On Sun, Aug 02, 2020 at 11:22:07AM -0700, Andrii Nakryiko wrote:

On Sun, Aug 2, 2020 at 9:11 AM Jiri Olsa [off-list ref] wrote:

quoted

On Thu, Jul 30, 2020 at 07:42:44PM -0700, Andrii Nakryiko wrote:

quoted

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.

nice, thanks for digging into this,
any idea why is the failure masked?

Two things.

1. In make, assume you have command f = a in one function and g = b; c
in another. If you write f && g, you end up with (a && b); c, right?

2. Try this shell script:

set -ex
false && true
true

It will return success. It won't execute the first true command, as
expected, but won't terminate the shell as you'd expect from set -e.

So basically, having a "logical operator" in a sequence of commands
negates the effect of `set -e`. Intuitively I'd expect that from ||,
but seems like && does that as well. if [] has similar effect -- any
failing command in an if check doesn't trigger an early termination of
a script.

nice, thanks for explanation

jirka

quoted

Acked-by: Jiri Olsa <redacted>

jirka

quoted

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 <redacted>
Fixes: 275e2d95591e ("tools build: Move dependency copy into function")
Signed-off-by: Andrii Nakryiko <redacted>
---

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