[PATCH bpf-next v2] bpftool: Clear errno after libcap's checks

[Quentin Monnet <quentin@isovalent.com>]

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.c?h=libcap-2.65#n20
[1] https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=f25a1b7e69f7b33e6afb58b3e38f3450b7d2d9a0

v2:
- Make the comment more meaningful
- Comment on v2.63 fixing the issue on libcap side
- Only reset errno if libcap is in use

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
---
 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 451cefc2d0da..ccd7457f92bf 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;
-- 
2.25.1