On Tue, 26 Oct 2021 10:18:51 +0800 Yafang Shao <laoar.shao@xxxxxxxxx> wrote: > > So, if we're ever going to copying these buffers out of the kernel (I > > don't know what the object lifetime here in bpf is for "e", etc), we > > should be zero-padding (as get_task_comm() does). > > > > Should this, instead, be using a bounce buffer? > > The comment in bpf_probe_read_kernel_str_common() says > > : /* > : * The strncpy_from_kernel_nofault() call will likely not fill the > : * entire buffer, but that's okay in this circumstance as we're probing > : * arbitrary memory anyway similar to bpf_probe_read_*() and might > : * as well probe the stack. Thus, memory is explicitly cleared > : * only in error case, so that improper users ignoring return > : * code altogether don't copy garbage; otherwise length of string > : * is returned that can be used for bpf_perf_event_output() et al. > : */ > > It seems that it doesn't matter if the buffer is filled as that is > probing arbitrary memory. > > > > > get_task_comm(comm, task->group_leader); > > This helper can't be used by the BPF programs, as it is not exported to BPF. > > > bpf_probe_read_kernel_str(&e.comm, sizeof(e.comm), comm); I guess Kees is worried that e.comm will have something exported to user space that it shouldn't. But since e is part of the BPF program, does the BPF JIT take care to make sure everything on its stack is zero'd out, such that a user BPF couldn't just read various items off its stack and by doing so, see kernel memory it shouldn't be seeing? I'm guessing it does, otherwise this would be a bigger issue than this patch series. -- Steve
