On Tue, Sep 21, 2021 at 9:06 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Hi Lorenz (Cc. the other people who participated in today's discussion) > > Following our discussion at the LPC session today, I dug up my previous > summary of the issue and some possible solutions[0]. Seems no on > actually replied last time, which is why we went with the "do nothing" > approach, I suppose. I'm including the full text of the original email > below; please take a look, and let's see if we can converge on a > consensus here. > > First off, a problem description: If an existing XDP program is exposed > to an xdp_buff that is really a multi-buffer, while it will continue to > run, it may end up with subtle and hard-to-debug bugs: If it's parsing > the packet it'll only see part of the payload and not be aware of that > fact, and if it's calculating the packet length, that will also only be > wrong (only counting the first fragment). > > So what to do about this? First of all, to do anything about it, XDP > programs need to be able to declare themselves "multi-buffer aware" (but > see point 1 below). We could try to auto-detect it in the verifier by > which helpers the program is using, but since existing programs could be > perfectly happy to just keep running, it probably needs to be something > the program communicates explicitly. One option is to use the > expected_attach_type to encode this; programs can then declare it in the > source by section name, or the userspace loader can set the type for > existing programs if needed. > > With this, the kernel will know if a given XDP program is multi-buff > aware and can decide what to do with that information. For this we came > up with basically three options: > > 1. Do nothing. This would make it up to users / sysadmins to avoid > anything breaking by manually making sure to not enable multi-buffer > support while loading any XDP programs that will malfunction if > presented with an mb frame. This will probably break in interesting > ways, but it's nice and simple from an implementation PoV. With this > we don't need the declaration discussed above either. > > 2. Add a check at runtime and drop the frames if they are mb-enabled and > the program doesn't understand it. This is relatively simple to > implement, but it also makes for difficult-to-understand issues (why > are my packets suddenly being dropped?), and it will incur runtime > overhead. > > 3. Reject loading of programs that are not MB-aware when running in an > MB-enabled mode. This would make things break in more obvious ways, > and still allow a userspace loader to declare a program "MB-aware" to > force it to run if necessary. The problem then becomes at what level > to block this? > I think there's another potential problem with this as well: what happens to already loaded programs that are not MB-aware? Are they forcibly unloaded? > Doing this at the driver level is not enough: while a particular > driver knows if it's running in multi-buff mode, we can't know for > sure if a particular XDP program is multi-buff aware at attach time: > it could be tail-calling other programs, or redirecting packets to > another interface where it will be processed by a non-MB aware > program. > > So another option is to make it a global toggle: e.g., create a new > sysctl to enable multi-buffer. If this is set, reject loading any XDP > program that doesn't support multi-buffer mode, and if it's unset, > disable multi-buffer mode in all drivers. This will make it explicit > when the multi-buffer mode is used, and prevent any accidental subtle > malfunction of existing XDP programs. The drawback is that it's a > mode switch, so more configuration complexity. > Could we combine the last two bits here into a global toggle that doesn't require a sysctl? If any driver is put into multi-buffer mode, then the system switches to requiring all programs be multi-buffer? When the last multi-buffer enabled driver switches out of multi-buffer, remove the system-wide restriction? Regarding my above question, if non-MB-aware XDP programs are not forcibly unloaded, then a global toggle is also insufficient. An existing non-MB-aware XDP program would still beed to be rejected at attach time by the driver. > None of these options are ideal, of course, but I hope the above > explanation at least makes sense. If anyone has any better ideas (or can > spot any flaws in the reasoning above) please don't hesitate to let us > know! > > -Toke > > [0] https://lore.kernel.org/r/8735srxglb.fsf@xxxxxxx >
