[lwip-users] Alignment problem i pbuf_alloc()

[Lars Thorup]

Hello,

We are trying to get lwip to run on our Arm-7 board; however we are
experiencing occasional memory overwrites, and we think that we have tracked
the problem down to a bug in pbuf_alloc(). We use 4 byte alignment (so
MEM_ALIGNMENT is 4).

In pbuf_alloc() we find the following code fragment:

> p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf) + length + offset));
> ...
> p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf) +
offset));

"sizeof(struct pbuf)" is 16, and offset in this case is 20+20+6 = 46

When "length" is 21, "sizeof(struct pbuf) + length + offset" becomes 83
which gets (correctly) rounded up by MEM_ALIGN_SIZE to 84.

Then "p + sizeof(struct pbuf) + offset" becomes p+62, and since p is already
allocated on an alignment boundary this gets (correctly) rounded up by
MEM_ALIGN to p+64.

However now "p->payload" points at the 64'th byte in a 84 byte buffer, and
that does not leave room enough for the 21 bytes of payload (length) that we
asked for. A short time later in the code we will get a memory overwrite.

Are we correct in seeing this as bug?

Best regards
Lars Thorup