Re: [PATCH bpf v2 2/2] bpf: ensure main program has an extable

[Alexei Starovoitov]

On Wed, Jun 7, 2023 at 2:04 PM Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx> wrote:
>
> When bpf subprograms are in use, the main program is not jit'd after the
> subprograms because jit_subprogs sets a value for prog->bpf_func upon
> success.  Subsequent calls to the JIT are bypassed when this value is
> non-NULL.  This leads to a situation where the main program and its
> func[0] counterpart are both in the bpf kallsyms tree, but only func[0]
> has an extable.  Extables are only created during JIT.  Now there are
> two nearly identical program ksym entries in the tree, but only one has
> an extable.  Depending upon how the entries are placed, there's a chance
> that a fault will call search_extable on the aux with the NULL entry.
>
> Since jit_subprogs already copies state from func[0] to the main
> program, include the extable pointer in this state duplication.  The
> alternative is to skip adding the main program to the bpf_kallsyms
> table, but that would mean adding a check for subprograms into the
> middle of bpf_prog_load.

adding a check to bpf_prog_load() isn't great. that's true, but...

> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 1c2a088a6626 ("bpf: x64: add JIT support for multi-function programs")
> Signed-off-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/bpf/verifier.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 5871aa78d01a..d6939db9fbf9 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -17242,6 +17242,7 @@ static int jit_subprogs(struct bpf_verifier_env *env)
>         prog->jited = 1;
>         prog->bpf_func = func[0]->bpf_func;
>         prog->jited_len = func[0]->jited_len;
> +       prog->aux->extable = func[0]->aux->extable;

Why not to do this hunk and what I suggested earlier: start from func=1 ?
That will address double ksym insertion that Yonghong mentioned.