Bug 237676

Summary:	LLD Filesize and default option suggestions
Product:	Base System	Reporter:	pprocacci
Component:	bin	Assignee:	freebsd-toolchain (Nobody) <toolchain>
Status:	Open ---
Severity:	Affects Some People	CC:	dim, emacsray, emaste
Priority:	---	Keywords:	needs-qa
Version:	12.0-RELEASE
Hardware:	Any
OS:	Any

Description pprocacci 2019-05-01 03:44:51 UTC

The .comment section added by lld should be removed and made optional.

The file size using lld is unacceptable (some still use assembly) for embedded stuff.  The default is 1200% larger by default and should be minimal like gnu ld.

# /usr/local/bin/ld -s -o porcupine porcupine.o
# ld --discard-all -o porcupine.lld -m elf_amd64_fbsd porcupine.o
# strip porcupine.lld
# objcopy --remove-section .comment porcupine.lld
# ls -l porcupine porcupine.lld
-rwxr-xr-x  1 root  wheel   352 Apr 30 23:41 porcupine
-rwxr-xr-x  1 root  wheel  4320 Apr 30 23:41 porcupine.lld


Specifying `-m elf_amd64_fbsd` or similar shouldn't be necessary and should be the default (depending on platform).

Comment 1 Ed Maste freebsd_committer

2019-05-02 13:25:35 UTC

As you note you can remove .comment with objcopy, I suspect the lld authors will feel the complexity of a --comment / --no-comment or similar flag is warranted.

lld is inherently a cross-platform linker and intentionally has no built-in default target. Normally the format is identified from the first object file - if your ELF .o has an EI_OSABI of FreeBSD the output will as well.

Would you compare readelf -lS porcupine and porcupine.lld?

Comment 2 Ed Maste freebsd_committer

2019-05-02 13:26:10 UTC

(In reply to Ed Maste from comment #1)
The complexity is not warranted, rather.

Comment 3 pprocacci 2019-05-03 02:59:09 UTC

# readelf -lS porcupine

Elf file type is EXEC (Executable file)
Entry point 0x400080
There are 1 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flg    Align
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x000000000000009c 0x000000000000009c  R E    0x200000

 Section to Segment mapping:
  Segment Sections...
   00     .text
There are 3 section headers, starting at offset 0xb0:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000400080  00000080
       000000000000001c  0000000000000000  AX       0     0     16
  [ 2] .shstrtab         STRTAB           0000000000000000  0000009c
       0000000000000011  0000000000000000           0     0     1






# readelf -lS porcupine.lld

Elf file type is EXEC (Executable file)
Entry point 0x201000
There are 4 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flg    Align
  PHDR           0x0000000000000040 0x0000000000200040 0x0000000000200040
                 0x00000000000000e0 0x00000000000000e0  R      0x8
  LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                 0x0000000000000120 0x0000000000000120  R      0x1000
  LOAD           0x0000000000001000 0x0000000000201000 0x0000000000201000
                 0x000000000000001c 0x000000000000001c  R E    0x1000
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0

 Section to Segment mapping:
  Segment Sections...
   00
   01
   02     .text
   03
There are 3 section headers, starting at offset 0x1030:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000201000  00001000
       000000000000001c  0000000000000000  AX       0     0     16
  [ 2] .shstrtab         STRTAB           0000000000000000  0000101c
       0000000000000011  0000000000000000           0     0     1



Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings)
  I (info), L (link order), G (group), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

Comment 4 maskray 2019-05-18 15:42:10 UTC

Use --strip-all if you don't want .symtab
Use --no-rosegment to merge R PT_LOAD and RE PT_LOAD
To discard PT_GNU_STACK and .comment, you have to use a linker script, e.g.

PHDRS { text PT_LOAD; }
SECTIONS { /DISCARD/ : {*(.comment)} .text : {*(.text)} :text }

(objcopy options can be combined, e.g. objcopy -S --remove-section=.comment porcupine.lld