Hi Tony,
On Mon, 3 Jun 2024, Tony Ambardar wrote:
BPF kfuncs are often not directly referenced and may be inadvertently
removed by optimization steps during kernel builds, thus the __bpf_kfunc
tag mitigates against this removal by including the __used macro. However,
this macro alone does not prevent removal during linking, and may still
yield build warnings (e.g. on mips64el):
LD vmlinux
BTFIDS vmlinux
WARN: resolve_btfids: unresolved symbol bpf_verify_pkcs7_signature
WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
WARN: resolve_btfids: unresolved symbol bpf_key_put
WARN: resolve_btfids: unresolved symbol bpf_iter_task_next
WARN: resolve_btfids: unresolved symbol bpf_iter_css_task_new
WARN: resolve_btfids: unresolved symbol bpf_get_file_xattr
WARN: resolve_btfids: unresolved symbol bpf_ct_insert_entry
WARN: resolve_btfids: unresolved symbol bpf_cgroup_release
WARN: resolve_btfids: unresolved symbol bpf_cgroup_from_id
WARN: resolve_btfids: unresolved symbol bpf_cgroup_acquire
WARN: resolve_btfids: unresolved symbol bpf_arena_free_pages
NM System.map
SORTTAB vmlinux
OBJCOPY vmlinux.32
Update the __bpf_kfunc tag to better guard against linker optimization by
including the new __retain compiler macro, which fixes the warnings above.
Verify the __retain macro with readelf by checking object flags for 'R':
$ readelf -Wa kernel/trace/bpf_trace.o
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
...
[178] .text.bpf_key_put PROGBITS 00000000 6420 0050 00 AXR 0 0 8
...
Key to Flags:
...
R (retain), D (mbind), p (processor specific)
Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/r/202401211357.OCX9yllM-lkp@xxxxxxxxx/
Fixes: 57e7c169cd6a ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
Cc: stable@xxxxxxxxxxxxxxx # v6.6+
Signed-off-by: Tony Ambardar <Tony.Ambardar@xxxxxxxxx>
Thanks for your patch, which is now commit 7bdcedd5c8fb88e7
("bpf: Harden __bpf_kfunc tag against linker kfunc removal") in
v6.10-rc5.
This is causing build failures on ARM with
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y:
net/core/filter.c:11859:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
11859 | {
| ^
net/core/filter.c:11872:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
11872 | {
| ^
net/core/filter.c:11885:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
11885 | {
| ^
net/core/filter.c:11906:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
11906 | {
| ^
net/core/filter.c:12092:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
12092 | {
| ^
net/core/xdp.c:713:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
713 | {
| ^
net/core/xdp.c:736:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
736 | {
| ^
net/core/xdp.c:769:1: error: ʽretainʼ attribute ignored [-Werror=attributes]
769 | {
| ^
[...]
My compiler is arm-linux-gnueabihf-gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04).
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -82,7 +82,7 @@
* as to avoid issues such as the compiler inlining or eliding either a static
* kfunc, or a global kfunc in an LTO build.
*/
-#define __bpf_kfunc __used noinline
+#define __bpf_kfunc __used __retain noinline
#define __bpf_kfunc_start_defs() \
__diag_push(); \
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
