Apply realpath(3) when hooking readlink("/proc/self/exe")

Author: fornwall

.gitignore

@@ -8,3 +8,4 @@ test-binary
 tests/fexecve
 tests/popen
 tests/system-uname
+tests/readlink-proc-self-exe

Makefile

@@ -27,6 +27,9 @@ tests/popen: tests/popen.c
 tests/system-uname: tests/system-uname.c
 	$(CC) $(CFLAGS) -DTERMUX_BASE_DIR=\"$(TERMUX_BASE_DIR)\" $< -o $@
 
+tests/readlink-proc-self-exe: tests/readlink-proc-self-exe.c
+	$(CC) $(CFLAGS) -DTERMUX_BASE_DIR=\"$(TERMUX_BASE_DIR)\" $< -o $@
+
 $(TERMUX_BASE_DIR)/usr/bin/termux-exec-test-print-argv0: tests/print-argv0.c
 	$(CC) $(CFLAGS) $< -o $@
 
@@ -43,7 +46,7 @@ on-device-tests:
 	make clean
 	ASAN_OPTIONS=symbolize=0,detect_leaks=0 make on-device-tests-internal
 
-on-device-tests-internal: libtermux-exec.so tests/fexecve tests/popen tests/system-uname $(TERMUX_BASE_DIR)/usr/bin/termux-exec-test-print-argv0
+on-device-tests-internal: libtermux-exec.so tests/fexecve tests/popen tests/system-uname tests/readlink-proc-self-exe $(TERMUX_BASE_DIR)/usr/bin/termux-exec-test-print-argv0
 	@LD_PRELOAD=${CURDIR}/libtermux-exec.so ./run-tests.sh
 
 format:

src/termux-readlink.c

@@ -10,6 +10,11 @@ __attribute__((visibility("default"))) ssize_t readlink(const char *restrict pat
   if (strcmp(pathname, "/proc/self/exe") == 0) {
     const char *termux_self_exe = getenv("TERMUX_EXEC__PROC_SELF_EXE");
     if (termux_self_exe) {
+      char resolved_path_buf[PATH_MAX];
+      char *resolved_path = realpath(termux_self_exe, resolved_path_buf);
+      if (resolved_path) {
+        termux_self_exe = resolved_path_buf;
+      }
       size_t termux_self_exe_len = strlen(termux_self_exe);
       size_t bytes_to_copy = (termux_self_exe_len < bufsiz) ? termux_self_exe_len : bufsiz;
       memcpy(buf, termux_self_exe, bytes_to_copy);

tests/popen.c

@@ -1,10 +1,10 @@
 #include <stdio.h>
 
 int main() {
-    FILE* file = popen("uname", "r");
-    char buffer[101];
-    fscanf(file, "%100s", buffer);
-    pclose(file);
-    printf("%s\n", buffer);
-    return 0;
+  FILE *file = popen("uname", "r");
+  char buffer[101];
+  fscanf(file, "%100s", buffer);
+  pclose(file);
+  printf("%s\n", buffer);
+  return 0;
 }

tests/readlink-proc-self-exe.c

@@ -0,0 +1,18 @@
+#define _DEFAULT_SOURCE
+#include <linux/limits.h>
+#include <stdio.h>
+#include <unistd.h>
+
+int main() {
+  char buf[PATH_MAX + 1];
+
+  ssize_t res = readlink("/proc/self/exe", buf, PATH_MAX);
+  if (res < 0) {
+    perror("readlink()");
+    return 1;
+  }
+  buf[res] = 0;
+  printf("%s\n", buf);
+  return 0;
+}
+

tests/readlink-proc-self-exe.sh

@@ -0,0 +1,33 @@
+TMPDIR=$(mktemp -d)
+
+cp tests/readlink-proc-self-exe $TMPDIR
+
+cd $TMPDIR
+
+ACTUAL_PATH_TO_SELF=$(./readlink-proc-self-exe)
+EXPECTED_PATH_TO_SELF=$TMPDIR/readlink-proc-self-exe
+
+if [ "$ACTUAL_PATH_TO_SELF" != "$EXPECTED_PATH_TO_SELF" ]; then
+  echo "ERROR(1): Expected '$EXPECTED_PATH_TO_SELF', was '$ACTUAL_PATH_TO_SELF'"
+  exit 1
+fi
+
+ln -s readlink-proc-self-exe symlinked-binary
+ACTUAL_PATH_TO_SELF=$(./symlinked-binary)
+if [ "$ACTUAL_PATH_TO_SELF" != "$EXPECTED_PATH_TO_SELF" ]; then
+  echo "ERROR(2): Expected '$EXPECTED_PATH_TO_SELF', was '$ACTUAL_PATH_TO_SELF'"
+  exit 1
+fi
+
+ln -s symlinked-binary nested-symlinked-binary
+ACTUAL_PATH_TO_SELF=$(./nested-symlinked-binary)
+if [ "$ACTUAL_PATH_TO_SELF" != "$EXPECTED_PATH_TO_SELF" ]; then
+  echo "ERROR(3): Expected '$EXPECTED_PATH_TO_SELF', was '$ACTUAL_PATH_TO_SELF'"
+  exit 1
+fi
+
+cd - > /dev/null
+
+rm -rf $TMPDIR
+
+echo ok

tests/readlink-proc-self-exe.sh-expected

@@ -0,0 +1 @@
+ok