Re: [PATCH] kbuild, rust: use -fremap-path-prefix to make paths relative

[Thomas Weißschuh]

On 2025-03-12 04:07:51+0900, Masahiro Yamada wrote:
> On Wed, Mar 12, 2025 at 2:36 AM Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, 2025-03-11 at 20:03 +0900, Masahiro Yamada wrote:
> > > On Mon, Feb 17, 2025 at 10:23 PM Miguel Ojeda
> > > <miguel.ojeda.sandonis@xxxxxxxxx> wrote:
> > > >
> > > > On Mon, Feb 10, 2025 at 6:11 PM Thomas Weißschuh <linux@xxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > Remap source path prefixes in all output, including compiler
> > > > > diagnostics, debug information, macro expansions, etc.
> > > >
> > > > Hmm... We don't do all the cases in the C side -- the docs ask to use
> > > > `KCFLAGS` when one wants to remove them in the debug info:
> > > >
> > > >     https://docs.kernel.org/kbuild/reproducible-builds.html#absolute-filenames
> > > >
> > > > I am not sure if there is a reason not to cover all cases in C (Cc'ing Ben).
> >
> > I think we should use the prefix-map options by default, for both C and
> > Rust code.
> 
> A patch is appreciated.
> (top-level Makefile change with updates
> in Documentation/kbuild/reproducible-builds.rst)

I'm happy to send a patch.
The new patch will conflict with the rust patch.
Could you push the current kbuild tree to git.kernel.org so the new
patch can be based on top of it?

> > The default of using absolute filenames only works when the build and
> > debug systems have their sources in the same absolute directory.  For
> > some developers this will always be true because they're the same
> > system.  In the general case, and particularly in production, it's
> > unlikely to be true.  See below.
> >
> > > GCC manual mentions the below about the -fdebug-prefix-map=old=new
> > >
> > >
> > > "It can also be used to change an absolute path to
> > > a relative path by using . for new.
> > > This can give more reproducible builds, which are location
> > > independent, but may require an extra command to tell GDB
> > > where to find the source files."
> > >
> > >
> > > I guess "the extra command" might be a bit annoying.
> >
> > The command in question is "dir <source-root-directory>".  It's not
> > particulary annoying.  It also isn't needed when invoking gdb with the
> > kernel source root as its current directory.
> >
> > Running that command adds the directory to gdb's source search path,
> > causing it to look for sources referenced in debug info by:
> >
> > 1. Appending the full filename to that directory
> > 2. Appending the base filename to that directory
> >
> > (This is slightly simplified from the docuemntation at:
> > <https://sourceware.org/gdb/current/onlinedocs/gdb.html/Source-Path.html#Source-Path>.)
> >
> > When the debug info has relative filenames, case 1 should always work.
> >
> > When the debug info has absolute filenames, case 2 can work if all
> > sources are in the same directory.  But the kernel has source files
> > spread across many directories, so there's no general way to make gdb
> > find them all.
> >
> > Ben.
> >
> > --
> > Ben Hutchings
> > A free society is one where it is safe to be unpopular.
> >                                                       - Adlai Stevenson