[PATCH bpf-next] tools build: propagate build failures from tools/build/Makefile.build

[Andrii Nakryiko <andriin@fb.com>]

The '&&' command seems to have a bad effect when $(cmd_$(1)) exits with
non-zero effect: the command failure is masked (despite `set -e`) and all but
the first command of $(dep-cmd) is executed (successfully, as they are mostly
printfs), thus overall returning 0 in the end.

This means in practice that despite compilation errors, tools's build Makefile
will return success. We see this very reliably with libbpf's Makefile, which
doesn't get compilation error propagated properly. This in turns causes issues
with selftests build, as well as bpftool and other projects that rely on
building libbpf.

The fix is simple: don't use &&. Given `set -e`, we don't need to chain
commands with &&. The shell will exit on first failure, giving desired
behavior and propagating error properly.

Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Fixes: 275e2d95591e ("tools build: Move dependency copy into function")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---

I'm sending this against bpf-next tree, given libbpf is affected enough for me
to debug this fun problem that no one seemed to notice (or care, at least) in
almost 5 years. If there is a better kernel tree, please let me know.

 tools/build/Build.include | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/build/Build.include b/tools/build/Build.include
index 9ec01f4454f9..585486e40995 100644
--- a/tools/build/Build.include
+++ b/tools/build/Build.include
@@ -74,7 +74,8 @@ dep-cmd = $(if $(wildcard $(fixdep)),
 #                   dependencies in the cmd file
 if_changed_dep = $(if $(strip $(any-prereq) $(arg-check)),         \
                   @set -e;                                         \
-                  $(echo-cmd) $(cmd_$(1)) && $(dep-cmd))
+                  $(echo-cmd) $(cmd_$(1));                         \
+                  $(dep-cmd))
 
 # if_changed      - execute command if any prerequisite is newer than
 #                   target, or command line has changed
-- 
2.24.1