurposed to carry a
> negative error code on the same_thread_group() failure path.
> 
> Introduce a local len for the truncated length and a separate ssize_t
> ret for the return value.

Looks right to me.

> Fixes: 4614a696bd1c ("procfs: allow threads to rename siblings via /proc/pid/tasks/tid/comm")

December 2009.

Hopefully no userspace is depending on the broken return value of a
write to /proc/pid/comm.

Arguably we should leave the code as-is and add an apologetic comment
explaining the situation.