On Fri, Jan 29, 2021 at 1:11 AM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > On Thu, Jan 28, 2021 at 03:51:13PM -0800, Andy Lutomirski wrote: > > Okay, so I guess you're trying to inline probe_read_kernel(). But > > that means you have to inline a valid implementation. In particular, > > you need to check that you're accessing *kernel* memory. Just like > > That check is on the verifier side. It only does it for kernel > pointers with known types. > In a sequnce a->b->c the verifier guarantees that 'a' is valid > kernel pointer and it's also !null. Then it guarantees that offsetof(b) > points to valid kernel field which is also a pointer. > What it doesn't check that b != null, so > that users don't have to write silly code with 'if (p)' after every > dereference. How is that supposed to work? If I e.g. have a pointer to a task_struct, and I do something like: task->mm->mmap->vm_file->f_inode and another thread concurrently mutates the VMA tree and frees the VMA that we're traversing here, how can BPF guarantee that task->mm->mmap->vm_file is a valid pointer and not whatever garbage we read from freed memory?
