[Valgrind-developers] Switching PT_INTERP (helps Debian unstable)

[Stephen M. <sm...@CS...>]

Current versions of Valgrind work less-than-perfectly with the latest
version of glibc in Debian unstable (Debian version 2.3.5-6). Part of
the problem seems to be that this build of libc6 manages to include
even less symbol information for /lib/ld-linux.so.2 than the 2.3.2
build from sarge did (or it's in a format that Valgrind can't
handle). One of the relevant symbols is _dl_relocate_object, so this
may just be a result of the gratuitous "sanitizing" that John Reiser
was complaining about a few weeks ago
(http://sourceforge.net/mailarchive/message.php?msg_id=12915644); the
new libc is also compiled with a newer GCC ("Debian 4.0.1-3 /
pre-4.0.2 20050725", instead of 3.3.5).

One obvious symptom of this is that suppressions don't work. For
instance, there's a longstanding problem whose correct stack trace is:

 Conditional jump or move depends on uninitialised value(s)
    at 0x40086B6: _dl_relocate_object (do-rel.h:65)
    by 0x4002376: dl_main (rtld.c:1916)
    by 0x400EBDD: _dl_sysdep_start (dl-sysdep.c:237)
    by 0x4003675: _dl_start (rtld.c:307)
    by 0x40007C6: (within /usr/lib/debug/ld-2.3.5.so)

With the sarge libc, Valgrind can at least see the name of the
function on top of the stack:

 Conditional jump or move depends on uninitialised value(s)
    at 0x4009620: _dl_relocate_object (in /lib/ld-2.3.2.so)
    by 0x400211C: (within /lib/ld-2.3.2.so)
    by 0x400F1F6: (within /lib/ld-2.3.2.so)
    by 0x4000F3A: (within /lib/ld-2.3.2.so)
    by 0x4000C26: (within /lib/ld-2.3.2.so)

With the latest libc version, no symbol information shows up at all:

 Conditional jump or move depends on uninitialised value(s)
    at 0x1B8ECB13: (within /lib/ld-2.3.5.so)
    by 0x1B8E631C: (within /lib/ld-2.3.5.so)
    by 0x1B8F2BDD: (within /lib/ld-2.3.5.so)
    by 0x1B8E7675: (within /lib/ld-2.3.5.so)
    by 0x1B8E47C6: (within /lib/ld-2.3.5.so)

A similar problem seems to affect the
{strlen,index}-not-intercepted-early-enough-HACK-* suppressions. These
unsuppressed errors are the subject of Debian bug #326823, I believe.

One potential fix for this problem is to use the debugging version of
the dynamic linker, which has full static symbols (you can see it's
where I got the first stack trace above). Unfortunately, I don't
believe there's any standard way (equivalent to LD_LIBRARY_PATH) to
tell programs to use a different dynamic linker: "/lib/ld-linux.so.2"
is hardcoded in binaries. (This is especially annoying because the
dynamic linkers from different versions of glibc are generally neither
forward nor backward compatible with other versions of libc.so, so you
can't have more than one glibc version installed at once. But that's a
different rant.) It's possible to start a program with a different
linker by saying something like "/usr/lib/debug/ld-linux.so.2 /bin/cat
/proc/self/maps", but that isn't inherited by child processes, and it
makes Valgrind report even more errors (I haven't investigated why).

Since Valgrind loads ld-linux.so.2 for its guests, it would be helpful
if there was a way it could be told to switch in a different version.
I've appended a proof-of-concept patch that hard-codes the standard
location of the debugging version. In brief testing, this change
allows the suppressions to work correctly for me, and also seems to
help Valgrind run better on a large program (XEmacs), perhaps because
it makes some redirects work.

By the way, are any of the Debian valgrind maintainers regular readers
of this list? If not, I'll forward this information to the BTS.

 -- Stephen

Index: coregrind/m_ume.c
===================================================================
--- coregrind/m_ume.c	(revision 4894)
+++ coregrind/m_ume.c	(working copy)
@@ -530,19 +530,26 @@
       case PT_INTERP: {
 	 char *buf = VG_(malloc)(ph->p_filesz+1);
 	 int j;
-	 int intfd;
+	 int intfd = -1;
 	 int baseaddr_set;
 
          vg_assert(buf);
 	 VG_(pread)(fd, buf, ph->p_filesz, ph->p_offset);
 	 buf[ph->p_filesz] = '\0';
 
-	 sres = VG_(open)(buf, VKI_O_RDONLY, 0);
-         if (sres.isError) {
-	    VG_(printf)("valgrind: m_ume.c: can't open interpreter\n");
-	    VG_(exit)(1);
+	 if (!VG_(strcmp)(buf, "/lib/ld-linux.so.2")) {
+	    sres = VG_(open)("/usr/lib/debug/ld-linux.so.2", VKI_O_RDONLY, 0);
+	    if (!sres.isError) 
+	       intfd = sres.val;
 	 }
-         intfd = sres.val;
+	 if (intfd == -1) {
+	    sres = VG_(open)(buf, VKI_O_RDONLY, 0);
+	    if (sres.isError) {
+	       VG_(printf)("valgrind: m_ume.c: can't open interpreter\n");
+	       VG_(exit)(1);
+	    }
+	    intfd = sres.val;
+	 }
 
 	 interp = readelf(intfd, buf);
 	 if (interp == NULL) {