Petr,
Below the changelog entry and diff that I actually applied.
Cheers,
Michael
commit ce02f233b3e884fe1fa303d5694ead012b298410
Author: Petr Baudis <pasky@xxxxxxx>
Date: Fri Dec 5 17:45:53 2008 -0500
dlopen.3: Describe confusing dladdr() behavior
dladdr() will act unexpectedly if called from non-pic code on a
compile-time-generated function pointer:
/* test_dladdr.c */
#define _GNU_SOURCE
#include <dlfcn.h>
#include <stdio.h>
int
main(void)
{
void *func;
Dl_info info = {};
func = printf;
dladdr(func, &info);
printf("%s at %p resolved from %s\n", info.dli_sname,
func, info.dli_fname);
return 0;
}
$ cc test_dladdr.c -ldl
$ ./a.out
printf at 0x804838c resolved from ./a.out
$ cc -fPIC test_dladdr.c -ldl
$ ./a.out
_IO_printf at 0xb7f71c30 resolved from /lib/libc.so.6
In the long term, it might make sense to make dladdr() recognize
plt pointers and recurse, but I'm too afraid of Ulrich ;-)
(and he seems to be heavy proponent of pic code anyway, so
the chances for that to be accepted probably aren't high).
Signed-off-by: Petr Baudis <pasky@xxxxxxx>
Signed-off-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
diff --git a/man3/dlopen.3 b/man3/dlopen.3
index d302360..43fba7c 100644
--- a/man3/dlopen.3
+++ b/man3/dlopen.3
@@ -30,8 +30,9 @@
.\" Modified by Matt Domsch, 2003-04-09: _init and _fini obsolete
.\" Modified by Michael Kerrisk <mtk.manpages@xxxxxxxxx> 2003-05-16.
.\" Modified by Walter Harms: dladdr, dlvsym
+.\" Modified by Petr Baudis <pasky@xxxxxxx>, 2008-12-04: dladdr caveat
.\"
-.TH DLOPEN 3 2008-10-27 "Linux" "Linux Programmer's Manual"
+.TH DLOPEN 3 2008-12-05 "Linux" "Linux Programmer's Manual"
.SH NAME
dladdr, dlclose, dlerror, dlopen, dlsym, dlvsym \- programming interface to
dynamic linking loader
@@ -414,6 +415,34 @@ That system also has
.BR dladdr (),
but not
.BR dlvsym ().
+.SH BUGS
+Sometimes, the function pointers you pass to
+.BR dladdr ()
+may surprise you.
+On some architectures (notably i386 and x86_64),
+.I dli_fname
+and
+.I dli_fbase
+may end up pointing back at the object from which you called
+.BR dladdr (),
+even if the function used as an argument should come from
+a dynamically linked library.
+.PP
+The problem is that the function pointer will still be resolved
+at compile time, but merely point to the
+.I plt
+(procedure linkage table)
+section of the original object (which dispatches the call after
+asking the dynamic linker to resolve the symbol).
+To work this around, you can try to compile the code to be
+position-independent
+.RI ( "cc -fPIC" ): then, the compiler cannot prepare the pointer
+at compile time anymore and today's
+.BR gcc (1)
+will just load the final symbol address from the
+.I GOT
+(Global Offset Table) at run time before passing it to
+.BR dladdr ().
.SH EXAMPLE
Load the math library, and print the cosine of 2.0:
.nf
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