Skip to content

L
linux
Switch branch/tag
  1. 30 Jan, 2024 2 commits
    • Jose E. Marchesi's avatar
      bpf: Generate const static pointers for kernel helpers · ff2071a7
      Jose E. Marchesi authored 
      The generated bpf_helper_defs.h file currently contains definitions
like this for the kernel helpers, which are static objects:

  static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;

These work well in both clang and GCC because both compilers do
constant propagation with -O1 and higher optimization, resulting in
`call 1' BPF instructions being generated, which are calls to kernel
helpers.

However, there is a discrepancy on how the -Wunused-variable
warning (activated by -Wall) is handled in these compilers:

- clang will not emit -Wunused-variable warnings for static variables
  defined in C header files, be them constant or not constant.

- GCC will not emit -Wunused-variable warnings for _constant_ static
  variables defined in header files, but it will emit warnings for
  non-constant static variables defined in header files.

There is no reason for these bpf_helpers_def.h pointers to not be
declared constant, and it is actually desirable to do so, since their
values are not to be changed.  So this patch modifies bpf_doc.py to
generate prototypes like:

  static void *(* const bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;

This allows GCC to not error while compiling BPF programs with `-Wall
-Werror', while still being able to detect and error on legitimate
unused variables in the program themselves.

This change doesn't impact the desired constant propagation in neither
Clang nor GCC with -O1 and higher.  On the contrary, being declared as
constant may increase the odds they get constant folded when
used/referred to in certain circumstances.

Tested in bpf-next master.
No regressions.
Signed-off-by: default avatarJose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarYonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240127185031.29854-1-jose.marchesi@oracle.com
      ff2071a7
    • Ian Rogers's avatar
      libbpf: Add some details for BTF parsing failures · f2e4040c
      Ian Rogers authored 
      As CONFIG_DEBUG_INFO_BTF is default off the existing "failed to find
valid kernel BTF" message makes diagnosing the kernel build issue somewhat
cryptic. Add a little more detail with the hope of helping users.

Before:
```
libbpf: failed to find valid kernel BTF
libbpf: Error loading vmlinux BTF: -3
```

After not accessible:
```
libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
libbpf: failed to find valid kernel BTF
libbpf: Error loading vmlinux BTF: -3
```

After not readable:
```
libbpf: failed to read kernel BTF from (/sys/kernel/btf/vmlinux): -1
```

Closes: https://lore.kernel.org/bpf/CAP-5=fU+DN_+Y=Y4gtELUsJxKNDDCOvJzPHvjUVaUoeFAzNnig@mail.gmail.com/Signed-off-by: default avatarIan Rogers <irogers@google.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240125231840.1647951-1-irogers@google.com
      f2e4040c
  3. 29 Jan, 2024 34 commits
  5. 27 Jan, 2024 4 commits
GitLab Nexedi Edition | About GitLab | About Nexedi | 沪ICP备2021021310号-2 | 沪ICP备2021021310号-7