4:05 a.m.
On 2014/12/05, 3:41 PM, "Tristan Lelong" <tristan(a)lelong.xyz> wrote:
On Fri, Dec 05, 2014 at 01:27:23PM -0800, Greg KH wrote:
> On Fri, Dec 05, 2014 at 12:03:47AM -0800, Tristan Lelong wrote:
> > static ssize_t
> > -fld_proc_hash_seq_write(struct file *file, const char *buffer,
> > - size_t count, loff_t *off)
> > +fld_proc_hash_seq_write(struct file *file,
> > + const char __user *buffer,
> > + size_t count, loff_t *off)
> > {
> > struct lu_client_fld *fld;
> > struct lu_fld_hash *hash = NULL;
> > + char name[80];
> > int i;
> >
> > + if (count > 80)
> > + return -ENAMETOOLONG;
> > +
> > + if (copy_from_user(name, buffer, count) != 0)
> > + return -EFAULT;
>
> How was this code ever working before?
I have no idea, and was actually surprised that this was there.
>
> And I know Joe asked, but how do you know that 80 is ok? And why on the
> stack?
80 is the sizeof(struct lu_fld_hash.fh_name) and there is no define for
that. A few other structure members are using this 80 value internally,
and as I told Joe, I will analyze if they are all related and submit a
patch to use a define instead.
Sorry, but I don't see where you get 80 from? fh_name is declared as a
"const char *", and initialized in the declaration of fld_hash[]. I'd
thought to reply that sizeof(fh_name) would even be better than a #define,
but sizeof(const char *) doesn't actually make sense.
The longest declared fh_name is 4 characters, but I'm not sure of an easy
way to determine this at compile time. I guess one option is to change
the declaration of struct lu_fld_hash to use "const char fh_name[4];" and
then use sizeof(fh_name), but I don't know if that is better than just
declaring a small buffer (8 chars) for this usage. IMHO that is small
enough to fit on the stack, since it is at the top of a very short
callchain (userspace->sys_write->vfs_write->fld_proc_hash_seq_write())
that just saves the value so the chance of stack overflow is basically nil.
>
> Shouldn't you just compare count to strlen(fld_hash[i].fh_name)? like
>you
> do later on?
>
This is actually done in the for loop already. I first compare with the
maximum size, then the loop use the strlen of each entries in the table,
and finally does the strncmp.
>
> Anyway, I don't like large stack variables like this, can you make it
> dynamic instead?
>
I can definitely do this with a kmalloc, I'll submit a v2 tonight.
Thanks
Cheers, Andreas
--
Andreas Dilger
Lustre Software Architect
Intel High Performance Data Division