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>

3 files changed, 62 insertions, 43 deletions

diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index b68e922..d3733d0 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -59,6 +59,7 @@ headers := \
   # headers
 
 routines := \
+  _fitoa_word \
   _itoa \
   _itowa \
   asprintf \
@@ -663,6 +664,8 @@ CFLAGS-dprintf.c += $(config-cflags-wno-ignored-attributes)
 # off for non-shared builds.
 CFLAGS-_itoa.o = $(no-stack-protector)
 CFLAGS-_itoa.op = $(no-stack-protector)
+CFLAGS-_fitoa_word.o = $(no-stack-protector)
+CFLAGS-_fitoa_word.op = $(no-stack-protector)
 
 CFLAGS-scanf13.c += $(test-config-cflags-wno-fortify-source)
 
diff --git a/stdio-common/_fitoa_word.c b/stdio-common/_fitoa_word.c
new file mode 100644
index 0000000..8064b1e
--- /dev/null
+++ b/stdio-common/_fitoa_word.c
@@ -0,0 +1,59 @@
+/* Internal function for converting integers to ASCII.
+   Copyright (C) 1994-2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <_itoa.h>
+
+char *
+_itoa_word (_ITOA_WORD_TYPE value, char *buflim,
+	    unsigned int base, int upper_case)
+{
+  const char *digits = (upper_case
+			? _itoa_upper_digits
+			: _itoa_lower_digits);
+
+  switch (base)
+    {
+#define SPECIAL(Base)							      \
+    case Base:								      \
+      do								      \
+	*--buflim = digits[value % Base];				      \
+      while ((value /= Base) != 0);					      \
+      break
+
+      SPECIAL (10);
+      SPECIAL (16);
+      SPECIAL (8);
+    default:
+      do
+	*--buflim = digits[value % base];
+      while ((value /= base) != 0);
+    }
+  return buflim;
+}
+#undef SPECIAL
+
+char *
+_fitoa_word (_ITOA_WORD_TYPE value, char *buf, unsigned int base,
+	     int upper_case)
+{
+  char tmpbuf[sizeof (value) * 4];	      /* Worst case length: base 2.  */
+  char *cp = _itoa_word (value, tmpbuf + sizeof (value) * 4, base, upper_case);
+  while (cp < tmpbuf + sizeof (value) * 4)
+    *buf++ = *cp++;
+  return buf;
+}
diff --git a/stdio-common/_itoa.c b/stdio-common/_itoa.c
index 51c3ab9..08859f0 100644
--- a/stdio-common/_itoa.c
+++ b/stdio-common/_itoa.c
@@ -162,38 +162,6 @@ const struct base_table_t _itoa_base_table[] attribute_hidden =
 };
 #endif
 
-#if IS_IN (libc)
-char *
-_itoa_word (_ITOA_WORD_TYPE value, char *buflim,
-	    unsigned int base, int upper_case)
-{
-  const char *digits = (upper_case
-			? _itoa_upper_digits
-			: _itoa_lower_digits);
-
-  switch (base)
-    {
-#define SPECIAL(Base)							      \
-    case Base:								      \
-      do								      \
-	*--buflim = digits[value % Base];				      \
-      while ((value /= Base) != 0);					      \
-      break
-
-      SPECIAL (10);
-      SPECIAL (16);
-      SPECIAL (8);
-    default:
-      do
-	*--buflim = digits[value % base];
-      while ((value /= base) != 0);
-    }
-  return buflim;
-}
-#undef SPECIAL
-#endif /* IS_IN (libc) */
-
-
 #if _ITOA_NEEDED
 char *
 _itoa (unsigned long long int value, char *buflim, unsigned int base,
@@ -460,17 +428,6 @@ _itoa (unsigned long long int value, char *buflim, unsigned int base,
 }
 #endif
 
-char *
-_fitoa_word (_ITOA_WORD_TYPE value, char *buf, unsigned int base,
-	     int upper_case)
-{
-  char tmpbuf[sizeof (value) * 4];	      /* Worst case length: base 2.  */
-  char *cp = _itoa_word (value, tmpbuf + sizeof (value) * 4, base, upper_case);
-  while (cp < tmpbuf + sizeof (value) * 4)
-    *buf++ = *cp++;
-  return buf;
-}
-
 #if _ITOA_NEEDED
 char *
 _fitoa (unsigned long long value, char *buf, unsigned int base, int upper_case)