** Description changed:
[ Impact ]
- - On Noble systems testing 6.17 backport kernels (OEM/HWE/clouds, etc.)
built
- with dwarves/pahole 1.25, the BPF selftests (TARGETS=bpf, e.g. ubuntu_bpf)
- fail to build. This was hinted for a while.
+ - On Noble systems testing 6.17 backport kernels (OEM/HWE/clouds, etc.)
built
+ with dwarves/pahole 1.25, the BPF selftests (TARGETS=bpf, e.g. ubuntu_bpf)
+ fail to build. They were hinted for a while.
- - bpf_iter_tasks.bpf.o hits an implicit declaration of
- `bpf_copy_from_user_task_str`. The generated vmlinux.h is missing kfunc
- prototypes.
+ - bpf_iter_tasks.bpf.o hits an implicit declaration of
+ `bpf_copy_from_user_task_str`. The generated vmlinux.h is missing kfunc
+ prototypes.
- - bpftool generates vmlinux.h from the kernel’s BTF. With dwarves 1.25 the
BTF
- lacks `DECL_TAG bpf_kfunc`, so the kfunc section is empty and selftests
see
- no prototypes.
+ - bpftool generates vmlinux.h from the kernel’s BTF. With dwarves 1.25 the
BTF
+ lacks `DECL_TAG bpf_kfunc`, so the kfunc section is empty and selftests
see
+ no prototypes.
- - Newer `dwarves` (1.26+) would add these tags, but Noble stays on 1.25.
This
- was discussed (at least for older series) years ago: Bug #1926330.
+ - Newer `dwarves` (1.26+) would add these tags, but Noble stays on 1.25.
This
+ was discussed (at least for older series) years ago: Bug #1926330.
- - Backporting a newer `dwarves` is avoided, so an explicit tagging
workaround
- in the kernel source is needed for 6.17 backport kernels.
+ - Adding `__attribute__((btf_decl_tag("bpf_kfunc")))` to `__bpf_kfunc`
+ macro would probably solve this problem but we're using
+ GCC to build kernels and using clang to build selftests...
- [ Test Plan ]
+ - Backporting a newer `dwarves` is avoided, so an explicit tagging
workaround
+ in the kernel source is needed for 6.17 backport kernels.
- 1. On an unpatched 6.17 kernel built with dwarves 1.25, run:
+ - Use of clang in pipeline should be considered for future.
- $ make -C linux/tools/testing/selftests TARGETS=bpf SKIP_TARGETS= clean
all \
- KDIR=/usr/src/linux-headers-$(uname -r) \
- VMLINUX_BTF=/sys/kernel/btf/vmlinux
+ [ Test Plan ]
- Expect an implicit-decl error at progs/bpf_iter_tasks.c:98.
- 2. Apply the patch rebuild the 6.17 kernel package, install, and reboot.
- 3. Verify on the running kernel:
+ 1. On an unpatched 6.17 kernel built with dwarves 1.25, run:
- $ bpftool btf dump file /sys/kernel/btf/vmlinux format c | grep \
- bpf_copy_from_user_task_str
+ $ make -C linux/tools/testing/selftests TARGETS=bpf \
+ SKIP_TARGETS= clean all \
+ KDIR=/usr/src/linux-headers-$(uname -r) \
+ VMLINUX_BTF=/sys/kernel/btf/vmlinux
- should show an extern prototype.
- 4. Re-run the selftest build as in step 1; it should succeed.
- 5. Optionally run the autotest suite for ubuntu_bpf to confirm tests run.
- 6. Verified in a Noble VM: clang 18 + dwarves 1.25
+ Expect an implicit-decl error at progs/bpf_iter_tasks.c:98.
+ 2. Apply the patch rebuild the 6.17 kernel package with clang, install, and
reboot.
+ 3. Verify on the running kernel:
- [ Where problems could occur ]
+ $ bpftool btf dump file /sys/kernel/btf/vmlinux format c | grep \
+ bpf_copy_from_user_task_str
- - The change adds decl tags to all kfuncs in BTF. If wrong, vmlinux.h could
- gain unexpected kfunc prototypes, and other selftests might start failing
at
- build time if their kfuncs are missing. If BTF generation broke, bpftool
or
- selftest builds would fail.
- - Runtime behavior and ABI are unchanged.
+ should show an extern prototype.
+ 4. Re-run the selftest build as in step 1; it should succeed.
+ 5. Optionally run the autotest suite for ubuntu_bpf to confirm tests run.
+ 6. Verified in a Noble VM: clang 18 + dwarves 1.25
- [ Other Info ]
+ [ Other Info ]
- - Upstream addresses this by passing `decl_tag_kfuncs` to a newer `dwarves`
- (commit ebb79e96f1ea, "kbuild: bpf: Tell pahole to DECL_TAG kfuncs", June
- 12, 2024) and by having bpftool emit kfunc prototypes (commit
770abbb5a25a,
- "bpftool: Support dumping kfunc prototypes from BTF").
+ - Upstream addresses this by passing `decl_tag_kfuncs` to a newer `dwarves`
+ (commit ebb79e96f1ea, "kbuild: bpf: Tell pahole to DECL_TAG kfuncs", June
+ 12, 2024) and by having bpftool emit kfunc prototypes (commit
770abbb5a25a,
+ "bpftool: Support dumping kfunc prototypes from BTF").
- - Questing (24.10) ships dwarves 1.27, so this resolves there
+ - Questing (24.10) ships dwarves 1.27, so this resolves there
automatically.
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https://bugs.launchpad.net/bugs/2136848
Title:
selftests/ubuntu_bpf: kfunc prototypes missing in vmlinux.h on Noble
6.17 kernels built with dwarves 1.25
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