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Since the BPF_PROG_TYPE_EXT program type on itself is just a >> placeholder for those, we need this extended check for those target >> programs to actually work while using this option. >> >> Tested this with a freplace xdp program. Without this patch, the >> verifier fails with error 'cannot write into packet'. >> >> Signed-off-by: Udip Pant >> --- >>   kernel/bpf/verifier.c | 6 +++++- >>   1 file changed, 5 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> index ef938f17b944..4d7604430994 100644 >> --- a/kernel/bpf/verifier.c >> +++ b/kernel/bpf/verifier.c >> @@ -2629,7 +2629,11 @@ static bool may_access_direct_pkt_data(struct >> bpf_verifier_env *env, >>                          const struct bpf_call_arg_meta *meta, >>                          enum bpf_access_type t) >>   { >> -    switch (env->prog->type) { >> +    struct bpf_prog *prog = env->prog; >> +    enum bpf_prog_type prog_type = prog->aux->linked_prog ? >> +          prog->aux->linked_prog->type : prog->type; > > I checked the verifier code. There are several places where > prog->type is checked and EXT program type will behave differently > from the linked program. > > Maybe abstract the the above logic to one static function like > > static enum bpf_prog_type resolved_prog_type(struct bpf_prog *prog) > { >     return prog->aux->linked_prog ? prog->aux->linked_prog->type >                       : prog->type; > } > > This function can then be used in different places to give the resolved > prog type. > > Besides here checking pkt access permission, > another possible places to consider is return value > in function check_return_code(). Currently, > for EXT program, the result value can be anything. This may need to > be enforced. Could you take a look? It could be others as well. > You can take a look at verifier.c by searching "prog->type". Note that if the EXT program tries to replace a global subprogram, then return value cannot be enforced, just as what Patch #2 example shows. > >> + >> +    switch (prog_type) { >>       /* Program types only with direct read access go here! */ >>       case BPF_PROG_TYPE_LWT_IN: >>       case BPF_PROG_TYPE_LWT_OUT: >>