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Message-ID: <06b68b24-63f3-79d2-2d1e-d61516a84bfb@huawei.com> Date: Sat, 26 Nov 2022 18:46:09 +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 2022/11/26 17:49, Leizhen (ThunderTown) wrote: > > > 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? Sorry,I looked at some of the other functions and they didn't check either. > >>>> >>>> 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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