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bpf: Harden __bpf_kfunc tag against linker kfunc removal

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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)

Fixes: 57e7c169cd6a ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Closes: https://lore.kernel.org/r/202401211357.OCX9yllM-lkp@intel.com/
Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
Link: https://lore.kernel.org/bpf/e9c64e9b5c073dabd457ff45128aabcab7630098.1717477560.git.Tony.Ambardar@gmail.com
This commit is contained in:
Tony Ambardar 2024-06-03 22:23:16 -07:00 committed by Daniel Borkmann
parent 0a5d3258d7
commit 7bdcedd5c8
1 changed files with 1 additions and 1 deletions
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2
include/linux/btf.h
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@ -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(); \