Using vanilla kernels headers to do backport testing -- issues with memcpy()

Tue Aug 7 19:28:37 UTC 2012

A while ago we reviewed the idea of whether or not to use a set of
distribution vanilla kernels to allow us to do compilation testing of
modules on the compat backport project [0] with the benefit being that
at least one Linux distribution was doing this work already and given
that this work was using plain vanilla kernels directly from
kernel.org, with no modifications. The other option that existed was
that each distro could provide their own set of kernel packages along
with their own kernel headers but it didn't seem like other Linux
distributions were doing this and the information was not easy to
gather. So we went with the Ubuntu vanilla kernels and Ozan later
figured out a way to extract deb packages in a distribution agnostic
manner [1] and later added an option to enable a way to install and
use this for ckmake which test compiles against 21 kernels without
requiring root [2].

This works swell as it allows anyone on any Linux distribution to test
compile against a series of older kernels, today 21 older kernels and
all vanilla kernels which means we get to focus and prioritize
upstream.

This has started working well except for the fact that as of the >=
v3.4 vanilla kernel releases, there are a few files which are binary
in the kernel headers which use dynamic libraries. Of interest to us
here are the ones required to build external modules. An example:

/lib/modules/3.4.4-030404-generic/build/scripts/genksyms/genksyms

It just so happens to be that the folks building these packages are
using a glibc >= 2.14 and although the *only* symbol I see being
required from glibc >= 2.14 being memcpy():

mcgrof at garbanzo ~/compat (git::master)$ objdump -T
/home/mcgrof/compat-ksrc/lib/modules/3.4.4-030404-generic/build/scripts/genksyms/genksyms

/home/mcgrof/compat-ksrc/lib/modules/3.4.4-030404-generic/build/scripts/genksyms/genksyms:
    file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.3.4 __snprintf_chk
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 free
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.3.4 __vfprintf_chk
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __errno_location
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 ferror
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 isatty
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fread
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fclose
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 strlen
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.4   __stack_chk_fail
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 getopt_long
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 strchr
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 _IO_putc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fputs
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fgetc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fputc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __strdup
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __libc_start_main
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 strcmp
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 feof
0000000000000000  w   D  *UND*  0000000000000000              __gmon_start__
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 strtol
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 clearerr

Bleh:

0000000000000000      DF *UND*  0000000000000000  GLIBC_2.14  memcpy

0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fileno
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 malloc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 _IO_getc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 ungetc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 realloc
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.3.4 __printf_chk
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fopen
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 perror
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 exit
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 fwrite
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.3.4 __fprintf_chk
000000000060b160 g    DO .bss   0000000000000008  GLIBC_2.2.5 stdout
000000000060b168 g    DO .bss   0000000000000008  GLIBC_2.2.5 stdin
000000000060b178 g    DO .bss   0000000000000008  GLIBC_2.2.5 stderr
000000000060b170 g    DO .bss   0000000000000008  GLIBC_2.2.5 optarg

It would be reasonable to simply require the users of ckmake to update
their libc version but even Debian sid does not yet seem to have libc
>= 2.14, not sure of others. The gains for requiring a glibc >= 2.14
only for memcpy() would seem rather small for this particular case so
I wanted to review the options to fix this.

---------------
1) Get the kernel headers to statically link against the required libraries

This options would enable any Linux distribution to use the packages provided

2) Add a LD_PRELOAD hack for mempcy() for running ckmake and friends
on the compat package

This doesn't seem so.. clean, but its the only thing I can currently control.

3) Simply require libc > 2.14 for folks who really want to help do
backport testing for kernels >= 3.4

I suppose this is the easiest but we likely could run into other
similar issues later.

---------------

Are there other options?

[0] https://lkml.org/lkml/2012/2/6/558
[1] https://github.com/mcgrof/compat/commit/fdbea5a6ce2cba6d6c8fb3eb998cffa5a9e3bf78
[2] https://github.com/mcgrof/compat/commit/e7d88d67040bbaa79720fa1b2dfc4fc24e4d004f

  Luis