kheaders: make it possible to override TAR

Commit 86cdd2f ("kheaders: make headers archive reproducible")
introduced a number of options specific to GNU tar to the `tar`
invocation in `gen_kheaders.sh` script. This causes the script to fail
to work on systems where `tar` is not GNU tar. This can occur e.g.
on recent Gentoo Linux installations that support using bsdtar from
libarchive instead.

Add a `TAR` make variable to make it possible to override the tar
executable used, e.g. by specifying:

  make TAR=gtar

Link: https://bugs.gentoo.org/884061
Reported-by: Sam James <[email protected]>
Tested-by: Sam James <[email protected]>
Co-developed-by: Masahiro Yamada <[email protected]>
Signed-off-by: Michał Górny <[email protected]>
Signed-off-by: Sam James <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

Author: mgorny

Makefile

@@ -543,6 +543,7 @@ LZMA		= lzma
 LZ4		= lz4
 XZ		= xz
 ZSTD		= zstd
+TAR		= tar
 
 CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
 		  -Wbitwise -Wno-return-void -Wno-unknown-attribute $(CF)
@@ -622,7 +623,7 @@ export RUSTC RUSTDOC RUSTFMT RUSTC_OR_CLIPPY_QUIET RUSTC_OR_CLIPPY BINDGEN
 export HOSTRUSTC KBUILD_HOSTRUSTFLAGS
 export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
 export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
-export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
+export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD TAR
 export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS KBUILD_PROCMACROLDFLAGS LDFLAGS_MODULE
 export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS
 

kernel/gen_kheaders.sh

@@ -30,8 +30,8 @@ rm -rf "${tmpdir}"
 mkdir "${tmpdir}"
 
 # shellcheck disable=SC2154 # srctree is passed as an env variable
-sed "s:^${srctree}/::" "${srclist}" | tar -c -f - -C "${srctree}" -T - | tar -xf - -C "${tmpdir}"
-tar -c -f - -T "${objlist}" | tar -xf - -C "${tmpdir}"
+sed "s:^${srctree}/::" "${srclist}" | ${TAR} -c -f - -C "${srctree}" -T - | ${TAR} -xf - -C "${tmpdir}"
+${TAR} -c -f - -T "${objlist}" | ${TAR} -xf - -C "${tmpdir}"
 
 # Remove comments except SDPX lines
 # Use a temporary file to store directory contents to prevent find/xargs from
@@ -43,7 +43,7 @@ xargs -0 -P8 -n1 \
 rm -f "${tmpdir}.contents.txt"
 
 # Create archive and try to normalize metadata for reproducibility.
-tar "${timestamp:+--mtime=$timestamp}" \
+${TAR} "${timestamp:+--mtime=$timestamp}" \
     --owner=0 --group=0 --sort=name --numeric-owner --mode=u=rw,go=r,a+X \
     -I "${XZ}" -cf "${tarfile}" -C "${tmpdir}/" . > /dev/null