Require and document using kernel headers from "make headers_install"

["Joseph S. Myers" <joseph at codesourcery dot com>]

This patch makes glibc (for GNU/Linux) require kernel headers
installed by "make headers_install" (which reduces the headers to just
the parts describing the interface between the kernel and userspace),
rather than the older approach of using headers copied directly from
the kernel source tree.  (That older approach was not in any case
supported at all by kernel developers.)  Headers from a newer kernel
can always be used to build a library working on older kernels.  "make
headers_install" was added around Linux 2.6.18 but didn't work well
enough for building glibc until 2.6.19.1, which is documented as the
minimum version (the configure check is for 2.6.19).  I think that
2.6.19.1 was long enough ago that there is no need to continue to
"support" the old approach (such support is in any case hardly likely
now to receive much testing).

The old 2.0.10 minimum for kernel headers has been there since 28 Jul
1996.

(Since the only concerns expressed about removing actual support for
pre-2.6 kernels related to the possibility that some people might be
using current glibc with 2.4 kernels (presumably with the features
required for NPTL backported), I think it will be safe to remove
support for 2.0-2.3 kernels.  2.4 and 2.5 kernel support is on the
deprecation list to be reconsidered for removal after the 2.16
release.  Note that we do have an actual user bug report now, 13717,
about older kernels being accepted but not working.  I don't see any
reason to remove support for using early 2.6 kernels
(pre-headers_install), just for not claiming to be able to use their
headers.)

Comments?  Everything changed apart from the bit documenting
--with-headers is Linux-specific; I don't know if --with-headers is
relevant for Hurd, but think the wording I used there should be
generic enough anyway.

2012-02-23  Joseph Myers  <joseph@codesourcery.com>

	* manual/install.texi (--with-headers): Describe headers as
	interface headers, not private headers.
	(Specific advice for GNU/Linux systems): Describe use of headers
	from "make headers_install", not private headers from older
	kernels.
	* sysdeps/unix/sysv/linux/configure.in (LIBC_LINUX_VERSION):
	Change to 2.6.19.

diff --git a/manual/install.texi b/manual/install.texi
index 1656c5e..793cc8d 100644
--- a/manual/install.texi
+++ b/manual/install.texi
@@ -79,8 +79,9 @@ directory if that option is specified, or @file{/usr/local} otherwise.
 
 @item --with-headers=@var{directory}
 Look for kernel header files in @var{directory}, not
-@file{/usr/include}.  Glibc needs information from the kernel's private
-header files.  Glibc will normally look in @file{/usr/include} for them,
+@file{/usr/include}.  Glibc needs information from the kernel's header
+files describing the interface to the kernel.  Glibc will normally
+look in @file{/usr/include} for them,
 but if you specify this option, it will look in @var{DIRECTORY} instead.
 
 This option is primarily of use on a system where the headers in
@@ -393,39 +394,37 @@ patches, although we try to avoid this.
 @appendixsec Specific advice for GNU/Linux systems
 @cindex kernel header files
 
-If you are installing GNU libc on a GNU/Linux system, you need to have the
-header files from a 2.2 or newer kernel around for reference.  For some
-architectures, like ia64, sh and hppa, you need at least headers from
-kernel 2.3.99 (sh and hppa) or 2.4.0 (ia64).  You do not need to use
-that kernel, just have its headers where glibc can access at them.  The
-easiest way to do this is to unpack it in a directory such as
-@file{/usr/src/linux-2.2.1}.  In that directory, run @samp{make config}
-and accept all the defaults.  Then run @samp{make
-include/linux/version.h}.  Finally, configure glibc with the option
-@samp{--with-headers=/usr/src/linux-2.2.1/include}.  Use the most recent
-kernel you can get your hands on.
-
-An alternate tactic is to unpack the 2.2 kernel and run @samp{make
-config} as above; then, rename or delete @file{/usr/include}, create a
-new @file{/usr/include}, and make symbolic links of
-@file{/usr/include/linux} and @file{/usr/include/asm} into the kernel
-sources.  You can then configure glibc with no special options.
+If you are installing GNU libc on a GNU/Linux system, you need to have
+the header files from a 2.6.19.1 or newer kernel around for reference.
+These headers must be installed using @samp{make headers_install}; the
+headers present in the kernel source directory are not suitable for
+direct use by GNU libc.  You do not need to use that kernel, just have
+its headers installed where glibc can access them, referred to here as
+@var{install-directory}.  The easiest way to do this is to unpack it
+in a directory such as @file{/usr/src/linux-@var{version}}.  In that
+directory, run @samp{make headers_install
+INSTALL_HDR_PATH=@var{install-directory}}.  Finally, configure glibc
+with the option @samp{--with-headers=@var{install-directory}/include}.
+Use the most recent kernel you can get your hands on.  (If you are
+cross-compiling GNU libc, you need to specify
+@samp{ARCH=@var{architecture}} in the @samp{make headers_install}
+command, where @var{architecture} is the architecture name used by the
+Linux kernel, such as @samp{x86} or @samp{powerpc}.)
 
 After installing GNU libc, you may need to remove or rename
-@file{/usr/include/linux} and @file{/usr/include/asm}, and replace them
-with copies of @file{include/linux} and
-@file{include/asm-$@var{ARCHITECTURE}} taken from the Linux source
-package which supplied kernel headers for building the library.
-@var{ARCHITECTURE} will be the machine architecture for which the
-library was built, such as @samp{i386} or @samp{alpha}.  You do not need
-to do this if you did not specify an alternate kernel header source
-using @samp{--with-headers}.  The intent here is that these directories
-should be copies of, @strong{not} symlinks to, the kernel headers used to
-build the library.
-
-Note that @file{/usr/include/net} and @file{/usr/include/scsi} should
-@strong{not} be symlinks into the kernel sources.  GNU libc provides its
-own versions of these files.
+directories such as @file{/usr/include/linux} and
+@file{/usr/include/asm}, and replace them with copies of directories
+such as @file{linux} and @file{asm} from
+@file{@var{install-directory}/include}.  All directories present in
+@file{@var{install-directory}/include} should be copied, except that
+GNU libc provides its own version of @file{/usr/include/scsi}; the
+files provided by the kernel should be copied without replacing those
+provided by GNU libc.  The @file{linux}, @file{asm} and
+@file{asm-generic} directories are required to compile programs using
+GNU libc; the other directories describe interfaces to the kernel but
+are not required if not compiling programs using those interfaces.
+You do not need to copy kernel headers if you did not specify an
+alternate kernel header source using @samp{--with-headers}.
 
 GNU/Linux expects some components of the libc installation to be in
 @file{/lib} and some in @file{/usr/lib}.  This is handled automatically
diff --git a/sysdeps/unix/sysv/linux/configure.in b/sysdeps/unix/sysv/linux/configure.in
index a73c3e5..59a1311 100644
--- a/sysdeps/unix/sysv/linux/configure.in
+++ b/sysdeps/unix/sysv/linux/configure.in
@@ -5,7 +5,7 @@ GLIBC_PROVIDES dnl See aclocal.m4 in the top level source directory.
 # existing system library, because we are the only system library.
 inhibit_glue=yes
 
-define([LIBC_LINUX_VERSION],[2.0.10])dnl
+define([LIBC_LINUX_VERSION],[2.6.19])dnl
 if test -n "$sysheaders"; then
   OLD_CPPFLAGS=$CPPFLAGS
   CPPFLAGS="$CPPFLAGS $SYSINCLUDES"

-- 
Joseph S. Myers
joseph@codesourcery.com