svn commit: r250306 - in stable/8/sys: net sys

[John Baldwin]

On Tuesday, May 07, 2013 2:59:57 pm Dimitry Andric wrote:
> On 2013-05-07 01:13, Julian Elischer wrote:
> > On 5/6/13 12:59 PM, Dimitry Andric wrote:
> >> Author: dim
> >> Date: Mon May  6 19:59:13 2013
> >> New Revision: 250306
> >> URL: http://svnweb.freebsd.org/changeset/base/250306
> >>
> >> Log:
> >>     MFC r215137:
> >>
> >>       Revert r103230, which depended on ld preserving the __start_xxx and
> >>       __stop_xxx symbols for custom sections, even when these were not
> >>       referenced (at link time).  This behaviour was changed again in binutils
> >>       commit 0b8ed435c3fe8bd09a08c23920e65bfb03251221.
> >>
> >>       This time, put the __GLOBL macro definition in cdefs.h, so it can be
> >>       reused in a few other places where it will be needed.
> >
> > Is this stuff documented anywhere?
> 
> If you mean the specifics of <sys/linker_set.h>, I don't think so.  The
> documentation for the things that use it, are mostly in SYSINIT(9),
> DECLARE_MODULE(9) and module(9).
> 
> In any case, the idea is that you tell the compiler and linker to add a
> few custom sections to the output, which contain metadata, for example
> to initialize a kernel module.
> 
> Take the section headers of mpt.ko, for instance:
> 
> Section Headers:
>    [Nr] Name                Type            Addr     Off    Size   ES Flg Lk Inf Al
>    [ 0]                     NULL            00000000 000000 000000 00      0   0  0
>    [ 1] .hash               HASH            000000b4 0000b4 00038c 04   A  2   0  4
>    [ 2] .dynsym             DYNSYM          00000440 000440 000800 10   A  3   1  4
>    [ 3] .dynstr             STRTAB          00000c40 000c40 000792 00   A  0   0  1
>    [ 4] .rel.dyn            REL             000013d4 0013d4 004b80 08   A  2   0  4
>    [ 5] .text               PROGBITS        00005f60 005f60 01258a 00  AX  0   0 16
>    [ 6] .rodata             PROGBITS        000184f0 0184f0 00561d 00   A  0   0 16
>    [ 7] set_modmetadata_set PROGBITS        0001db10 01db10 000054 00   A  0   0  4
>    [ 8] set_sysinit_set     PROGBITS        0001db64 01db64 000020 00   A  0   0  4
>    [ 9] set_sysuninit_set   PROGBITS        0001db84 01db84 000004 00   A  0   0  4
>    [10] .dynamic            DYNAMIC         0001e000 01e000 000078 08  WA  3   0  4
>    [11] .got.plt            PROGBITS        0001e078 01e078 00000c 04  WA  0   0  4
>    [12] .data               PROGBITS        0001e084 01e084 000478 00  WA  0   0  4
>    [13] .bss                NOBITS          0001e4fc 01e4fc 000310 00  WA  0   0  4
>    [14] .comment            PROGBITS        00000000 01e4fc 0001b3 01  MS  0   0  1
>    [15] .SUNW_ctf           PROGBITS        00000000 01e6b0 0065f7 00      0   0  4
>    [16] .gnu_debuglink      PROGBITS        00000000 024ca7 000014 00      0   0  1
>    [17] .shstrtab           STRTAB          00000000 024cbb 0000bd 00      0   0  1
>    [18] .symtab             SYMTAB          00000000 025098 0018d0 10     19 270  4
>    [19] .strtab             STRTAB          00000000 026968 001e9f 00      0   0  1
> Key to Flags:
>    W (write), A (alloc), X (execute), M (merge), S (strings)
>    I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
>    O (extra OS processing required) o (OS specific), p (processor specific)
> 
> The 'set_*' sections are added by the linker_set macros, and are marked
> as used, so the compiler will not optimize them away.
> 
> However, in some versions of binutils, the linker still optimizes them
> away, so we have to resort to inserting assembler directives to force it
> to keep them.  This is what r103230 and r215137 were about.

You can use linker sets in userland as well.  I used them in mfiutil and mptutil
to build nested command tables.  I've also used linker sets to build lists of
unit tests in kernel modules.

-- 
John Baldwin