llvm dev - Oct 2019 - contributing llvm-install-name-tool

Hey everyone!
Recently there has been some progress on LLVM-based tools for manipulating
MachO binaries: llvm-objcopy has been gaining a lot of important bits to
support MachO (it's relatively close to the point where one can implement
the strip-like functionality), llvm-lipo is functional and supports most of
cctools' lipo options (https://llvm.org/docs/CommandGuide/llvm-lipo.html).
There is another useful utility called install_name_tool (see e.g.
https://www.unix.com/man-page/osx/1/install_name_tool/ ),
this tool is capable of changing the rpaths, the names of the dependent
shared libraries, etc. The way it works -  install-name-tool (from cctools)
generates the new list of MachO load commands and the new MachO header, the
other parts of the binary are copied over (in particular, if I'm not
mistaken, it assumes that the new "prefix" (MachO header + load
commands)
fits into the binary so the offsets don't need to be recalculated,
otherwise suggests to relink the binary with -headerpad /
-headerpad_max_install_names / reports an error).  One possible (and
simple) approach is to implement llvm-install-name-tool in a similar
fashion: use libObject to parse the input binary and rebuild the list of
load commands (analogously to what cctools' install_name_tool does (and
with the same limitations)). I'd like to ask for your general feedback /
thoughts / suggestions on contributing llvm-install-name-tool to LLVM with
the goal to be a drop-in replacement for cctools' install_name_tool.
Thanks,
Alex
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Hi there,

This tool has been around for a long time. Originally it gave people a way to
move a dylib to a new location without rebuilding, and then as a way to make an
existing dylib embeddable (@rpath). I wonder if this tool is really necessary.
It’s always better to make the dylib with the correct install name at build
time, including @rpath, etc.. Most dylibs we see any more are either installed
in fixed file system locations by OS vendors (i.e., Apple) or are embedded
@rpath dylibs installed by everyone else.

Is there a use-case for install_name_tool that I have overlooked?

Or Is that small set of dylibs meant to be installed in fixed file system
locations large / complex enough that people really need to install them in many
different locations without simply relinking the files?

Turning to practical matters, is your intent to have the load-command rewriting
code live in the tool? Or are you proposing adding support for
building/swapping/rewriting Object files to libObject?

MDT

> On Oct 11, 2019, at 10:31 AM, Alexander Shaposhnikov
<alexander.v.shaposhnikov at gmail.com> wrote:
> 
> Hey everyone!  
> Recently there has been some progress on LLVM-based tools for manipulating
MachO binaries: llvm-objcopy has been gaining a lot of important bits to support
MachO (it's relatively close to the point where one can implement the
strip-like functionality), llvm-lipo is functional and supports most of
cctools' lipo options (https://llvm.org/docs/CommandGuide/llvm-lipo.html
<https://llvm.org/docs/CommandGuide/llvm-lipo.html>).
> There is another useful utility called install_name_tool (see e.g.
https://www.unix.com/man-page/osx/1/install_name_tool/
<https://www.unix.com/man-page/osx/1/install_name_tool/> ),
> this tool is capable of changing the rpaths, the names of the dependent
shared libraries, etc. The way it works -  install-name-tool (from cctools)
generates the new list of MachO load commands and the new MachO header, the
other parts of the binary are copied over (in particular, if I'm not
mistaken, it assumes that the new "prefix" (MachO header + load
commands) fits into the binary so the offsets don't need to be recalculated,
otherwise suggests to relink the binary with -headerpad /
-headerpad_max_install_names / reports an error).  One possible (and simple)
approach is to implement llvm-install-name-tool in a similar fashion: use
libObject to parse the input binary and rebuild the list of load commands
(analogously to what cctools' install_name_tool does (and with the same
limitations)). I'd like to ask for your general feedback / thoughts /
suggestions on contributing llvm-install-name-tool to LLVM with the goal to be a
drop-in replacement for cctools' install_name_tool.
> Thanks,
> Alex
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Hey Michael,

I completely agree that setting the rpath properly the first time around is
much preferred.  But, changes to the binary, particularly during
development is much quicker.

Prebuilt libraries which are being repackaged is one use case that is
missed.

In the past, I’ve even used it to repair am incorrectly built library which
was missing the library name.

There are a few different approaches to implementing this, and I am not
particularly tied to one.  I think that selecting the one that is in the
best interest of the project and the most maintainable long term would be
best.

On Fri, Oct 11, 2019 at 10:12 PM Michael Trent <mtrent at apple.com>
wrote:
> Hi there,
>
> This tool has been around for a long time. Originally it gave people a way
> to move a dylib to a new location without rebuilding, and then as a way to
> make an existing dylib embeddable (@rpath). I wonder if this tool is really
> necessary. It’s always better to make the dylib with the correct install
> name at build time, including @rpath, etc.. Most dylibs we see any more are
> either installed in fixed file system locations by OS vendors (i.e., Apple)
> or are embedded @rpath dylibs installed by everyone else.
>
> Is there a use-case for install_name_tool that I have overlooked?
>
> Or Is that small set of dylibs meant to be installed in fixed file system
> locations large / complex enough that people really need to install them in
> many different locations without simply relinking the files?
>
> Turning to practical matters, is your intent to have the load-command
> rewriting code live in the tool? Or are you proposing adding support for
> building/swapping/rewriting Object files to libObject?
>
> MDT
>
>
> On Oct 11, 2019, at 10:31 AM, Alexander Shaposhnikov <
> alexander.v.shaposhnikov at gmail.com> wrote:
>
> Hey everyone!
> Recently there has been some progress on LLVM-based tools for manipulating
> MachO binaries: llvm-objcopy has been gaining a lot of important bits to
> support MachO (it's relatively close to the point where one can
implement
> the strip-like functionality), llvm-lipo is functional and supports most of
> cctools' lipo options
(https://llvm.org/docs/CommandGuide/llvm-lipo.html
> ).
> There is another useful utility called install_name_tool (see e.g.
> https://www.unix.com/man-page/osx/1/install_name_tool/ ),
> this tool is capable of changing the rpaths, the names of the dependent
> shared libraries, etc. The way it works -  install-name-tool (from cctools)
> generates the new list of MachO load commands and the new MachO header, the
> other parts of the binary are copied over (in particular, if I'm not
> mistaken, it assumes that the new "prefix" (MachO header + load
commands)
> fits into the binary so the offsets don't need to be recalculated,
> otherwise suggests to relink the binary with -headerpad /
> -headerpad_max_install_names / reports an error).  One possible (and
> simple) approach is to implement llvm-install-name-tool in a similar
> fashion: use libObject to parse the input binary and rebuild the list of
> load commands (analogously to what cctools' install_name_tool does (and
> with the same limitations)). I'd like to ask for your general feedback
/
> thoughts / suggestions on contributing llvm-install-name-tool to LLVM with
> the goal to be a drop-in replacement for cctools' install_name_tool.
> Thanks,
> Alex
>
>
> --
Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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