From: Quentin Monnet quentin@isovalent.com
[ Upstream commit cea558855c39b7f1f02ff50dcf701ca6596bc964 ]
When bpftool is linked against libcap, the library runs a "constructor" function to compute the number of capabilities of the running kernel [0], at the beginning of the execution of the program. As part of this, it performs multiple calls to prctl(). Some of these may fail, and set errno to a non-zero value:
# strace -e prctl ./bpftool version prctl(PR_CAPBSET_READ, CAP_MAC_OVERRIDE) = 1 prctl(PR_CAPBSET_READ, 0x30 /* CAP_??? */) = -1 EINVAL (Invalid argument) prctl(PR_CAPBSET_READ, CAP_CHECKPOINT_RESTORE) = 1 prctl(PR_CAPBSET_READ, 0x2c /* CAP_??? */) = -1 EINVAL (Invalid argument) prctl(PR_CAPBSET_READ, 0x2a /* CAP_??? */) = -1 EINVAL (Invalid argument) prctl(PR_CAPBSET_READ, 0x29 /* CAP_??? */) = -1 EINVAL (Invalid argument) ** fprintf added at the top of main(): we have errno == 1 ./bpftool v7.0.0 using libbpf v1.0 features: libbfd, libbpf_strict, skeletons +++ exited with 0 +++
This has been addressed in libcap 2.63 [1], but until this version is available everywhere, we can fix it on bpftool side.
Let's clean errno at the beginning of the main() function, to make sure that these checks do not interfere with the batch mode, where we error out if errno is set after a bpftool command.
[0] https://git.kernel.org/pub/scm/libs/libcap/libcap.git/tree/libcap/cap_alloc.... [1] https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=f25a1b7e69f...
Signed-off-by: Quentin Monnet quentin@isovalent.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Link: https://lore.kernel.org/bpf/20220815162205.45043-1-quentin@isovalent.com Signed-off-by: Sasha Levin sashal@kernel.org --- tools/bpf/bpftool/main.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c index 9062ef2b8767..0881437587ba 100644 --- a/tools/bpf/bpftool/main.c +++ b/tools/bpf/bpftool/main.c @@ -435,6 +435,16 @@ int main(int argc, char **argv)
setlinebuf(stdout);
+#ifdef USE_LIBCAP + /* Libcap < 2.63 hooks before main() to compute the number of + * capabilities of the running kernel, and doing so it calls prctl() + * which may fail and set errno to non-zero. + * Let's reset errno to make sure this does not interfere with the + * batch mode. + */ + errno = 0; +#endif + last_do_help = do_help; pretty_output = false; json_output = false;