On 10/2/24 18:55, Alexei Starovoitov wrote: > 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? Yes, it does. The strings must be read via the str() call (which internally calls bpf_probe_read_str) and there's an upper bound on the size of each string. > I would think __get_kernel_nofault() approach is user friendlier. That's probably true but isn't there still the problem that strings are not necessarily null-terminated? And in such case, unbounded string functions may not terminate which is not allowed in BPF?
