[llvm-dev] Proposal for Mach-O support in llvm-objcopy: section renaming

Thu May 23 22:33:12 PDT 2019

Hi Michael,

Thank you for your comments. Now I think the following behavior is
more appropriate:

 --*-section behaves like GNU objcopy:
WRONG: llvm-objcopy --only-secton=__TEXT,__text a.out
WRONG: llvm-objcopy --only-secton=__TEXT.__text a.out
OK: llvm-objcopy --only-secton=.text a.out a.out2

--*-macho-section accepts "two arguments pattern":
WRONG: llvm-objcopy --only-macho-secton=.text a.out a.out2
ERROR: llvm-objcopy --only-secton=__TEXT,__text  --only-macho-secton
__TEXT __text a.out (error: --only-section and --only-macho-section
are exclusive)
OK: llvm-objcopy --only-macho-secton __TEXT __text a.out (use two
arguments to specify the segment/section name)

> How will people use llvm-objcopy when the segment and section names legitimately contain “.” or “,” characters? Will these be escapable?
I think the "two arguments pattern" as you described sounds the best
way to solve this.

> The rest of the behavior, especially around “__TEXT.__text is bad but __TEXT.__unwind_info is good”, is pretty confusing. Can we define our own “canonical names” for canonical sections such as __unwind_info?
While it's confusing, for the compatibility with GNU obcopy, I think
we should inherit canonical names hard-coded in it [1].

> Can you explain what LC_SEGMENT means here? What happens if the segment name (in the file) does not begin with a “_”?
If a segment name doesn't start with a "_" (they call it "a weird
name"), GNU objcopy adds the prefix "LC_SEGMENT." to the BFD section
name. For example, running "objdump -h" to an object file which
contains a section named "foo,bar" prints as follows. I'm unclear why
they do so though.

foo:     file format mach-o-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         002ffec7  0000000100001dd4  0000000100001dd4  00000dd4  2**2
                  CONTENTS, ALLOC, LOAD, CODE
...
 13 LC_SEGMENT.foo.bar 0000000d  0000000000000000  0000000000000000
0043d000  2**0
                  CONTENTS, ALLOC, LOAD

[1]: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/mach-o.c;h=d9edef2871d83b53280b613935c068e4327f3270;hb=HEAD#l90

Thank you,
Seiya

On Fri, May 24, 2019 at 3:19 AM Michael Trent <mtrent at apple.com> wrote:
>
>
> On May 23, 2019, at 2:05 AM, James Henderson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> I discussed this with Seiya off the mailing list yesterday, and this was the suggestion we came up with, on the basis that GNU objcopy has support for the renaming for GDB support, but it might be confusing to people who are new to the system, so we provide a more expected output option. I'm not experienced with MachO at all, though, so we'd appreciate any feedback from any MachO users.
>
>
> Generally, Mach-O tools separate the segment name and the section name as different entries on the command line. The “<Segment Name>,<Section Name>” string is almost always an output format. Some examples include:
>
> otool -s __TEXT __text /bin/ls
> ld -sectcreate __EXAMPLE __example /dev/zero ...
>
> In my opinion, that would be ideal from a "Mach-O users" point of view.
>
> That said, the “two arguments” pattern isn’t very common in llvm, although it does appear in places such as llvm-nm. llvm-objdump has a -section option that takes a single string in the  “<Segment Name>,<Section Name>” format. This option only applies when a Mach-O specific flag, “-macho” or “-m", appears on the command line. And that’s basically the proposal here. So while not ideal, it’s certainly familiar.
>
> How will people use llvm-objcopy when the segment and section names legitimately contain “.” or “,” characters? Will these be escapable?
>
> The rest of the behavior, especially around “__TEXT.__text is bad but __TEXT.__unwind_info is good”, is pretty confusing. Can we define our own “canonical names” for canonical sections such as __unwind_info?
>
>
> Thanks,
>
> James
>
> On Thu, 23 May 2019 at 05:43, Seiya Nuta <nuta at seiya.me> wrote:
>>
>> Hi,
>>
>> I'm going to implement Mach-O support in llvm-objcopy. Before working
>> on this, I'd like to hear your thoughts how llvm-objcopy should handle
>> Mach-O section names.
>>
>> By convention, Mach-O section names are denoted by "<segment
>> name>,<section name>". However, GNU objcopy renames them in the
>> following rule [1]:
>>
>> - If the section name is well-known, rename it to an "canonical" name [2].
>> - Otherwise:
>>     - Rename to "<segment name>.<section name>" (the separator is `.' not `,')
>>     - If the segment name does not start with `_', prefix it with `LC_SEGMENT.’
>
>
> Can you explain what LC_SEGMENT means here? What happens if the segment name (in the file) does not begin with a “_”?
>
> Thanks!
>
> MDT
>
>>
>> For example, __TEXT,__text is renamed to .text and
>> __TEXT,__unwind_info is renamed to __TEXT.__unwind_info. For that
>> reason, specifying a section in command line options is rather
>> nonintuitive:
>>
>> WRONG: objcopy --only-secton=__TEXT,__text a.out
>> WRONG: objcopy --only-secton=__TEXT.__text a.out
>> OK: objcopy --only-secton=.text a.out
>>
>> WRONG: objcopy --only-secton=__TEXT,__unwind_info a.out
>> WRONG: objcopy --only-secton=.unwind_info a.out
>> OK: objcopy --only-secton=__TEXT.__unwind_info a.out
>>
>> For the compatibility with GNU binutils, I propose to make this
>> section renaming rule as default in llvm-objcopy and implement a flag
>> named —macho-names to use conventional section names:
>>
>> WRONG: llvm-objcopy --only-secton=__TEXT,__text a.out
>> WRONG: llvm-objcopy --only-secton=__TEXT.__text a.out
>> OK: llvm-objcopy --only-secton=.text a.out a.out2
>>
>> WRONG: llvm-objcopy --macho-names --only-secton=.text a.out
>> WRONG: llvm-objcopy --macho-names --only-secton=__TEXT.__text a.out
>> OK: llvm-objcopy --macho-names --only-secton=__TEXT,__text a.out
>>
>> What do you think about this behavior?
>>
>> Thanks,
>> Seiya
>>
>> [1]: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/mach-o.c;h=d9edef2871d83b53280b613935c068e4327f3270;hb=HEAD#l364
>> [2]: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/mach-o.c;h=d9edef2871d83b53280b613935c068e4327f3270;hb=HEAD#l90
>
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