[PATCH bpf-next v2] libbpf: prevent overwriting of log_level in bpf_object__load_progs()

[PATCH bpf-next v2] libbpf: prevent overwriting of log_level in bpf_object__load_progs()

[Quentin Monnet]

There are two functions in libbpf that support passing a log_level
parameter for the verifier for loading programs:
bpf_object__load_xattr() and bpf_prog_load_xattr(). Both accept an
attribute object containing the log_level, and apply it to the programs
to load.

It turns out that to effectively load the programs, the latter function
eventually relies on the former. This was not taken into account when
adding support for log_level in bpf_object__load_xattr(), and the
log_level passed to bpf_prog_load_xattr() later gets overwritten with a
zero value, thus disabling verifier logs for the program in all cases:

bpf_prog_load_xattr()             // prog->log_level = attr1->log_level;
-> bpf_object__load()             // attr2->log_level = 0;
   -> bpf_object__load_xattr()    // <pass prog and attr2>
      -> bpf_object__load_progs() // prog->log_level = attr2->log_level;

Fix this by OR-ing the log_level in bpf_object__load_progs(), instead of
overwriting it.

v2: Fix commit log description (confusion on function names in v1).

Fixes: 60276f984998 ("libbpf: add bpf_object__load_xattr() API function to pass log_level")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
---
 tools/lib/bpf/libbpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ca4432f5b067..30cb08e2eb75 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2232,7 +2232,7 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
 	for (i = 0; i < obj->nr_programs; i++) {
 		if (bpf_program__is_function_storage(&obj->programs[i], obj))
 			continue;
-		obj->programs[i].log_level = log_level;
+		obj->programs[i].log_level |= log_level;
 		err = bpf_program__load(&obj->programs[i],
 					obj->license,
 					obj->kern_version);
-- 
2.17.1

Re: [PATCH bpf-next v2] libbpf: prevent overwriting of log_level in bpf_object__load_progs()

[Daniel Borkmann]

On 05/29/2019 04:26 PM, Quentin Monnet wrote:
> There are two functions in libbpf that support passing a log_level
> parameter for the verifier for loading programs:
> bpf_object__load_xattr() and bpf_prog_load_xattr(). Both accept an
> attribute object containing the log_level, and apply it to the programs
> to load.
> 
> It turns out that to effectively load the programs, the latter function
> eventually relies on the former. This was not taken into account when
> adding support for log_level in bpf_object__load_xattr(), and the
> log_level passed to bpf_prog_load_xattr() later gets overwritten with a
> zero value, thus disabling verifier logs for the program in all cases:
> 
> bpf_prog_load_xattr()             // prog->log_level = attr1->log_level;
> -> bpf_object__load()             // attr2->log_level = 0;
>    -> bpf_object__load_xattr()    // <pass prog and attr2>
>       -> bpf_object__load_progs() // prog->log_level = attr2->log_level;
> 
> Fix this by OR-ing the log_level in bpf_object__load_progs(), instead of
> overwriting it.
> 
> v2: Fix commit log description (confusion on function names in v1).
> 
> Fixes: 60276f984998 ("libbpf: add bpf_object__load_xattr() API function to pass log_level")
> Reported-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>

That's better, applied, thanks!