From f272aaa8886e2db3c208df32889ee4e2fe04169b Mon Sep 17 00:00:00 2001
From: Fredrik Fornwall <fredrik@fornwall.net>
Date: Wed, 18 Sep 2024 11:37:36 +0200
Subject: [PATCH] Apply realpath(3) when hooking readlink("/proc/self/exe")

---
 .gitignore                            |  1 +
 Makefile                              |  5 ++++-
 src/termux-readlink.c                 |  5 +++++
 tests/popen.c                         | 12 ++++++------
 tests/readlink-proc-self-exe.c        | 16 ++++++++++++++++
 tests/readlink-proc-self-exe.expected |  1 +
 tests/readlink-proc-self-exe.sh       | 22 ++++++++++++++++++++++
 7 files changed, 55 insertions(+), 7 deletions(-)
 create mode 100644 tests/readlink-proc-self-exe.c
 create mode 100644 tests/readlink-proc-self-exe.expected
 create mode 100755 tests/readlink-proc-self-exe.sh

diff --git a/.gitignore b/.gitignore
index 8caebef..f85e6ed 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,4 @@ test-binary
 tests/fexecve
 tests/popen
 tests/system-uname
+tests/readlink-proc-self-exe
diff --git a/Makefile b/Makefile
index 3fe778b..43f6ca6 100644
--- a/Makefile
+++ b/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:
diff --git a/src/termux-readlink.c b/src/termux-readlink.c
index 63a50d2..9edf2db 100644
--- a/src/termux-readlink.c
+++ b/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);
diff --git a/tests/popen.c b/tests/popen.c
index 5126ed9..a5cbf20 100644
--- a/tests/popen.c
+++ b/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;
 }
diff --git a/tests/readlink-proc-self-exe.c b/tests/readlink-proc-self-exe.c
new file mode 100644
index 0000000..16a1f35
--- /dev/null
+++ b/tests/readlink-proc-self-exe.c
@@ -0,0 +1,16 @@
+#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;
+}
+
diff --git a/tests/readlink-proc-self-exe.expected b/tests/readlink-proc-self-exe.expected
new file mode 100644
index 0000000..9766475
--- /dev/null
+++ b/tests/readlink-proc-self-exe.expected
@@ -0,0 +1 @@
+ok
diff --git a/tests/readlink-proc-self-exe.sh b/tests/readlink-proc-self-exe.sh
new file mode 100755
index 0000000..006a6cd
--- /dev/null
+++ b/tests/readlink-proc-self-exe.sh
@@ -0,0 +1,22 @@
+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
+
+echo ok