[lwip-users] Strange values appearing inside an LWIP packet buffer

[Peter]

Hi All,

Amazingly, I managed to catch one instance fairly quickly

https://peter-ftp.co.uk/screenshots/202207150014414908.jpg

The code is

>      if ((!mem->used) &&
>          (mem->next - (ptr + SIZEOF_STRUCT_MEM)) >= size) {
>        /* mem is not used and at least perfect fit is possible:
>         * mem->next - (ptr + SIZEOF_STRUCT_MEM) gives us the 'user data size' 
> of mem */
>
>        if (mem->next - (ptr + SIZEOF_STRUCT_MEM) >= (size + SIZEOF_STRUCT_MEM 
> + MIN_SIZE_ALIGNED)) {
>          /* (in addition to the above, we test if another struct mem 
> (SIZEOF_STRUCT_MEM) containing
>           * at least MIN_SIZE_ALIGNED of data also fits in the 'user data 
> space' of 'mem')
>           * -> split large block, create empty remainder,
>           * remainder must be large enough to contain MIN_SIZE_ALIGNED data: 
> if
>           * mem->next - (ptr + (2*SIZEOF_STRUCT_MEM)) == size,
>           * struct mem would fit in but no data between mem2 and mem2->next
>           * @todo we could leave out MIN_SIZE_ALIGNED. We would create an 
> empty
>           *       region that couldn't hold data, but when mem->next gets 
> freed,
>           *       the 2 regions would be combined, resulting in more free 
> memory
>           */
>          ptr2 = ptr + SIZEOF_STRUCT_MEM + size;
>          /* create mem2 struct */
>          mem2 = (struct mem *)(void *)&ram[ptr2];
>          mem2->used = 0;
>          mem2->next = mem->next;
>          mem2->prev = ptr;

so it looks genuine. It does suggest that the PBUFs are used for stuff
other than packet data. Some sort of struct is being filled-in.
Strange place for it though.

Peter