On Tue, Sep 28, 2021 at 10:06:09AM IST, Andrii Nakryiko wrote:

On Mon, Sep 27, 2021 at 9:16 PM Kumar Kartikeya Dwivedi
[off-list ref] wrote:

quoted

Generate vmlinux.h only from the in-tree vmlinux, and remove enum
declarations that would cause a build failure in case of version
mismatches.

There are now two options when building the samples:
1. Compile the kernel to use in-tree vmlinux for vmlinux.h
2. Override VMLINUX_BTF for samples using something like this:
   make VMLINUX_BTF=/sys/kernel/btf/vmlinux -C samples/bpf

This change was tested with relative builds, e.g. cases like:
 * make O=build -C samples/bpf
 * make KBUILD_OUTPUT=build -C samples/bpf
 * make -C samples/bpf
 * cd samples/bpf && make

When a suitable VMLINUX_BTF is not found, the following message is
printed:
/home/kkd/src/linux/samples/bpf/Makefile:333: *** Cannot find a vmlinux
for VMLINUX_BTF at any of "  ./vmlinux", build the kernel or set
VMLINUX_BTF variable.  Stop.

Cc: Toke Høiland-Jørgensen <redacted>
Fixes: 384b6b3bbf0d (samples: bpf: Add vmlinux.h generation support)
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 samples/bpf/Makefile                     | 13 ++++++-------
 samples/bpf/xdp_redirect_map_multi.bpf.c |  5 -----
 2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 4dc20be5fb96..a05130e91403 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile

@@ -324,15 +324,9 @@ $(obj)/hbm_edt_kern.o: $(src)/hbm.h $(src)/hbm_kern.h

 VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)                           \
                     $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)    \
-                    ../../../../vmlinux                                \
-                    /sys/kernel/btf/vmlinux                            \
-                    /boot/vmlinux-$(shell uname -r)
+                    ./vmlinux

isn't this relative to samples/bpf subdirectory, so shouldn't it be
../../vmlinux?

I was confused about this too; but it executes this when you call make:

# Trick to allow make to be run from this directory
all:
        $(MAKE) -C ../../ M=$(CURDIR) BPF_SAMPLES_PATH=$(CURDIR)

so it fails if the path isn't relative to kernel source root.

quoted

 VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))

-ifeq ($(VMLINUX_BTF),)
-$(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)")
-endif
-

[...]

--
Kartikeya