[PATCH bpf-next] bpftool: Clear errno after libcap's checks

[Quentin Monnet <quentin@xxxxxxxxxxxxx>]

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 +++

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.c?h=v1.2.65#n20

Signed-off-by: Quentin Monnet <quentin@xxxxxxxxxxxxx>
---
 tools/bpf/bpftool/main.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
index 451cefc2d0da..c0e2e4fedbe8 100644
--- a/tools/bpf/bpftool/main.c
+++ b/tools/bpf/bpftool/main.c
@@ -435,6 +435,9 @@ int main(int argc, char **argv)
 
 	setlinebuf(stdout);
 
+	/* Libcap */
+	errno = 0;
+
 	last_do_help = do_help;
 	pretty_output = false;
 	json_output = false;
-- 
2.25.1