Hi Pablo, I sent this email (explanation of how the system works) before I saw your email asking for that explanation. Then I replied to that email of yours before I saw this one. On Wed, Apr 29, 2020 at 09:16:43PM +0200, Pablo Neira Ayuso wrote: > On Thu, Apr 30, 2020 at 05:10:47AM +1000, Duncan Roe wrote: > [...] > > Sorry, I should have explained a bit more how the system would work: > > > > struct pkt_buff has 3 new members: > > > > bool copy_done; > > uint32_t extra; > > uint8_t *copy_buf; > > > > When extra > 0, pktb_alloc2 verifies that buflen is >= len + extra. It then > > stores extra and copy_buf in pktb, ready for use by pktb_mangle() (all the other > > manglers call this eventually). > > > > So that's how pktb_mangle() doesn't need to allocate a buffer. > > Thanks for the explaining. Given this is in userspace, it is easier if Tiny nit - this could be userspace on an embedded device where memory is really tight. So perhaps document with "If memory is at a premium, you really only need len + extra" otherwise a big buf is fine. > the user allocates the maximum packet length that is possible: > > 0xffff + (MNL_SOCKET_BUFFER_SIZE/2); > > We can probably expose this to the header so they can pre-allocate a > buffer that is large enough and, hence, _mangle() is guaranteed to > have always enough room to add extra bytes. Yes I saw that expression in examples/nf-queue.c. How about #define COPY_BUF_SIZE (0xffff + (MNL_SOCKET_BUFFER_SIZE/2)) or what other name would you like? --- Off-topic I'm intrigued that you ccan use MNL_SOCKET_BUFFER_SIZE when dimensioning static variables. The expansion is #define MNL_SOCKET_BUFFER_SIZE (sysconf(_SC_PAGESIZE) < 8192L ? sysconf(_SC_PAGESIZE) : 8192L) and sysconf is an actual function: unistd.h:622:extern long int sysconf (int __name) __THROW; If I try to dimension a static variable using pktb_head_size(), the compiler throws an error. Why is sysconf() ok but not pktb_head_size()? Cheers ... Duncan.
