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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 <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240624192903.854261-1-alan.maguire@oracle.com
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