On 64 bit architectures, you can run 32 bit executables and the rtld can trace them, but ldd(1) doesn't know yet how to detect them:

[/] root@ed-exigent>ldd `which httpd`
    ldd: /usr/local/sbin/httpd: can't read program header
    ldd: /usr/local/sbin/httpd: not a dynamic executable

    But...

    [/] root@ed-exigent>LD_32_TRACE_LOADED_OBJECTS==1 `which httpd`
    libm.so.4 => /lib32//libm.so.4 (0x280c8000)
    libaprutil-1.so.2 => /usr/local/lib/libaprutil-1.so.2 (0x280de000)
    libexpat.so.6 => /usr/local/lib/libexpat.so.6 (0x280f2000)
    libiconv.so.3 => /usr/local/lib/libiconv.so.3 (0x28110000)
    libapr-1.so.2 => /usr/local/lib/libapr-1.so.2 (0x281fd000)
    libcrypt.so.3 => /lib32//libcrypt.so.3 (0x2821d000)
    libpthread.so.2 => not found (0x0)
    libc.so.6 => /lib32//libc.so.6 (0x28235000)
    libpthread.so.2 => /usr/lib32/libpthread.so.2 (0x2830d000)

Added support in ldd(1) for the LD_32_xxx environment variables if
the architecture of the machine is >32 bits. If we ever go to 128
bit architectures this excercise will have to be repeated but thanks
to earlier commits today it will be relative simple.

PR:		bin/124906
Submitted by:	edwin
Approved by:	bde (mentor)
MFC after:	1 week

usr.bin/ldd/ldd.1

@@ -64,6 +64,12 @@ option.
 It will print a report of all ELF binaries in the current directory,
 which link against libc.so.6:
 .Dl "find . -type f | xargs -n1 file -F " " | grep ELF | cut -f1 -d' ' | xargs ldd -f '%A %o\en' | grep libc.so.6"
+.Sh BUGS
+On 64 bit architectures, dlopen() cannot open 32 bit dynamic libraries,
+so
+.Nm
+will show the error
+.Qq "unsupported file layout" .
 .Sh SEE ALSO
 .Xr ld 1 ,
 .Xr nm 1 ,

usr.bin/ldd/ldd.c

@@ -40,6 +40,7 @@ __FBSDID("$FreeBSD$");
 #include <a.out.h>
 #include <dlfcn.h>
 #include <err.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -54,6 +55,9 @@ static void	usage(void);
 #define	TYPE_UNKNOWN	0
 #define	TYPE_AOUT	1
 #define	TYPE_ELF	2	/* Architecture default */
+#if __ELF_WORD_SIZE > 32
+#define	TYPE_ELF32	3	/* Explicit 32 bits on architectures >32 bits */
+#endif
 
 #define	ENV_OBJECTS		0
 #define	ENV_OBJECTS_FMT1	1
@@ -69,6 +73,15 @@ const char	*envdef[ENV_LAST] = {
 	"LD_TRACE_LOADED_OBJECTS_PROGNAME",
 	"LD_TRACE_LOADED_OBJECTS_ALL",
 };
+#if __ELF_WORD_SIZE > 32
+const char	*env32[ENV_LAST] = {
+	"LD_32_TRACE_LOADED_OBJECTS",
+	"LD_32_TRACE_LOADED_OBJECTS_FMT1",
+	"LD_32_TRACE_LOADED_OBJECTS_FMT2",
+	"LD_32_TRACE_LOADED_OBJECTS_PROGNAME",
+	"LD_32_TRACE_LOADED_OBJECTS_ALL",
+};
+#endif
 
 int
 main(int argc, char *argv[])
@@ -124,8 +137,6 @@ main(int argc, char *argv[])
 		int fd, status, is_shlib, rv, type;
 		const char **env;
 
-		env = envdef;	/* Temporary placeholder */
-
 		if ((fd = open(*argv, O_RDONLY, 0)) < 0) {
 			warn("%s", *argv);
 			rval |= 1;
@@ -138,6 +149,25 @@ main(int argc, char *argv[])
 			continue;
 		}
 
+		switch (type) {
+		case TYPE_ELF:
+		case TYPE_AOUT:
+			env = envdef;
+			break;
+#if __ELF_WORD_SIZE > 32
+		case TYPE_ELF32:
+			env = env32;
+			break;
+#endif
+		case TYPE_UNKNOWN:
+		default:
+			/*
+			 * This shouldn't happen unless is_executable()
+			 * is broken.
+			 */
+			errx(EDOOFUS, "unknown executable type");
+		}
+
 		/* ld.so magic */
 		setenv(env[ENV_OBJECTS], "yes", 1);
 		if (fmt1 != NULL)
@@ -199,6 +229,7 @@ is_executable(const char *fname, int fd, int *is_shlib, int *type)
 {
 	union {
 		struct exec aout;
+		Elf32_Ehdr elf32;
 		Elf_Ehdr elf;
 	} hdr;
 	int n;
@@ -225,6 +256,49 @@ is_executable(const char *fname, int fd, int *is_shlib, int *type)
 		return (1);
 	}
 
+#if __ELF_WORD_SIZE > 32
+	if ((size_t)n >= sizeof(hdr.elf32) && IS_ELF(hdr.elf32) &&
+	    hdr.elf32.e_ident[EI_CLASS] == ELFCLASS32) {
+		/* Handle 32 bit ELF objects */
+		Elf32_Phdr phdr;
+		int dynamic, i;
+
+		dynamic = 0;
+		*type = TYPE_ELF32;
+
+		if (lseek(fd, hdr.elf32.e_phoff, SEEK_SET) == -1) {
+			warnx("%s: header too short", fname);
+			return (0);
+		}
+		for (i = 0; i < hdr.elf32.e_phnum; i++) {
+			if (read(fd, &phdr, hdr.elf32.e_phentsize) !=
+			    sizeof(phdr)) {
+				warnx("%s: can't read program header", fname);
+				return (0);
+			}
+			if (phdr.p_type == PT_DYNAMIC) {
+				dynamic = 1;
+				break;
+			}
+		}
+
+		if (!dynamic) {
+			warnx("%s: not a dynamic ELF executable", fname);
+			return (0);
+		}
+		if (hdr.elf32.e_type == ET_DYN) {
+			if (hdr.elf32.e_ident[EI_OSABI] & ELFOSABI_FREEBSD) {
+				*is_shlib = 1;
+				return (1);
+			}
+			warnx("%s: not a FreeBSD ELF shared object", fname);
+			return (0);
+		}
+
+		return (1);
+	}
+#endif
+
 	if ((size_t)n >= sizeof(hdr.elf) && IS_ELF(hdr.elf) &&
 	    hdr.elf.e_ident[EI_CLASS] == ELF_TARG_CLASS) {
 		/* Handle default ELF objects on this architecture */