Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args

梦龙董
2024-03-12T02:49:21Z

On Tue, Mar 12, 2024 at 10:42 AM 梦龙董 <dongmenglong.8@bytedance.com> wrote:
>
> On Tue, Mar 12, 2024 at 10:09 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Mon, Mar 11, 2024 at 7:01 PM 梦龙董 <dongmenglong.8@bytedance.com> wrote:
> > >
> > > On Tue, Mar 12, 2024 at 9:46 AM Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Mon, Mar 11, 2024 at 2:34 AM Menglong Dong
> > > > <dongmenglong.8@bytedance.com> wrote:
> > > > >
> > > > > In this commit, we add the 'accessed_args' field to struct bpf_prog_aux,
> > > > > which is used to record the accessed index of the function args in
> > > > > btf_ctx_access().
> > > > >
> > > > > Meanwhile, we add the function btf_check_func_part_match() to compare the
> > > > > accessed function args of two function prototype. This function will be
> > > > > used in the following commit.
> > > > >
> > > > > Signed-off-by: Menglong Dong <dongmenglong.8@bytedance.com>
> > > > > ---
> > > > > include/linux/bpf.h | 4 ++
> > > > > kernel/bpf/btf.c | 108 +++++++++++++++++++++++++++++++++++++++++++-
> > > > > 2 files changed, 110 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > > > index 95e07673cdc1..0f677fdcfcc7 100644
> > > > > --- a/include/linux/bpf.h
> > > > > +++ b/include/linux/bpf.h
> > > > > @@ -1461,6 +1461,7 @@ struct bpf_prog_aux {
> > > > > const struct btf_type *attach_func_proto;
> > > > > /* function name for valid attach_btf_id */
> > > > > const char *attach_func_name;
> > > > > + u64 accessed_args;
> > > > > struct bpf_prog **func;
> > > > > void *jit_data; /* JIT specific data. arch dependent */
> > > > > struct bpf_jit_poke_descriptor *poke_tab;
> > > > > @@ -2565,6 +2566,9 @@ struct bpf_reg_state;
> > > > > int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog);
> > > > > int btf_check_type_match(struct bpf_verifier_log *log, const struct bpf_prog *prog,
> > > > > struct btf *btf, const struct btf_type *t);
> > > > > +int btf_check_func_part_match(struct btf *btf1, const struct btf_type *t1,
> > > > > + struct btf *btf2, const struct btf_type *t2,
> > > > > + u64 func_args);
> > > > > const char *btf_find_decl_tag_value(const struct btf *btf, const struct btf_type *pt,
> > > > > int comp_idx, const char *tag_key);
> > > > > int btf_find_next_decl_tag(const struct btf *btf, const struct btf_type *pt,
> > > > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > > > index 170d017e8e4a..c2a0299d4358 100644
> > > > > --- a/kernel/bpf/btf.c
> > > > > +++ b/kernel/bpf/btf.c
> > > > > @@ -6125,19 +6125,24 @@ static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
> > > > > }
> > > > >
> > > > > static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
> > > > > - int off)
> > > > > + int off, int *aligned_idx)
> > > > > {
> > > > > const struct btf_param *args;
> > > > > const struct btf_type *t;
> > > > > u32 offset = 0, nr_args;
> > > > > int i;
> > > > >
> > > > > + if (aligned_idx)
> > > > > + *aligned_idx = -ENOENT;
> > > > > +
> > > > > if (!func_proto)
> > > > > return off / 8;
> > > > >
> > > > > nr_args = btf_type_vlen(func_proto);
> > > > > args = (const struct btf_param *)(func_proto + 1);
> > > > > for (i = 0; i < nr_args; i++) {
> > > > > + if (aligned_idx && offset == off)
> > > > > + *aligned_idx = i;
> > > > > t = btf_type_skip_modifiers(btf, args[i].type, NULL);
> > > > > offset += btf_type_is_ptr(t) ? 8 : roundup(t->size, 8);
> > > > > if (off < offset)
> > > > > @@ -6207,7 +6212,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > > > > tname, off);
> > > > > return false;
> > > > > }
> > > > > - arg = get_ctx_arg_idx(btf, t, off);
> > > > > + arg = get_ctx_arg_idx(btf, t, off, NULL);
> > > > > args = (const struct btf_param *)(t + 1);
> > > > > /* if (t == NULL) Fall back to default BPF prog with
> > > > > * MAX_BPF_FUNC_REG_ARGS u64 arguments.
> > > > > @@ -6217,6 +6222,9 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > > > > /* skip first 'void *__data' argument in btf_trace_##name typedef */
> > > > > args++;
> > > > > nr_args--;
> > > > > + prog->aux->accessed_args |= (1 << (arg + 1));
> > > > > + } else {
> > > > > + prog->aux->accessed_args |= (1 << arg);
> > > >
> > > > What do you need this aligned_idx for ?
> > > > I'd expect that above "accessed_args |= (1 << arg);" is enough.
> > > >
> > >
> > > Which aligned_idx? No aligned_idx in the btf_ctx_access(), and
> > > aligned_idx is only used in the btf_check_func_part_match().
> > >
> > > In the btf_check_func_part_match(), I need to compare the
> > > t1->args[i] and t2->args[j], which have the same offset. And
> > > the aligned_idx is to find the "j" according to the offset of
> > > t1->args[i].
> >
> > And that's my question.
> > Why you don't do the max of accessed_args across all attach
> > points and do btf_check_func_type_match() to that argno
> > instead of nargs1.
> > This 'offset += btf_type_is_ptr(t1) ? 8 : roundup...
> > is odd.
>
> Hi, I'm trying to make the bpf flexible enough. Let's take an example,
> now we have the bpf program:
>
> int test1_result = 0;
> int BPF_PROG(test1, int a, long b, char c)
> {
> test1_result = a + c;
> return 0;
> }
>
> In this program, only the 1st and 3rd arg is accessed. So all kernel
> functions whose 1st arg is int and 3rd arg is char can be attached
> by this bpf program, even if their 2nd arg is different.
>
> And let's take another example for struct. This is our bpf program:
>
> int test1_result = 0;
> int BPF_PROG(test1, long a, long b, char c)
> {
> test1_result = c;
> return 0;
> }
>
> Only the 3rd arg is accessed. And we have following kernel function:
>
> int kernel_function1(long a, long b, char c)
> {
> xxx
> }
>
> struct test1 {
> long a;
> long b;
> };
> int kernel_function2(struct test1 a, char b)
> {
> xxx
> }
>
> The kernel_function1 and kernel_function2 should be compatible,
> as the bpf program only accessed the ctx[2], whose offset is 16.
> And the arg in kernel_function1() with offset 16 is "char c", the
> arg in kernel_function2() with offset 16 is "char b", which is
> compatible.
>
> That's why we need to check the consistency of accessed args
> by offset instead of function arg index.
>

And that's why I didn't share the code with btf_check_func_type_match().
In btf_check_func_part_match(), I'm trying to check the "real" accessed
args of t1 and t2, not by the function index, which has quite a difference
with btf_check_func_type_match().

> I'm not sure if I express my idea clearly, is this what you are
> asking?
>
> Thanks!
> Menglong Dong