On Tue, Oct 1, 2024 at 11:12 PM Viktor Malik <vmalik@xxxxxxxxxx> wrote: > > On 10/1/24 19:40, Andrii Nakryiko wrote: > > On Tue, Oct 1, 2024 at 10:34 AM Alexei Starovoitov > > <alexei.starovoitov@xxxxxxxxx> wrote: > >> > >> On Tue, Oct 1, 2024 at 10:04 AM Andrii Nakryiko > >> <andrii.nakryiko@xxxxxxxxx> wrote: > >>> > >>> On Tue, Oct 1, 2024 at 7:48 AM Alexei Starovoitov > >>> <alexei.starovoitov@xxxxxxxxx> wrote: > >>>> > >>>> On Tue, Oct 1, 2024 at 4:26 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote: > >>>>> > >>>>> On Mon, 2024-09-30 at 15:00 -0700, Andrii Nakryiko wrote: > >>>>> > >>>>> [...] > >>>>> > >>>>>> Right now, the only way to pass dynamically sized anything is through > >>>>>> dynptr, AFAIU. > >>>>> > >>>>> But we do have 'is_kfunc_arg_mem_size()' that checks for __sz suffix, > >>>>> e.g. used for bpf_copy_from_user_str(): > >>>>> > >>>>> /** > >>>>> * bpf_copy_from_user_str() - Copy a string from an unsafe user address > >>>>> * @dst: Destination address, in kernel space. This buffer must be > >>>>> * at least @dst__sz bytes long. > >>>>> * @dst__sz: Maximum number of bytes to copy, includes the trailing NUL. > >>>>> * ... > >>>>> */ > >>>>> __bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__sz, const void __user *unsafe_ptr__ign, u64 flags) > >>>>> > >>>>> However, this suffix won't work for strnstr because of the arguments order. > >>>> > >>>> Stating the obvious... we don't need to keep the order exactly the same. > >>>> > >>>> Regarding all of these kfuncs... as Andrii pointed out 'const char *s' > >>>> means that the verifier will check that 's' points to a valid byte. > >>>> I think we can do a hybrid static + dynamic safety scheme here. > >>>> All of the kfunc signatures can stay the same, but we'd have to > >>>> open code all string helpers with __get_kernel_nofault() instead of > >>>> direct memory access. > >>>> Since the first byte is guaranteed to be valid by the verifier > >>>> we only need to make sure that the s+N bytes won't cause page faults > >>> > >>> You mean to just check that s[N-1] can be read? Given a large enough > >>> N, couldn't it be that some page between s[0] and s[N-1] still can be > >>> unmapped, defeating this check? > >> > >> Just checking s[0] and s[N-1] is not enough, obviously, and especially, > >> since the logic won't know where nul byte is, so N is unknown. > >> I meant to that all of str* kfuncs will be reading all bytes > >> via __get_kernel_nofault() until they find \0. > > > > Ah, ok, I see what you mean now. > > > >> It can be optimized to 8 byte access. > >> The open coding (aka copy-paste) is unfortunate, of course. > > > > Yep, this sucks. > > Yeah, that's quite annoying. I really wanted to avoid doing that. Also, > we won't be able to use arch-optimized versions of the functions. > > Just to make sure I understand things correctly - can we do what Eduard > suggested and add explicit sizes for all arguments using the __sz > suffix? So something like: > > const char *bpf_strnstr(const char *s1, u32 s1__sz, const char *s2, u32 s2__sz); That's ok-ish, but you probably want: const char *bpf_strnstr(void *s1, u32 s1__sz, void *s2, u32 s2__sz); and then to call strnstr() you still need to strnlen(s2, s2__sz). But a more general question... how always passing size will work for bpftrace ? Does it always know the upper bound of storage where strings are stored? I would think __get_kernel_nofault() approach is user friendlier.
