Re: [PATCH bpf-next v3 02/11] tools/bpf/runqslower: Fix override option for VMLINUX_BTF

Andrii Nakryiko [off-list ref] writes:

On Thu, Jan 16, 2020 at 5:23 AM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

From: Toke Høiland-Jørgensen <redacted>

The runqslower tool refuses to build without a file to read vmlinux BTF
from. The build fails with an error message to override the location by
setting the VMLINUX_BTF variable if autodetection fails. However, the
Makefile doesn't actually work with that override - the error message is
still emitted.

Fix this by including the value of VMLINUX_BTF in the expansion, and only
emitting the error message if the *result* is empty. Also permit running
'make clean' even though no VMLINUX_BTF is set.

Fixes: 9c01546d26d2 ("tools/bpf: Add runqslower tool to tools/bpf")
Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 tools/bpf/runqslower/Makefile |   18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefile
index cff2fbcd29a8..89fb7cd30f1a 100644
--- a/tools/bpf/runqslower/Makefile
+++ b/tools/bpf/runqslower/Makefile

@@ -10,12 +10,14 @@ CFLAGS := -g -Wall

 # Try to detect best kernel BTF source
 KERNEL_REL := $(shell uname -r)
-ifneq ("$(wildcard /sys/kernel/btf/vmlinux)","")
-VMLINUX_BTF := /sys/kernel/btf/vmlinux
-else ifneq ("$(wildcard /boot/vmlinux-$(KERNEL_REL))","")
-VMLINUX_BTF := /boot/vmlinux-$(KERNEL_REL)
-else
-$(error "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly")
+VMLINUX_BTF_PATHS := $(VMLINUX_BTF) /sys/kernel/btf/vmlinux /boot/vmlinux-$(KERNEL_REL)
+VMLINUX_BTF_PATH := $(firstword $(wildcard $(VMLINUX_BTF_PATHS)))

If user specifies VMLINUX_BTF pointing to non-existing file, but the
system has /sys/kernel/btf/vmlinux, the latter will still be used,
which is a very surprising behavior.

Hmm, yeah, good point.

Also MAKECMDGOALS feels like a fragile hack to me. How about we move
this VMLINUX_BTF guessing (without $(error)) into vmlinux.h rule
itself and use shell if conditional after it to check for file
existance and print nice error. That way we'll be checking VMLINUX_BTF
only when it's really needed.

OK, sure, can do.

-Toke