On Thu, Dec 13, 2018 at 1:20 PM Michal Kubecek <mkube...@suse.cz> wrote: > > On Thu, Dec 13, 2018 at 12:59:36PM +0100, Michal Kubecek wrote: > > On Thu, Dec 13, 2018 at 12:00:59PM +0100, Alexander Potapenko wrote: > > > Hi BPF maintainers, > > > > > > some time ago KMSAN found an issue in BPF code which we decided to > > > suppress at that point, but now I'd like to bring it to your > > > attention. > > > Namely, some BPF programs may contain instructions that XOR a register > > > with itself. > > > This effectively results in the following C code: > > > regs[BPF_REG_A] = regs[BPF_REG_A] ^ regs[BPF_REG_A]; > > > or > > > regs[BPF_REG_X] = regs[BPF_REG_X] ^ regs[BPF_REG_X]; > > > being executed. > > > > > > According to the C11 standard this is undefined behavior, so KMSAN > > > reports an error in this case. > > > > Can you quote the part of the standard saying this is undefined > > behavior? I couldn't find anything else than > > > > If the value being stored in an object is read from another object > > that overlaps in any way the storage of the first object, then the > > overlap shall be exact and the two objects shall have qualified or > > unqualified versions of a compatible type; otherwise, the behavior > > is undefined. > > > > (but I only have a draft for obvious reasons). I'm not sure what exactly > > they mean by "exact overlap" and the standard doesn't seem to define > > the term but if the two objects are actually the same, they certainly > > have compatible types. > > > I think I understand now. You didn't want to say that the statement > > regs[BPF_REG_A] = regs[BPF_REG_A] ^ regs[BPF_REG_A]; > > as such is undefined behavior but that it's UB when regs[BPF_REG_A] is > uninitialized. Right? Yes. Sorry for being unclear. By default regs[] is uninitialized, so we need to initialize it before using the register values. I am also wondering if it's possible to simply copy the uninitialized register values from regs[] to the userspace via maps. > Michal Kubecek
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