From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-lf1-f50.google.com (mail-lf1-f50.google.com [209.85.167.50]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1D22B36D500 for ; Fri, 24 Apr 2026 13:26:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.167.50 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777037161; cv=none; b=P5kxT4UHmIV7df7/g9zoXmMTM9W2uj65typP896EoepFO2ajzzgI0qXh3cY2Nhgmn5bkylJH5hFXZfuatLAI+vQXF/LwYqaUOwpA5+kq0pgOghXIRhNED6lkBGukuEvW4nNtIJ6VhovJZvbNkUlba2reKyt2cmvnXtjNiL5ldJM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777037161; c=relaxed/simple; bh=AwARQ4HT7CzsrHCJ4iHvttcyWJy5GfGt9dkoJ/GPL6I=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TZeBmPy2ITxp4Ud/2U46VD+aQ6rcd7GHd7zkhRYrZyUvl0i84N7NTFATk5aIceJCnXTgiYtAvUk/hPxY4XLp7nUgSka30nQl3KbccBd+XxxbNfJpp9xq320dk3QD5jU6V4sgbqkwQhFg27WmvY/tFJ6EScEqHd3ig/ACXKjfutg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=oaOU0vbZ; arc=none smtp.client-ip=209.85.167.50 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="oaOU0vbZ" Received: by mail-lf1-f50.google.com with SMTP id 2adb3069b0e04-5a402dea4a5so8277395e87.2 for ; Fri, 24 Apr 2026 06:25:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1777037158; x=1777641958; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=ff0pvFENIRBXsx36gJRS/Pi3Sb7+mMJWoywzMmXc4yA=; b=oaOU0vbZOUTZkXOYvxNZdqfvqP72DWpjlr4ubCZMPegrTQRQAUQlJoQ0gTAKnDrL3q 4pZTSdHioXGDLdjLae6WvMYd5gvWaPDAfqOcO0ITuWwtrV+YuNbMj0kxNpLsajwQuoMu oBpqBXE4FgU9z3oqoHPezIMDbdATSf0Xxwb96luLiGkat6lWieZ7WSbPumgGthqC89J+ m/7vw+DAFAM25ga4qm7cK+1D07XIAf8M4XPdw+PVHpL2MBReyUYA8rc2U5RO8pW2SXzD 9jKjGmGbYUDmeRjvfhOh1yLrwiD5I9xw/y2svJSlDFEayDkz4za0o64Odl7MB8zA1iKK PrEA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1777037158; x=1777641958; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=ff0pvFENIRBXsx36gJRS/Pi3Sb7+mMJWoywzMmXc4yA=; b=J56NaP4i1zgAOE6/UfzmTQ2bsmF/ljnVPFrLSBwFhAeVrfR0eiwSc2BMKs1T1zkjg3 hK7A5l+ZJjh6fHtU0UPO+P8vN6qK3H3wqmUA2Afj+kGnMJVv5OQhLV+4SUKlcQa/xRUr 03whLqVCvO6QOJBL/xYwNx3A1yEpueN5Ib1dMY4+rJOJVNV1krlV3wMRA3+hbS9P279o F8RvKoUkx5LFiaRIRtF39BXLrVKye//KxnVr3Rl8Dpa0s9X1TASTqfs2xYREJfIX7986 VsbzawVHPM9vzNaqeXrVyfEjHwBy9uACs/ZEOK3ztsNqw3ZW++1rmKYlJkNMQRypKFP7 NZ/g== X-Forwarded-Encrypted: i=1; AFNElJ8kOZjiqpUs66R9/1ZWjOoClrh3p1ICTOyLm6QREiDGwOnGQqWxQjSZru1D1THnCGK3M3LH17lKupItrAkQ@vger.kernel.org X-Gm-Message-State: AOJu0YyEKipnsOV1nGuS4KOgscHfkvZm/4DRD1H5xpNRJm0JBsaNCnPO MENv4b5DrgXWoKCygquJDaG0HPxNWk3Hha48RyJWwfGkjE9ZVUWdbWA= X-Gm-Gg: AeBDievHBdr9q1XWQ5rlRO6ylr7Pk3yMQIptrsZeTvoyZhS5l7ShsSwAyaU1Z9tCkOU /9KSBre3V/W+xqT6FMwjfSR2cwFhEjW9d44N065eAfB+rRIQPEPSmNDmJlUWgThCgvjRwtZyGln ZgoE2eoMRUVMWeb5URYDM1SwatV+2yKyq5mRvyLjjVRWd1txWNNqz3duE9tIzm1XcChlFAZ8PaV dZv2g6ZpBq4G5H0+hKiS3D41RkcNAu39RW3gFAywqeaKITgA9qErcw7Av+9F06S6h0zXmhLYlRP Zgro/T2iKzBrIFuXGRSBGvpr3llM8Srd9temWeixySzRghum/uQ8FJ7RkWhpK4UjtuSXIrGhnJV wD+lN67hbjbNo9i8093sbG46NHNa/VZdQymH6ohgV+hfR+YTMkXf5mM32e0gJAXlmKNJ5EjcIVy mWjrOptPNS26iFaYHktBQFYQQ= X-Received: by 2002:a05:6512:3f0b:b0:5a1:1496:922 with SMTP id 2adb3069b0e04-5a4172eecccmr10206146e87.33.1777037157964; Fri, 24 Apr 2026 06:25:57 -0700 (PDT) Received: from p183 ([178.172.147.205]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5a4187e12e0sm5990946e87.51.2026.04.24.06.25.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Apr 2026 06:25:57 -0700 (PDT) Date: Fri, 24 Apr 2026 16:28:08 +0300 From: Alexey Dobriyan To: Andrew Morton Cc: Shengzhuo Wei , John Stultz , Yao Zi , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] proc: fix comm_write return value when truncated or error Message-ID: <3f91ed95-b38e-4049-b69c-13335b13ae44@p183> References: <20260424-fix_proc_write_return-v1-1-7a793c2aad32@cherr.cc> <20260424035028.a811dd6bc8248b640a9fd7c2@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260424035028.a811dd6bc8248b640a9fd7c2@linux-foundation.org> On Fri, Apr 24, 2026 at 03:50:28AM -0700, Andrew Morton wrote: > On Fri, 24 Apr 2026 04:06:21 +0800 "Shengzhuo Wei" 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