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Message-ID: <164f759d-23b8-0a68-e68e-2f0a46318e94@huawei.com> Date: Sat, 26 Nov 2022 17:49:50 +0800 From: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com> To: Tejun Heo <tj@...nel.org> CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] kernfs: fix potential null-ptr-deref in kernfs_path_from_node_locked() On 2022/11/24 10:52, Leizhen (ThunderTown) wrote: > > > On 2022/11/24 10:28, Leizhen (ThunderTown) wrote: >> >> >> On 2022/11/24 10:24, Leizhen (ThunderTown) wrote: >>> >>> >>> On 2022/11/24 0:55, Tejun Heo wrote: >>>> On Wed, Nov 23, 2022 at 10:04:19AM +0800, Zhen Lei wrote: >>>>> Ensure that the 'buf' is not empty before strlcpy() uses it. >>>>> >>>>> Commit bbe70e4e4211 ("fs: kernfs: Fix possible null-pointer dereferences >>>>> in kernfs_path_from_node_locked()") first noticed this, but it didn't >>>>> fix it completely. >>>>> >>>>> Fixes: 9f6df573a404 ("kernfs: Add API to generate relative kernfs path") >>>>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com> >>>> >>>> I think the right thing to do is removing that if. It makes no sense to call >>>> that function with NULL buf and the fact that nobody reported crashes on >>>> NULL buf indicates that we in fact never do. kernfs_path_from_node -->kernfs_path_from_node_locked EXPORT_SYMBOL_GPL(kernfs_path_from_node) I've rethought it. The export APIs need to do null pointer check, right? >>> >>> OK. >>> >>> How about I remove "buf[0] = '\0';" too? It seems to be a useless operation. >>> When 'kn_from' and 'kn_to' have a common ancestor, there must be a path from >>> 'kn_from' to 'kn_to', and strlcpy() always fills in the terminator correctly, >>> even if the buf is too small to save the first path node. >> >> Sorry, I misanalyzed. The length used by "len < buflen ? buflen - len : 0" may >> be zero. > > Ah, my brain is unstable today. The initial value of len is 0. So "buf[0] = '\0';" > can still be safely removed. > >> >>> >>> static void test(void) >>> { >>> char buf[4]; >>> int i, n, buflen; >>> >>> buflen = 1; >>> n = strlcpy(buf, "string", buflen); >>> for (i = 0; i < buflen; i++) >>> printk("%d: %02x\n", i, buf[i]); >>> printk("n=%d\n\n", n); >>> >>> buflen = sizeof(buf); >>> n = strlcpy(buf, "string", buflen); >>> for (i = 0; i < buflen; i++) >>> printk("%d: %02x\n", i, buf[i]); >>> printk("n=%d\n", n); >>> } >>> >>> Output: >>> [ 33.691497] 0: 00 >>> [ 33.691569] n=6 >>> >>> [ 33.691595] 0: 73 >>> [ 33.691622] 1: 74 >>> [ 33.691630] 2: 72 >>> [ 33.691637] 3: 00 >>> [ 33.691650] n=6 >>> >>>> >>>> Thanks. >>>> >>> >> > -- Regards, Zhen Lei
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