Gilad Reti wrote: > On Thu, Apr 2, 2020 at 6:38 PM Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > re-added mailing list back. pls don't remove it from cc. > Sorry, this wasn't on purpose. > > > > On Thu, Apr 2, 2020 at 1:05 AM Gilad Reti <gilad.reti@xxxxxxxxx> wrote: > > > > > > On Thu, Apr 2, 2020 at 2:28 AM Alexei Starovoitov > > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > > > On Tue, Mar 31, 2020 at 07:16:28PM +0300, Gilad Reti wrote: > > > > > When I try to probe_write_common into a writable location (e.g a > > > > > memory address on a usermode stack) which is not yet mapped or mapped > > > > > as read only to the memory, the function sometimes return a EFAULT > > > > > (bad address) error. This is happening since the pagefault handler was > > > > > disabled and thus this memory location won't be mapped when the > > > > > function tries to write into it, an error will be returned and no data > > > > > will be written. > > > > > Is that behavior intended? Did you want those functions to have as > > > > > less side-effects are possible? > > > > > > > > You mean bpf_probe_write_user() helper? > > > Well yes, but it calls probe_write_common which disables the pagefault > > > handler so I asked about it. > > > > Yes it's a non-faulting helper that will fail if prog is trying to > > > > write into a valid memory that could have been served with minor fault. > > > > The main reason for this is that bpf progs are not allowed to sleep. > > > > We're working on sleepable bpf progs that will be able to do copy_from/to_user > > > > from the context where it is safe to sleep. Like syscall entry. > > > Thanks! > > > > Could you please share more about your use case, so we can make sure > > > > that it will be covered by upcoming work? > > > Sure. I was playing with modifying kprobed syscall parameters (for > > > example, changing the path of an openat syscall etc). > > > > yes, but what is the use case? > > Why do you want to modify path of openat syscall? > I had no specific use case. I have seen that eBPF can modify usermode > memory and wanted to experiment with that... Late reply but figured its worth adding a concrete example. We have a use case to read user memory from syscall (args when they are pointers to memory for example) where we can cause a page fault. Sleepable bpf progs with copy helpers would solve this for us. Thanks, John
