Messages in this thread | | | Subject | Re: [PATCH] kernfs: fix potential null-ptr-deref in kernfs_path_from_node_locked() | From | "Leizhen (ThunderTown)" <> | Date | Thu, 24 Nov 2022 10:52:43 +0800 |
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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@huawei.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. >> >> 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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