elf: Canonicalize $ORIGIN in an explicit ld.so invocation [BZ 25263]

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.)

The __fd_to_filename requires _fitoa_word and _itoa_word, which for
32-bits pulls a lot of definitions from _itoa.c (due _ITOA_NEEDED
being defined).  To simplify the build move the required function
to a new file, _fitoa_word.c.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Co-authored-by: Geoffrey Thomas <geofft@ldpreload.com>
Reviewed-by: Geoffrey Thomas <geofft@ldpreload.com>
Tested-by: Geoffrey Thomas <geofft@ldpreload.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/dl-origin.c b/sysdeps/unix/sysv/linux/dl-origin.c
index decdd8a..3c52ba5 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 +73,25 @@ _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);
+
+  /* Check if the path was truncated.  */
+  if (size >= 0 && size < PATH_MAX - 1)
+    {
+      canonical[size] = '\0';
+      return __strdup (canonical);
+    }
+  return NULL;
+}