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

[Yonghong Song]

On 6/7/23 2:04 PM, Krister Johansen 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.

I think having two early identical program ksym entries is bad.
When people 'cat /proc/kallsyms | grep <their program name>',
they will find two programs with identical kernel address but different
hash value. This is just very confusing. I think removing the
duplicate in kallsyms is better from user's perspective.

> 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;
>   	prog->aux->func = func;
>   	prog->aux->func_cnt = env->subprog_cnt;
>   	bpf_prog_jit_attempt_done(prog);