Hi Richard,
On Mon, Mar 16, 2015 at 1:18 PM, Richard Weinberger <rich...@nod.at> wrote:
> Am 16.03.2015 um 13:03 schrieb Geert Uytterhoeven:
>> On Mon, Mar 16, 2015 at 12:41 PM, Richard Weinberger <rich...@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:
Oops, that's of course not true, as s is the destination, not the source,
of the copy operation.
>>> - 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().
Ah, so the path is stored in the end of the buffer...
> Therefore this bounds check works:
> if (len > p - name) {
... and if this is true, prefix and path would overlap, which means there's\
not enough space in the buffer.
> __putname(name);
> return NULL;
> }
>
> Is it now clearer or did I miss something?
> I agree that this code is tricky. :)
Yes, thanks for your explanation!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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