[PATCH bpf v2 2/2] bpf: Harden __bpf_kfunc tag against linker kfunc removal

[Tony Ambardar <tony.ambardar@xxxxxxxxx>]

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>
---
 include/linux/btf.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index f9e56fd12a9f..7c3e40c3295e 100644
--- 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();							       \
-- 
2.34.1