[META] Using LLD as FreeBSD's system linker

Quuxplusone commented 9 years ago


Bugzilla Link	PR23214
Status	RESOLVED FIXED
Importance	P normal
Reported by	emaste@freebsd.org
Reported on	2015-04-13 10:48:45 -0700
Last modified on	2020-11-04 12:16:23 -0800
Version	unspecified
Hardware	PC FreeBSD
CC	alfredo.junior@eldorado.org.br, anton@korobeynikov.info, ditaliano@apple.com, erik@cederstrand.dk, glaubitz@physik.fu-berlin.de, grimar@accesssoftek.com, jsweval@arxan.com, kevin.bowling@kev009.com, lffpires@ruabrasil.org, lists@eitanadler.com, llvm-bugs@lists.llvm.org, marklmi26-fbsd@yahoo.com, mpysw@vip.163.com, rafael@espindo.la, rdivacky@freebsd.org, rengolin@gmail.com, ruiu@google.com, rwindz0@gmail.com, shawn.webb@hardenedbsd.org, willdtz@gmail.com
Fixed by commit(s)
Attachments	`loader.zip` (1298516 bytes, application/x-zip-compressed)
Blocks
Blocked by	PR20976, PR22906, PR23024, PR23035, PR23231, PR23232, PR23233, PR25188, PR25528, PR25587, PR25790, PR25798, PR26676, PR26693, PR26699, PR26705, PR26712, PR26729, PR26730, PR26731, PR26732, PR26737, PR26813, PR26814, PR26818, PR26878, PR26968, PR27016, PR27228, PR27406, PR27407, PR27647, PR27893, PR28349, PR28357, PR28359, PR28414, PR28688, PR28689, PR28690, PR28720, PR28976, PR28999, PR29093, PR29115, PR30218, PR30221, PR30222, PR30237, PR30268, PR30269, PR30282, PR30311, PR30312, PR30406, PR30415, PR30891, PR31057, PR31148, PR31149, PR31159, PR31196, PR31213, PR31221, PR31277, PR31295, PR31495, PR31582, PR31619, PR31716, PR31762, PR32504, PR33422, PR33423, PR33482, PR33766, PR33820, PR33821, PR34164, PR34311, PR35570, PR35720, PR35751, PR35788, PR36009, PR36017
See also	https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229050, PR30267, PR25176

This is a meta-bug to track dependencies needed for LLD to be functional as FreeBSD's system linker.

Quuxplusone commented 9 years ago

Do you have a list of missing features for this bug?

Quuxplusone commented 9 years ago

There are the depends-on PRs:

* We need __start_<section> and __stop_<section> as we use linker sets in a few
programs
* -r relocatable link

I just closed PR23035 as the change was reapplied with a fixed test case, but
have found a new issue relating to dependency handling (no PR yet):

link main.o -l<static> -l<shared>, where libstatic.a depends on libshared.a.
LLD reports an undefined symbol from libstatic.a. GNU ld accepts this. Arguably
the link order ought to be changed around in our build, but other software will
likely encounter this too.

For the actual case in FreeBSD the static library was NTP - it builds a
libntp.a that depends on libedit, and a binary linked with -ledit -lntp.

I also observed one "error: unsupported emulation 'elf_i386_fbsd'."

PRs for these two and other issues we find will be added as dependencies of
this one.

Quuxplusone commented 9 years ago

We also need linker script support, although I don't have a list of the specific linker script features we rely on just yet.

Quuxplusone commented 9 years ago

Unknown arguments during world (userland) build:

warning: ignoring unknown argument: --version-script=Version.map
warning: ignoring unknown argument: -x
warning: ignoring unknown argument: --fatal-warnings
warning: ignoring unknown argument: --warn-shared-textrel

From FreeBSD arm64 kernel link:

--- kernel.debug ---
linking kernel.debug
warning: ignoring unknown argument: -Bdynamic
warning: ignoring unknown argument: --no-warn-mismatch
warning: ignoring unknown argument: --warn-common
warning: ignoring unknown argument: --dynamic-linker
warning: ignoring unknown argument: -X
SymbolTable: error while merging __bss_start
LLVM ERROR: duplicate symbol error

Quuxplusone commented 9 years ago

Note that this meta PR was created based on the first version of LLD ELF support, and needs to be revisited for elf2.

