Re: [PATCH 0/2] Build ORC fast lookup table in scripts/sorttable tool

changhuaixin
2020-06-03T13:48:20Z

> On Jun 2, 2020, at 1:38 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Sun, May 31, 2020 at 01:26:54PM +0800, changhuaixin wrote:
>> It turned out to be an alignment problem. If sh_size of previous section
>> orc_unwind is not 4-byte aligned, sh_offset of the following orc_lookup
>> section is not 4-byte aligned too. However, the VMA of section orc_lookup
>> is aligned to the nearest 4-byte. Thus, the orc_lookup section means two
>> different ares for scripts/sorttable tool and kernel.
>>
>> Sections headers look like this when it happens:
>>
>> 12 .orc_unwind_ip 00172124 ffffffff82573b28 0000000002573b28 01773b28
>> 2**0
>> CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
>> 13 .orc_unwind 0022b1b6 ffffffff826e5c4c 00000000026e5c4c 018e5c4c
>> 2**0
>> CONTENTS, ALLOC, LOAD, READONLY, DATA
>> 14 .orc_lookup 0003003c ffffffff82910e04 0000000002910e04 01b10e02
>> 2**0
>> ALLOC
>> 15 .vvar 00001000 ffffffff82941000 0000000002941000 01b41000
>> 2**4
>> CONTENTS, ALLOC, LOAD, DATA
>>
>> Sorttable tool uses the are starting with offset 0x01b10e02 for 0x0003003c
>> bytes. While kernel use the area starting with VMA at 0xffffffff82910e04
>> for 0x0003003c bytes, meaning that each entry in this table used by kernel
>> is actually 2 bytes behind the corresponding entry set from sorttable
>> tool.
>>
>> Any suggestion on fixing this？
>
> The VMA and LMA are both 4-byte aligned. The file offset alignment
> (0x01b10e02) shouldn't matter.
>
> Actually it looks like the problem is that the section doesn't have
> CONTENTS, so it's just loaded as a BSS section (all zeros). The section
> needs to be type SHT_PROGBITS instead of SHT_NOBITS.
>
> $ readelf -S vmlinux |grep orc_lookup
> [16] .orc_lookup NOBITS ffffffff82b68418 01d68418
>
> I tried to fix it with
>
> diff --git a/scripts/sorttable.h b/scripts/sorttable.h
> index a36c76c17be4..76adb1fb88f8 100644
> --- a/scripts/sorttable.h
> +++ b/scripts/sorttable.h
> @@ -341,6 +341,7 @@ static int do_sort(Elf_Ehdr *ehdr,
> param.lookup_table_size = s->sh_size;
> param.orc_lookup_table = (unsigned int *)
> ((void *)ehdr + s->sh_offset);
> + w(SHT_PROGBITS, &s->sh_type);
> }
> if (!strcmp(secstrings + idx, ".text")) {
> param.text_size = s->sh_size;
>
>
> But that makes kallsyms unhappy, so I guess we need to do it from the
> linker script where .orc_lookup is created.
>
> Linker script doesn't seem to allow manual specification of the section
> type, so this is the best I could come up with:
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index db600ef218d7..49f4f5bc6165 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -826,6 +826,8 @@
> . += (((SIZEOF(.text) + LOOKUP_BLOCK_SIZE - 1) / \
> LOOKUP_BLOCK_SIZE) + 1) * 4; \
> orc_lookup_end = .; \
> + /* HACK: force SHT_PROGBITS so sorttable can edit: */ \
> + BYTE(1); \
> }
> #else
> #define ORC_UNWIND_TABLE

Thanks! It works.