Re: [PATCH] btf_encoder: Check var type after checking var addr.

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Oct 26, 2020 at 3:18 PM Hao Luo <haoluo@google.com> wrote:
>
> Commit 2e719cc ("btf_encoder: revamp how per-CPU variables are
> encoded") adds percpu_var_exists() to filter out the symbols that
> are not percpu var. However, the check comes after checking the
> var's type. There can be symbols that are of zero type. If we hit
> that, btf_encoder will not work without '--btf_encode_force'.
> So we should check percpu_var_exists before checking var's type.
>
> Tested:
>  haoluo@haoluo:~/kernel/tip$ gcc --version
>  gcc (GCC) 10.2.0
>  Copyright (C) 2020 Free Software Foundation, Inc.
>  This is free software; see the source for copying conditions.  There is NO
>  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>
>  Before:
>
>  haoluo@haoluo:~/kernel/tip$ make clean -s
>  haoluo@haoluo:~/kernel/tip$ make -j 32 -s
>    LINK     resolve_btfids
>  error: found variable in CU 'kernel/bpf/btf.c' that has void type
>  Encountered error while encoding BTF.
>  FAILED: load BTF from vmlinux: Unknown error -2make: *** [Makefile:1164: vmlinux] Error 255
>
>  After:
>
>  haoluo@haoluo:~/kernel/tip$ make clean -s
>  haoluo@haoluo:~/kernel/tip$ make -j 32 -s
>    LINK     resolve_btfids
>  haoluo@haoluo:~/kernel/tip$
>
> Fixes: 2e719cc ("btf_encoder: revamp how per-CPU variables are encoded")
> Signed-off-by: Hao Luo <haoluo@google.com>
> ---

Looks good, thanks for the prompt fix!

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>  btf_encoder.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 2a6455b..b5f8f3e 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -441,9 +441,12 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>                 if (var->spec)
>                         var = var->spec;
>
> +               if (!percpu_var_exists(addr, &size, &name))
> +                       continue; /* not a per-CPU variable */
> +
>                 if (var->ip.tag.type == 0) {
> -                       fprintf(stderr, "error: found variable in CU '%s' that has void type\n",
> -                               cu->name);
> +                       fprintf(stderr, "error: found variable '%s' in CU '%s' that has void type\n",
> +                               name, cu->name);
>                         if (force)
>                                 continue;
>                         err = -1;
> @@ -452,8 +455,6 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>
>                 type = var->ip.tag.type + type_id_off;
>                 linkage = var->external ? BTF_VAR_GLOBAL_ALLOCATED : BTF_VAR_STATIC;
> -               if (!percpu_var_exists(addr, &size, &name))
> -                       continue; /* not a per-CPU variable */
>
>                 if (btf_elf__verbose) {
>                         printf("Variable '%s' from CU '%s' at address 0x%lx encoded\n",
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>