[v2,bpf-next] libbpf: fix clang compilation error in btf_relocate.c

Commit Message

Alan Maguire June 24, 2024, 7:29 p.m. UTC

When building with clang for ARCH=i386, the following errors are
observed:

  CC      kernel/bpf/btf_relocate.o
./tools/lib/bpf/btf_relocate.c:206:23: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
  206 |                 info[id].needs_size = true;
      |                                     ^ ~
./tools/lib/bpf/btf_relocate.c:256:25: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
  256 |                         base_info.needs_size = true;
      |                                              ^ ~
2 errors generated.

The problem is we use 1-bit, 31-bit bitfields in a signed int.
Changing to

	bool needs_size: 1;
	unsigned int size:31;

...resolves the error and pahole reports that 4 bytes are used
for the underlying representation:

$ pahole btf_name_info tools/lib/bpf/btf_relocate.o
struct btf_name_info {
	const char  *              name;                 /*     0     8 */
	unsigned int               needs_size:1;         /*     8: 0  4 */
	unsigned int               size:31;              /*     8: 1  4 */
	__u32                      id;                   /*    12     4 */

	/* size: 16, cachelines: 1, members: 4 */
	/* last cacheline: 16 bytes */
};

Signed-off-by: Alan Maguire <[e-mail]>
---
 tools/lib/bpf/btf_relocate.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

[e-mail] June 26, 2024, 11:40 p.m. UTC | #1

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <[e-mail]>:

On Mon, 24 Jun 2024 20:29:03 +0100 you wrote:
> When building with clang for ARCH=i386, the following errors are
> observed:
> 
>   CC      kernel/bpf/btf_relocate.o
> ./tools/lib/bpf/btf_relocate.c:206:23: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
>   206 |                 info[id].needs_size = true;
>       |                                     ^ ~
> ./tools/lib/bpf/btf_relocate.c:256:25: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
>   256 |                         base_info.needs_size = true;
>       |                                              ^ ~
> 2 errors generated.
> 
> [...]

Here is the summary with links:
  - [v2,bpf-next] libbpf: fix clang compilation error in btf_relocate.c
    https://git.kernel.org/bpf/bpf-next/c/0f31c2c61f69

You are awesome, thank you!

Patch

diff --git a/tools/lib/bpf/btf_relocate.c b/tools/lib/bpf/btf_relocate.c
index 2281dbbafa11..66c6204c4064 100644
--- a/tools/lib/bpf/btf_relocate.c
+++ b/tools/lib/bpf/btf_relocate.c
@@ -58,8 +58,8 @@  struct btf_relocate {
 struct btf_name_info {
 	const char *name;
 	/* set when search requires a size match */
-	int needs_size:1,
-	    size:31;
+	bool needs_size: 1;
+	unsigned int size: 31;
 	__u32 id;
 };