Quuxplusone commented 9 years ago

Trying with ELF2 turned up the issues below; we're not yet at the point in
ELF2's development where it makes sense to track individual PRs for them.

* PT_INTERP and ABI notes must be in the first page

ELF2 currently rounds up to a page and stores the data for PT_INTERP
and the NT_FREEBSD_ABI_TAG and NT_FREEBSD_NOINIT_TAG ELF notes there.
These have to be in the first page on FreeBSD.

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flg    Align
  INTERP         0x00000000000010cc 0x00000000000110cc 0x00000000000110cc
                 0x0000000000000015 0x0000000000000015  R      0x1
      [Requesting program interpreter: /libexec/ld-elf.so.1]

* Undefined sym in shared library provided by regular object

This is the FreeBSD __progname issue fixed in r218259 for the original
ELF linker.

* Options used in the FreeBSD base system

These are options that need to be ignored or implemented (excluding
those already ignored by ELF2)

--enable-new-dtags
--fatal-warnings and --no-fatal-warnings
--warn-shared-textrel
--output-filetype
--version-script
-Bsymbolic
-r
-znodelete
--no-warn-mismatch
--warn-common
-T

* Linker script support

The FreeBSD base system linker scripts use (at least) the following set:
ALIGN ENTRY GROUP KEEP OUTPUT_ARCH OUTPUT_FORMAT SEARCH_DIR SECTIONS PROVIDE

(http://lists.llvm.org/pipermail/llvm-dev/2015-October/091072.html)

Quuxplusone commented 9 years ago

--enable/disable-new-dtags is r249428

Quuxplusone commented 9 years ago

What kind of expressions are used in SECTIONS linker script directive?

Quuxplusone commented 9 years ago

> What kind of expressions are used in SECTIONS linker script directive?

Examples can be found here: https://svnweb.freebsd.org/base/head/sys/conf/
For amd64: https://svnweb.freebsd.org/base/head/sys/conf/ldscript.amd64?view=log

Some examples:

  kernphys = CONSTANT (MAXPAGESIZE);
  . = kernbase + kernphys + SIZEOF_HEADERS;
  .interp         : AT (kernphys + SIZEOF_HEADERS) { *(.interp) }
  .hash           : { *(.hash) }
  .rel.text       : { *(.rel.text .rel.text.* .rel.gnu.linkonce.t.*) }

  .init           :
  {
    KEEP (*(.init))
  } =0x90909090

  PROVIDE (__etext = .);

  .eh_frame       : ONLY_IF_RO { KEEP (*(.eh_frame)) }

  . = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1)); . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));

  .preinit_array     :
  {
    PROVIDE_HIDDEN (__preinit_array_start = .);
    KEEP (*(.preinit_array))
    PROVIDE_HIDDEN (__preinit_array_end = .);
  }

   . = ALIGN(. != 0 ? 64 / 8 : 1);

  .debug_pubtypes 0 : { *(.debug_pubtypes) }

Quuxplusone commented 9 years ago

With the change in http://reviews.llvm.org/D13637 LLD can now link a FreeBSD
"hello world."

The current userland world build fails due to unhandled R_X86_64_TLSGD:

--- libc.so.7 ---
unrecognized reloc 19
cc: error: linker command failed with exit code 1 (use -v to see invocation)

The kernel build fails in the linker script parser:

--- kernel.full ---
linking kernel.full
) expected, but got ,

Quuxplusone commented 9 years ago

(In reply to comment #10)
> With the change in http://reviews.llvm.org/D13637 LLD can now link a FreeBSD
> "hello world."
>
> The current userland world build fails due to unhandled R_X86_64_TLSGD:
>
> --- libc.so.7 ---
>
> unrecognized reloc 19
>
> cc: error: linker command failed with exit code 1 (use -v to see invocation)
>

Michael Spencer is working on it -- there's an initial patch under review so
that should be unlocked soon.

>
>
> The kernel build fails in the linker script parser:
>
> --- kernel.full ---
>
> linking kernel.full
>
> ) expected, but got ,

 http://reviews.llvm.org/D13668