Bug 220675

Created attachment 144322 [details]
LD_DEBUG=all LD_LIBRARY_PATH=/lib /bin/echo

This smells like a kernel bug:
objdump -s -j .note /tmp/vdso

/tmp/vdso:     file format elf32-i386

Contents of section .note:
 0460 06000000 04000000 00000000 4c696e75  ............Linu
 0470 78000000 12060200 04000000 12000000  x...............
 0480 02000000 474e5500 01000000 01000000  ....GNU.........
 0490 016e6f73 65676e65 67000000           .nosegneg...

Mask is 0x00000001, so bit 0 is set there.  Mask is what is used when ld.so.cache
comes to play, but when LD_LIBRARY_PATH is used, it needs the strings located
after it.  The format is always a uint8_t bit number followed by zero terminated
name, and ld.so excludes strings which are not set in mask:
#if defined NEED_DL_SYSINFO || defined NEED_DL_SYSINFO_DSO
  if (dsocaps != NULL)
    {
      const ElfW(Word) mask = ((const ElfW(Word) *) dsocaps)[-1];
      GLRO(dl_hwcap) |= (uint64_t) mask << _DL_FIRST_EXTRA;
      size_t len;
      for (const char *p = dsocaps; p < dsocaps + dsocapslen; p += len + 1)
        {
          uint_fast8_t bit = *p++;
          len = strlen (p);

          /* Skip entries that are not enabled in the mask word.  */
          if (__builtin_expect (mask & ((ElfW(Word)) 1 << bit), 1))
            {
              temp[m].str = p;
              temp[m].len = len;
              ++m;
            }
          else
            --cnt;
        }
    }
#endif

But, the kernel says that bit 1 is nosegneg, but only bit 0 is set in the mask.

vsyscall-note-xen.S has:
/*
 * This supplies .note.* sections to go into the PT_NOTE inside the vDSO text.
 * Here we can supply some information useful to userland.
 * First we get the vanilla i386 note that supplies the kernel version info.
 */

#include "vsyscall-note.S"

/*
 * Now we add a special note telling glibc's dynamic linker a fake hardware
 * flavor that it will use to choose the search path for libraries in the
 * same way it uses real hardware capabilities like "mmx".
 * We supply "nosegneg" as the fake capability, to indicate that we
 * do not like negative offsets in instructions using segment overrides,
 * since we implement those inefficiently.  This makes it possible to
 * install libraries optimized to avoid those access patterns in someplace
 * like /lib/i686/tls/nosegneg.  Note that an /etc/ld.so.conf.d/file
 * corresponding to the bits here is needed to make ldconfig work right.
 * It should contain:
 *      hwcap 0 nosegneg
 * to match the mapping of bit to name that we give here.
 */
#define NOTE_KERNELCAP_BEGIN(ncaps, mask) \
        ASM_ELF_NOTE_BEGIN(".note.kernelcap", "a", "GNU", 2) \
        .long ncaps, mask
#define NOTE_KERNELCAP(bit, name) \
        .byte bit; .asciz name
#define NOTE_KERNELCAP_END ASM_ELF_NOTE_END

NOTE_KERNELCAP_BEGIN(1, 1)
NOTE_KERNELCAP(1, "nosegneg")  /* Change 1 back to 0 when glibc is fixed! */
NOTE_KERNELCAP_END

There indeed was a glibc bug, but it has been fixed more than a year ago:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/sysdeps/generic/Attic/dl-sysdep.c.diff?r1=1.114&r2=1.116&cvsroot=glibc
(broken between April and September 2005, fixed since then).
So, the kernel should change.

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux maintenance release.  Product Management has requested
further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed
products.  This request is not yet committed for inclusion in an Update
release.

I think one option is:
-NOTE_KERNELCAP(1, "nosegneg")  /* Change 1 back to 0 when glibc is fixed! */
+NOTE_KERNELCAP(0, "nosegneg")
(i.e. reverting the hack), the other, if we really want to stay compatible
even with glibcs from the short period of time when it was broken, would be
changing /etc/ld.so.conf.d/kernel* to 1 nosegneg and
-NOTE_KERNELCAP_BEGIN(1, 1)
+NOTE_KERNELCAP_BEGIN(1, 2)
(i.e. keep using bit 1 rather than bit 0 and adjust the mask for that as well
as ldconfig's configuration).

nothing is lost by doing it that way, in case anyone is paranoid about bug compat

I am pretty sure I reverted that bit in the kernel back to zero a while ago.  I
have absolutely no idea why it went back to 1 :(((

QE ack for RHEL5.

Built into 2.6.18-1.3002.el5.

Created attachment 145488 [details]
"Documentation/kernel-parameters.txt" vdso sideeffects update.

Confirming kernel-xen-2.6.18-1.3002.el5.i686 fixes the problem.

Please consider updating the "Documentation/kernel-parameters.txt"
documentation as the problem was still present with non-default
"/proc/sys/kernel/vdso".

A package has been built which should help the problem described in 
this bug report. This report is therefore being closed with a resolution 
of CURRENTRELEASE. You may reopen this bug report if the solution does 
not work for you.