[dpdk-dev,v2] Correctly handle malloc_elem resize with padding

  Currently when a malloc_elem is split after resizing, any padding
present in the elem is ignored.  This causes the resized elem to be too
small when padding is present, and user data can overwrite the beginning
of the following malloc_elem.

Solve this by including the size of the padding when computing where to
split the malloc_elem.

Signed-off-by: Jamie Lavigne <lavignen@amazon.com>
---
 lib/librte_eal/common/malloc_elem.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
  

Hi Jamie,

On 31/05/2017 01:16, Jamie Lavigne wrote:
> Currently when a malloc_elem is split after resizing, any padding
> present in the elem is ignored.  This causes the resized elem to be too
> small when padding is present, and user data can overwrite the beginning
> of the following malloc_elem.
>
> Solve this by including the size of the padding when computing where to
> split the malloc_elem.

Nice catch!

Could you please rework commit format a bit:
- Add 'mem:' as prefix in your patch title
- I would mention in the title that this is a fix
- Provide 'Fixes' line in commit message

> Signed-off-by: Jamie Lavigne <lavignen@amazon.com>
> ---
>   lib/librte_eal/common/malloc_elem.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/lib/librte_eal/common/malloc_elem.c b/lib/librte_eal/common/malloc_elem.c
> index 42568e1..8766fa8 100644
> --- a/lib/librte_eal/common/malloc_elem.c
> +++ b/lib/librte_eal/common/malloc_elem.c
> @@ -333,9 +333,11 @@ malloc_elem_resize(struct malloc_elem *elem, size_t size)
>   	elem_free_list_remove(next);
>   	join_elem(elem, next);
>   
> -	if (elem->size - new_size >= MIN_DATA_SIZE + MALLOC_ELEM_OVERHEAD){
> +	const size_t new_total_size = new_size + elem->pad;
> +
> +	if (elem->size - new_total_size >= MIN_DATA_SIZE + MALLOC_ELEM_OVERHEAD) {
>   		/* now we have a big block together. Lets cut it down a bit, by splitting */
> -		struct malloc_elem *split_pt = RTE_PTR_ADD(elem, new_size);
> +		struct malloc_elem *split_pt = RTE_PTR_ADD(elem, new_total_size);
>   		split_pt = RTE_PTR_ALIGN_CEIL(split_pt, RTE_CACHE_LINE_SIZE);
>   		split_elem(elem, split_pt);
>   		malloc_elem_free_list_insert(split_pt);

This indeed fixes the issue you have mentioned. I was thinking of the 
following fix instead:
- Add elem->pad to new_size
- Remove current_size var and instead use elem->size

I think those changes should have the same result while removing a 
couple of vars from the function, which I hope would be easier to read.

What do you think?

Thanks,
Sergio
  

Hi Sergio,

> Hi Jamie, 
> 
> On 31/05/2017 01:16, Jamie Lavigne wrote: 
> > Currently when a malloc_elem is split after resizing, any padding 
> > present in the elem is ignored.  This causes the resized elem to be too 
> > small when padding is present, and user data can overwrite the beginning 
> > of the following malloc_elem. 
> > 
> > Solve this by including the size of the padding when computing where to 
> > split the malloc_elem. 
>  
> Nice catch! 
>  
> Could you please rework commit format a bit: 
> - Add 'mem:' as prefix in your patch title 
> - I would mention in the title that this is a fix 
> - Provide 'Fixes' line in commit message 

Updated.

>  
> > Signed-off-by: Jamie Lavigne <lavignen@amazon.com> 
> > --- 
> >   lib/librte_eal/common/malloc_elem.c | 6 ++++-- 
> >   1 file changed, 4 insertions(+), 2 deletions(-) 
> > 
> > diff --git a/lib/librte_eal/common/malloc_elem.c b/lib/librte_eal/common/malloc_elem.c 
> > index 42568e1..8766fa8 100644 
> > --- a/lib/librte_eal/common/malloc_elem.c 
> > +++ b/lib/librte_eal/common/malloc_elem.c 
> > @@ -333,9 +333,11 @@ malloc_elem_resize(struct malloc_elem *elem, size_t size) 
> >       elem_free_list_remove(next); 
> >       join_elem(elem, next); 
> > 
> > -     if (elem->size - new_size >= MIN_DATA_SIZE + MALLOC_ELEM_OVERHEAD){ 
> > +     const size_t new_total_size = new_size + elem->pad; 
> > + 
> > +     if (elem->size - new_total_size >= MIN_DATA_SIZE + MALLOC_ELEM_OVERHEAD) { 
> >               /* now we have a big block together. Lets cut it down a bit, by splitting */ 
> > -             struct malloc_elem *split_pt = RTE_PTR_ADD(elem, new_size); 
> > +             struct malloc_elem *split_pt = RTE_PTR_ADD(elem, new_total_size); 
> >               split_pt = RTE_PTR_ALIGN_CEIL(split_pt, RTE_CACHE_LINE_SIZE); 
> >               split_elem(elem, split_pt); 
> >               malloc_elem_free_list_insert(split_pt); 
>  
> This indeed fixes the issue you have mentioned. I was thinking of the 
> following fix instead: 
> - Add elem->pad to new_size 
> - Remove current_size var and instead use elem->size 
>  
> I think those changes should have the same result while removing a 
> couple of vars from the function, which I hope would be easier to read. 
>  
> What do you think? 

I like this.  It looks equivalent to my solution, but simpler.  I will post an updated patch.

Jamie