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

[Masahiro Yamada]

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)

>
> 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



-- 
Best Regards
Masahiro Yamada