linux-kernel - Re: [PATCH v4] checkpatch: do not apply "initialise globals to 0" check to BPF progs

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Message-ID: <2b4805f6ca2b44f4195b6fdba4f82d5e90ab1989.camel@perches.com>
Date:   Tue, 09 Feb 2021 18:10:22 -0800
From:   Joe Perches <joe@...ches.com>
To:     Song Liu <songliubraving@...com>, linux-kernel@...r.kernel.org
Cc:     bpf@...r.kernel.org, Andy Whitcroft <apw@...onical.com>
Subject: Re: [PATCH v4] checkpatch: do not apply "initialise globals to 0"
 check to BPF progs

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@...ches.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