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From: Richard Weinberger <richard@nod.at>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: uml-devel <user-mode-linux-devel@lists.sourceforge.net>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [uml-devel] [PATCH 07/15] hostfs: Remove open coded strcpy()
Date: Mon, 16 Mar 2015 13:18:43 +0100	[thread overview]
Message-ID: <5506CA23.8090106@nod.at> (raw)
In-Reply-To: <CAMuHMdXpkyvoTRYZ5X4dVNKoHA=seiYpS7pFSZc+fzxFRM=r2Q@mail.gmail.com>

Am 16.03.2015 um 13:03 schrieb Geert Uytterhoeven:
> On Mon, Mar 16, 2015 at 12:41 PM, Richard Weinberger <richard@nod.at> wrote:
>> --- a/fs/hostfs/hostfs_kern.c
>> +++ b/fs/hostfs/hostfs_kern.c
>> @@ -105,11 +105,10 @@ static char *__dentry_name(struct dentry *dentry, char *name)
> 
> This code looks fishy to me...
> 
> First we have:
> 
>     len = strlen(root);
>     strlcpy(name, root, PATH_MAX);
> 
> (I notice the code used strncpy() before. One difference with strlcpy()
>  is that strncpy() fills the remaining of the destination buffer with zeroes.)
> 
> Then:
> 
>>                 __putname(name);
>>                 return NULL;
>>         }
>> -       if (p > name + len) {
>> -               char *s = name + len;
> 
> Unless strlcpy() truncated the string (which is unlikely, as root
> cannot be longer
> than PATH_MAX?), s = name + len now points to the zero terminator.
> So the below would copy just one single byte:
> 
>> -               while ((*s++ = *p++) != '\0')
>> -                       ;
>> -       }
>> +
>> +       if (p > name + len)
>> +               strcpy(name + len, p);
>> +
> 
> What is this code really supposed to do?

Hostfs' __dentry_name() builds the real path. i.e, the prefix on the host side
plus the requested path in UML.

"strlcpy(name, root, PATH_MAX);" copies the host prefix into name and then
the "strcpy(name + len, p);" copies the requested path into it.

The trick is that both share the same buffer, allocated by dentry_path_raw().
Therefore this bounds check works:
        if (len > p - name) {
                __putname(name);
                return NULL;
        }

Is it now clearer or did I miss something?
I agree that this code is tricky. :)

Thanks,
//richard

  reply	other threads:[~2015-03-16 12:18 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-16 11:41 Hostfs updates Richard Weinberger
2015-03-16 11:41 ` [PATCH 01/15] hostfs: hostfs_file_open: Switch to data locking model Richard Weinberger
2015-03-16 11:41 ` [PATCH 02/15] hostfs: hostfs_file_open: Fix a fd leak in hostfs_file_open Richard Weinberger
2015-03-16 11:41 ` [PATCH 03/15] hostfs: Allow fsync on directories Richard Weinberger
2015-03-16 11:41 ` [PATCH 04/15] hostfs: Handle bogus st.mode Richard Weinberger
2015-03-16 11:41 ` [PATCH 05/15] hostfs: Make hostfs_readpage more readable Richard Weinberger
2015-03-16 11:41 ` [PATCH 06/15] hostfs: Add a BUG_ON to detect behavior changes of dentry_path_raw() Richard Weinberger
2015-03-16 11:41 ` [PATCH 07/15] hostfs: Remove open coded strcpy() Richard Weinberger
2015-03-16 12:03   ` [uml-devel] " Geert Uytterhoeven
2015-03-16 12:18     ` Richard Weinberger [this message]
2015-03-16 12:44       ` Geert Uytterhoeven
2015-03-16 11:41 ` [PATCH 08/15] hostfs: Use __getname() in follow_link Richard Weinberger
2015-03-16 11:41 ` [PATCH 09/15] hostfs: Report append flag in ->show_options() Richard Weinberger
2015-03-16 11:41 ` [PATCH 10/15] hostfs: Remove superfluous test in hostfs_open() Richard Weinberger
2015-03-16 11:41 ` [PATCH 11/15] hostfs: hostfs_open: Reset open flags upon each retry Richard Weinberger
2015-03-16 11:41 ` [PATCH 12/15] hostfs: Remove superfluous initializations in hostfs_open() Richard Weinberger
2015-03-16 11:41 ` [PATCH 13/15] hostfs: Set page flags in hostfs_readpage() correctly Richard Weinberger
2015-03-16 11:41 ` [PATCH 14/15] hostfs: Use page_offset() Richard Weinberger
2015-03-16 11:41 ` [PATCH 15/15] hostfs: No need to box and later unbox the file mode Richard Weinberger

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