From 50e19119064cdab850e74f5ee168df73e2b0061e Mon Sep 17 00:00:00 2001
From: Fredrik Fornwall <fredrik@fornwall.net>
Date: Tue, 8 Oct 2024 11:41:38 +0200
Subject: [PATCH] Hook realpath(3) for /proc/self/exe

---
 Makefile                   |  7 +++++--
 src/termux-realpath.c      | 21 +++++++++++++++++++++
 tests/realpath.c           | 20 ++++++++++++++++++++
 tests/realpath.sh          | 27 +++++++++++++++++++++++++++
 tests/realpath.sh-expected |  1 +
 5 files changed, 74 insertions(+), 2 deletions(-)
 create mode 100644 src/termux-realpath.c
 create mode 100644 tests/realpath.c
 create mode 100755 tests/realpath.sh
 create mode 100644 tests/realpath.sh-expected

diff --git a/Makefile b/Makefile
index 82f61f8..1ce6c7c 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,10 @@
 CC ?= clang
 TERMUX_BASE_DIR ?= /data/data/com.termux/files
 CFLAGS += -Wall -Wextra -Werror -Wshadow -fvisibility=hidden -std=c23 -D__USE_GNU
-C_SOURCE := src/termux-exec.c src/exec-variants.c src/termux-readlink.c
+C_SOURCE := src/termux-exec.c src/exec-variants.c src/termux-readlink.c src/termux-realpath.c
 CLANG_FORMAT := clang-format --sort-includes --style="{ColumnLimit: 120}" $(C_SOURCE) tests/fexecve.c tests/system-uname.c tests/print-argv0.c tests/popen.c
 CLANG_TIDY ?= clang-tidy
-TEST_BINARIES = tests/execl tests/exec-directory tests/fexecve tests/popen tests/system-uname tests/readlink-proc-self-exe
+TEST_BINARIES = tests/execl tests/exec-directory tests/fexecve tests/popen tests/system-uname tests/readlink-proc-self-exe tests/realpath
 
 ifeq ($(SANITIZE),1)
   CFLAGS += -O1 -g -fsanitize=address -fno-omit-frame-pointer
@@ -39,6 +39,9 @@ tests/system-uname: tests/system-uname.c
 tests/readlink-proc-self-exe: tests/readlink-proc-self-exe.c
 	$(CC) $(CFLAGS) -DTERMUX_BASE_DIR=\"$(TERMUX_BASE_DIR)\" $< -o $@
 
+tests/realpath: tests/realpath.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 $@
 
diff --git a/src/termux-realpath.c b/src/termux-realpath.c
new file mode 100644
index 0000000..3d9ec1c
--- /dev/null
+++ b/src/termux-realpath.c
@@ -0,0 +1,21 @@
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+__attribute__((visibility("default"))) char *_Nullable realpath(const char *_Nonnull path, char *_Nullable resolved) {
+  char *(*orig_realpath)(const char *, char *);
+  orig_realpath = dlsym(RTLD_NEXT, "realpath");
+
+  if (strcmp(path, "/proc/self/exe") == 0) {
+    const char *termux_self_exe = getenv("TERMUX_EXEC__PROC_SELF_EXE");
+    if (termux_self_exe) {
+      path = termux_self_exe;
+    }
+  }
+
+  return orig_realpath(path, resolved);
+}
diff --git a/tests/realpath.c b/tests/realpath.c
new file mode 100644
index 0000000..4ca5d72
--- /dev/null
+++ b/tests/realpath.c
@@ -0,0 +1,20 @@
+#define _POSIX_C_SOURCE 1
+#define _XOPEN_SOURCE 500
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv) {
+  if (argc != 2) {
+    fprintf(stderr, "Wrong arguments\n");
+  }
+  char buf[PATH_MAX];
+  char *res = realpath(argv[1], buf);
+  if (res) {
+    printf("%s\n", buf);
+  } else {
+    perror("realpath()");
+    exit(EXIT_FAILURE);
+  }
+  return 0;
+}
diff --git a/tests/realpath.sh b/tests/realpath.sh
new file mode 100755
index 0000000..699b78b
--- /dev/null
+++ b/tests/realpath.sh
@@ -0,0 +1,27 @@
+TMPDIR=$(mktemp -d)
+
+cp tests/realpath $TMPDIR
+
+cd $TMPDIR
+
+ACTUAL_PATH_TO_SELF=$(./realpath /proc/self/exe)
+EXPECTED_PATH_TO_SELF=$TMPDIR/realpath
+
+if [ "$ACTUAL_PATH_TO_SELF" != "$EXPECTED_PATH_TO_SELF" ]; then
+  echo "ERROR(1): Expected '$EXPECTED_PATH_TO_SELF', was '$ACTUAL_PATH_TO_SELF'"
+  exit 1
+fi
+
+ACTUAL_PATH_TO_SELF=$(./realpath /$TMPDIR)
+EXPECTED_PATH_TO_SELF=$TMPDIR
+
+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
+
+cd - > /dev/null
+
+rm -rf $TMPDIR
+
+echo ok
diff --git a/tests/realpath.sh-expected b/tests/realpath.sh-expected
new file mode 100644
index 0000000..9766475
--- /dev/null
+++ b/tests/realpath.sh-expected
@@ -0,0 +1 @@
+ok