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Wed, 29 Jan 2025 04:57:40 -0800 (PST) Received: from qasdev.system ([2a02:c7c:6696:8300:574c:cbc5:bae8:cfe0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c2a1763e9sm17475001f8f.16.2025.01.29.04.57.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Jan 2025 04:57:40 -0800 (PST) Date: Wed, 29 Jan 2025 12:57:34 +0000 From: Qasim Ijaz To: Hugh Dickins Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/shmem: Fix invalid PTR_ERR(NULL) call in shmem_xattr_handler_set() Message-ID: Reply-To: 16055fb0-bed3-e000-8c36-6e4886b08c9d@google.com Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Jan 28, 2025 at 08:37:51PM -0800, Hugh Dickins wrote: > On Tue, 28 Jan 2025, Qasim Ijaz wrote: > > > In shmem_xattr_handler_set() if simple_xattr_set() succeeds and the pointer > > returned is not an error pointer, then old_xattr will be set to NULL in > > the body of the following if statement: > > > > if (!IS_ERR(old_xattr)) > > > > Later on shmem_xattr_handler_set() calls: > > > > return PTR_ERR(old_xattr); > > > > The PTR_ERR macro is used to extract an error code from an error pointer > > and NULL is not an error pointer, PTR_ERR(NULL) simply results in 0. > > NULL pointer returns 0 for success: yes, that's what's wanted there. > > > > > To improve correctness and readability, > > Correctness? Please explain - I don't see any incorrectness. > > Readability? Perhaps - I should not be the judge of that. > > > refactor the error handling > > to have an explicit default return value of 0 (success) in "ret". > > If simple_xattr_set() returns an error pointer, store its > > error code in "ret". > > > > Signed-off-by: Qasim Ijaz > > --- > > mm/shmem.c | 9 +++++++-- > > 1 file changed, 7 insertions(+), 2 deletions(-) > > I prefer how it was written before. Hi Hugh, By correctness I mean that NULL is not an error pointer, here are some additional points: The code goes against the documentation of PTR_ERR() which mentions the argument to PTR_ERR() should be an error pointer. NULL is not an error pointer so I think it is inconsistent in that regard. Secondly the current code is hinging on a side effect of the PTR_ERR() API being passed NULL, sure it works but I think it would be better if the API was used clearly and consistently with how it is meant to be used. Also to add onto my points I decided to run the mm/shmem.c file through smatch (since one of its key features is detecting 0 being passed to PTR_ERR()) to see if it detects what I spotted and it brings up the following message: mm/shmem.c:4174 shmem_xattr_handler_set() warn: passing zero to 'PTR_ERR' Best regards, Qasim > > > > > diff --git a/mm/shmem.c b/mm/shmem.c > > index 532afd8e049c..3e97c7890aac 100644 > > --- a/mm/shmem.c > > +++ b/mm/shmem.c > > @@ -4143,6 +4143,7 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler, > > struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb); > > struct simple_xattr *old_xattr; > > size_t ispace = 0; > > + int ret = 0; > > > > name = xattr_full_name(handler, name); > > if (value && sbinfo->max_inodes) { > > @@ -4158,7 +4156,9 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler, > > } > > > > old_xattr = simple_xattr_set(&info->xattrs, name, value, size, flags); > > - if (!IS_ERR(old_xattr)) { > > + if (IS_ERR(old_xattr)) { > > + ret = PTR_ERR(old_xattr); > > + } else { > > ispace = 0; > > if (old_xattr && sbinfo->max_inodes) > > ispace = simple_xattr_space(old_xattr->name, > > @@ -4168,12 +4171,13 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler, > > inode_set_ctime_current(inode); > > inode_inc_iversion(inode); > > } > > + > > if (ispace) { > > raw_spin_lock(&sbinfo->stat_lock); > > sbinfo->free_ispace += ispace; > > raw_spin_unlock(&sbinfo->stat_lock); > > } > > - return PTR_ERR(old_xattr); > > + return ret; > > } > > > > static const struct xattr_handler shmem_security_xattr_handler = { > > -- > > 2.39.5