linux-kernel - Re: [PATCH v1] qnx4_match: do not over run the buffer

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Message-ID: <1645927.Y8UaVx19Bi@alarsen.net>
Date:   Sat, 21 Nov 2020 22:57:34 +0100
From:   Anders Larsen <al@...rsen.net>
To:     Tong Zhang <ztong0001@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] qnx4_match: do not over run the buffer

On Saturday, 2020-11-21 22:47 Tong Zhang wrote:
> 
> > On Nov 21, 2020, at 4:40 PM, Anders Larsen <al@...rsen.net> wrote:
> > 
> > On Friday, 2020-11-20 22:21 Tong Zhang wrote:
> >> the di_fname may not terminated by '\0', use strnlen to prevent buffer
> >> overrun
> >> 
> >> ---
> >> fs/qnx4/namei.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/fs/qnx4/namei.c b/fs/qnx4/namei.c
> >> index 8d72221735d7..c0e79094f578 100644
> >> --- a/fs/qnx4/namei.c
> >> +++ b/fs/qnx4/namei.c
> >> @@ -40,7 +40,7 @@ static int qnx4_match(int len, const char *name,
> >> 	} else {
> >> 		namelen = QNX4_SHORT_NAME_MAX;
> >> 	}
> >> -	thislen = strlen( de->di_fname );
> >> +	thislen = strnlen( de->di_fname, QNX4_SHORT_NAME_MAX );
> > 
> > that should be
> > +	thislen = strnlen( de->di_fname, namelen );
> > otherwise the length of a filename would always be limited to QNX4_SHORT_NAME_MAX (16) characters.
> > 
> Why should we put something bigger here if the size of qnx4_inode_entry->di_fname is QNX4_SHORT_NAME_MAX.
> Won’t that be a problem?

If QNX4_FILE_LINK is set in de->di_status (see line 38), 'de' actually points to a struct qnx4_link_info which can hold a longer name.
That's the reason for the namelen massage.
(Please don't ask why it is not a union)

Cheers
Anders