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SubjectRe: [PATCH v4] checkpatch: do not apply "initialise globals to 0" check to BPF progs
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On Tue, 2021-02-09 at 13:19 -0800, Song Liu wrote:
> BPF programs explicitly initialise global variables to 0 to make sure
> clang (v10 or older) do not put the variables in the common section.

Acked-by: Joe Perches <joe@perches.com>

So the patch is OK now, but I have a question about the concept:

Do you mean that these initialized to 0 global variables
should go into bss or another section?

Perhaps it'd be useful to somehow mark variables into specific
sections rather than bss when initialized to 0 and data when not
initialized to 0.

$ clang --version
clang version 10.0.0 (git://github.com/llvm/llvm-project.git 305b961f64b75e73110e309341535f6d5a48ed72)
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ cat t_common.c
int a = 0;
int b = 1;

int foo_a(void)
{
return a;
}

int foo_b(void)
{
return b;
}

$ clang -c -O3 t_common.c

$ objdump -x t_common.o

t_common.o: file format elf64-x86-64
t_common.o
architecture: i386:x86-64, flags 0x00000011:
HAS_RELOC, HAS_SYMS
start address 0x0000000000000000

Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000017 0000000000000000 0000000000000000 00000040 2**4
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .bss 00000004 0000000000000000 0000000000000000 00000058 2**2
ALLOC
2 .data 00000004 0000000000000000 0000000000000000 00000058 2**2
CONTENTS, ALLOC, LOAD, DATA
3 .comment 00000068 0000000000000000 0000000000000000 0000005c 2**0
CONTENTS, READONLY
4 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000c4 2**0
CONTENTS, READONLY
5 .eh_frame 00000040 0000000000000000 0000000000000000 000000c8 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
6 .llvm_addrsig 00000000 0000000000000000 0000000000000000 00000210 2**0
CONTENTS, READONLY, EXCLUDE
SYMBOL TABLE:
0000000000000000 l df *ABS* 0000000000000000 t_common.c
0000000000000000 l d .text 0000000000000000 .text
0000000000000000 g O .bss 0000000000000004 a
0000000000000000 g O .data 0000000000000004 b
0000000000000000 g F .text 0000000000000007 foo_a
0000000000000010 g F .text 0000000000000007 foo_b


RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000002 R_X86_64_PC32 a-0x0000000000000004
0000000000000012 R_X86_64_PC32 b-0x0000000000000004


RELOCATION RECORDS FOR [.eh_frame]:
OFFSET TYPE VALUE
0000000000000020 R_X86_64_PC32 .text
0000000000000034 R_X86_64_PC32 .text+0x0000000000000010


Perhaps instead something like:

$ cat t_common_bpf.c
__attribute__((__section__("bpf"))) int a = 0;
__attribute__((__section__("bpf"))) int b = 1;

int foo_a(void)
{
return a;
}

int foo_b(void)
{
return b;
}

$ clang -c -O3 t_common_bpf.c

$ objdump -x t_common_bpf.o

t_common_bpf.o: file format elf64-x86-64
t_common_bpf.o
architecture: i386:x86-64, flags 0x00000011:
HAS_RELOC, HAS_SYMS
start address 0x0000000000000000

Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000017 0000000000000000 0000000000000000 00000040 2**4
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 bpf 00000008 0000000000000000 0000000000000000 00000058 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .comment 00000068 0000000000000000 0000000000000000 00000060 2**0
CONTENTS, READONLY
3 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000c8 2**0
CONTENTS, READONLY
4 .eh_frame 00000040 0000000000000000 0000000000000000 000000c8 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
5 .llvm_addrsig 00000000 0000000000000000 0000000000000000 00000210 2**0
CONTENTS, READONLY, EXCLUDE
SYMBOL TABLE:
0000000000000000 l df *ABS* 0000000000000000 t_common_bpf.c
0000000000000000 l d .text 0000000000000000 .text
0000000000000000 g O bpf 0000000000000004 a
0000000000000004 g O bpf 0000000000000004 b
0000000000000000 g F .text 0000000000000007 foo_a
0000000000000010 g F .text 0000000000000007 foo_b


RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000002 R_X86_64_PC32 a-0x0000000000000004
0000000000000012 R_X86_64_PC32 b-0x0000000000000004


RELOCATION RECORDS FOR [.eh_frame]:
OFFSET TYPE VALUE
0000000000000020 R_X86_64_PC32 .text
0000000000000034 R_X86_64_PC32 .text+0x0000000000000010




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