elf: Canonicalize $ORIGIN in an explicit ld.so invocation [BZ 25263]azanella/bz25263-loader-realpath

When an executable is invoked directly, we calculate $ORIGIN by calling
readlink on /proc/self/exe, which the Linux kernel resolves to the
target of any symlinks.  However, if an executable is run through ld.so,
we cannot use /proc/self/exe and instead use the path given as an
argument. This leads to a different calculation of $ORIGIN, which is
most notable in that it causes ldd to behave differently (e.g., by not
finding a library) from directly running the program.

To make the behavior consistent, take advantage of the fact that the
kernel also resolves /proc/self/fd/ symlinks to the target of any
symlinks in the same manner, so once we have opened the main executable
in order to load it, replace the user-provided path with the result of
calling readlink("/proc/self/fd/N").

(On non-Linux platforms this resolution does not happen and so no
behavior change is needed.)

Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

3 files changed, 27 insertions, 0 deletions

diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 8465cbaa9b..19494b82ee 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1223,6 +1223,10 @@ extern struct link_map * _dl_get_dl_main_map (void) attribute_hidden;
 /* Find origin of the executable.  */
 extern const char *_dl_get_origin (void) attribute_hidden;
 
+/* Return the canonalized path name from the opened file descriptor FD,
+   or NULL otherwise.  */
+extern char * _dl_canonicalize (int fd) attribute_hidden;
+
 /* Count DSTs.  */
 extern size_t _dl_dst_count (const char *name) attribute_hidden;
 
diff --git a/sysdeps/mach/hurd/Makefile b/sysdeps/mach/hurd/Makefile
index 13e5cea4c2..4b69b40065 100644
--- a/sysdeps/mach/hurd/Makefile
+++ b/sysdeps/mach/hurd/Makefile
@@ -300,6 +300,8 @@ ifeq ($(subdir),elf)
 check-execstack-xfail += ld.so libc.so libpthread.so
 # We always create a thread for signals
 test-xfail-tst-single_threaded-pthread-static = yes
+# Bug 25263
+test-xfail-tst-origin = yes
 
 CFLAGS-tst-execstack.c += -DDEFAULT_RWX_STACK=1
 endif
diff --git a/sysdeps/unix/sysv/linux/dl-origin.c b/sysdeps/unix/sysv/linux/dl-origin.c
index decdd8ae9e..20e95f8bca 100644
--- a/sysdeps/unix/sysv/linux/dl-origin.c
+++ b/sysdeps/unix/sysv/linux/dl-origin.c
@@ -21,6 +21,7 @@
 #include <fcntl.h>
 #include <ldsodefs.h>
 #include <sysdep.h>
+#include <fd_to_filename.h>
 
 /* On Linux >= 2.1 systems which have the dcache implementation we can get
    the path of the application from the /proc/self/exe symlink.  Try this
@@ -72,3 +73,23 @@ _dl_get_origin (void)
 
   return result;
 }
+
+/* On Linux, readlink on the magic symlinks in /proc/self/fd also has
+   the same behavior of returning the canonical path from the dcache.
+   If it does not work, we do not bother to canonicalize. */
+
+char *
+_dl_canonicalize (int fd)
+{
+  struct fd_to_filename fdfilename;
+  char canonical[PATH_MAX];
+  char *path = __fd_to_filename (fd, &fdfilename);
+  int size = INTERNAL_SYSCALL_CALL (readlinkat, AT_FDCWD, path,
+                                    canonical, PATH_MAX - 1);
+  if (size >= 0)
+    {
+      canonical[size] = '\0';
+      return __strdup (canonical);
+    }
+  return NULL;
+}