Re: [PATCH] proc: fix comm_write return value when truncated or error

[Alexey Dobriyan]

On Fri, Apr 24, 2026 at 03:50:28AM -0700, Andrew Morton wrote:
> On Fri, 24 Apr 2026 04:06:21 +0800 "Shengzhuo Wei" <me@xxxxxxxx> 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.

Yes, this issue must all over virtual filesystems.

Patch may break stuff in the other direction too:

if process is doing "full write" loop, and write hook gets string
which is too long, then the last piece will be written, not truncated
first part.

	/proc/alexey