Re: [PATCH bpf] libbpf: Return btf_fd in libbpf__probe_raw_btf

On Wed, May 29, 2019 at 08:35:25AM -0700, Andrii Nakryiko wrote:

On Wed, May 29, 2019 at 1:30 AM Michal Rostecki [off-list ref] wrote:

quoted

Function load_sk_storage_btf expects that libbpf__probe_raw_btf is
returning a btf descriptor, but before this change it was returning
an information about whether the probe was successful (0 or 1).
load_sk_storage_btf was using that value as an argument to the close
function, which was resulting in closing stdout and thus terminating the
process which used that dunction.

That bug was visible in bpftool. `bpftool feature` subcommand was always
exiting too early (because of closed stdout) and it didn't display all
requested probes. `bpftool -j feature` or `bpftool -p feature` were not
returning a valid json object.

Thanks for the fix!

quoted

Fixes: d7c4b3980c18 ("libbpf: detect supported kernel BTF features and sanitize BTF")
Signed-off-by: Michal Rostecki <redacted>
---
 tools/lib/bpf/libbpf.c        | 36 +++++++++++++++++++++--------------
 tools/lib/bpf/libbpf_probes.c |  7 +------
 2 files changed, 23 insertions(+), 20 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 197b574406b3..bc2dca36bced 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -1645,15 +1645,19 @@ static int bpf_object__probe_btf_func(struct bpf_object *obj)
                /* FUNC x */                                    /* [3] */
                BTF_TYPE_ENC(5, BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0), 2),
        };
-       int res;
+       int btf_fd;
+       int ret;

-       res = libbpf__probe_raw_btf((char *)types, sizeof(types),
-                                   strs, sizeof(strs));
-       if (res < 0)
-               return res;
-       if (res > 0)
+       btf_fd = libbpf__probe_raw_btf((char *)types, sizeof(types),
+                                      strs, sizeof(strs));
+       if (btf_fd < 0)
+               ret = 0;
+       else {
+               ret = 1;

This whole ret variable seems unnecessary. Also if btf_fd is invalid,
we probably shouldn't close it. So just this should work:

btf_fd = libbpf__probe_raw_btf(...);
if (btf_fd >= 0) {
    obj->caps.btf_func = 1;
    close(btf_fd);
}
return btf_fd >= 0;

Makes sense, I will do it in v3.

quoted

                obj->caps.btf_func = 1;
-       return 0;
+       }
+       close(btf_fd);
+       return ret;
 }

 static int bpf_object__probe_btf_datasec(struct bpf_object *obj)

@@ -1670,15 +1674,19 @@ static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
                BTF_TYPE_ENC(3, BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 1), 4),
                BTF_VAR_SECINFO_ENC(2, 0, 4),
        };
-       int res;
+       int btf_fd;
+       int ret;

-       res = libbpf__probe_raw_btf((char *)types, sizeof(types),
-                                   strs, sizeof(strs));
-       if (res < 0)
-               return res;
-       if (res > 0)
+       btf_fd = libbpf__probe_raw_btf((char *)types, sizeof(types),
+                                      strs, sizeof(strs));
+       if (btf_fd < 0)
+               ret = 0;
+       else {
+               ret = 1;
                obj->caps.btf_datasec = 1;
-       return 0;
+       }
+       close(btf_fd);

Same as above.

quoted

+       return ret;
 }

 static int

diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 5e2aa83f637a..2c2828345514 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c

@@ -157,14 +157,9 @@ int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,

I'm wondering if it's better to rename this function to something like
libbpf__load_raw_btf? probe (at least to me) implies true/false
result, so feels like it might be easily misused.

Good idea.

quoted

        memcpy(raw_btf + hdr.hdr_len + hdr.type_len, str_sec, hdr.str_len);

        btf_fd = bpf_load_btf(raw_btf, btf_len, NULL, 0, false);
-       if (btf_fd < 0) {
-               free(raw_btf);
-               return 0;
-       }

-       close(btf_fd);
        free(raw_btf);
-       return 1;
+       return btf_fd;
 }

 static int load_sk_storage_btf(void)
--
2.21.0