Initial commit

Author: fornwall

.gitignore

@@ -0,0 +1,3 @@
+*.so
+*.o
+*-actual

Makefile

@@ -0,0 +1,16 @@
+libtermux-exec.so: termux-exec.c
+	$(CC) $(CFLAGS) -Wall -Wextra -Oz termux-exec.c -shared -fPIC -o libtermux-exec.so
+
+install: libtermux-exec.so
+	install libtermux-exec.so $(PREFIX)/lib/libtermux-exec.so
+
+uninstall:
+	rm -f $(PREFIX)/lib/libtermux-exec.so
+
+test: libtermux-exec.so
+	@LD_PRELOAD=${CURDIR}/libtermux-exec.so ./run-tests.sh
+
+clean:
+	rm -f libtermux-exec.so tests/*-actual
+
+.PHONY: clean install test uninstall

README.md

@@ -1,2 +1,24 @@
 # termux-exec
-A execve() wrapper to fix problem with shebangs.
+A `execve()` wrapper to fix problem with shebangs when running in Termux
+
+# Problem
+A lot of Linux software is written with the assumption that `/bin/sh`, `/usr/bin/env`
+and similar file exists. This is not the case on Android where neither `/bin/` or `/usr/`
+exists.
+
+When building packages for Termux those hard-coded assumptions are patched away, but this
+does not help with installing scripts and programs during runtime.
+
+# Solution
+Create an `execve()` wrapper that rewrites calls to execute files under `/bin/` and `/usr/bin`
+into the matching Termux executables under `$PREFIX/bin/` and injecat that into processes
+using `LD_PRELOAD`.
+
+# How to install
+1. Install with `pkg install termux-exec`.
+2. Exit your current session and start a new one.
+3. From now on shebangs such as `/bin/sh` and `/usr/bin/env python` should work.
+
+# Where is LD_PRELOAD set?
+The `$PREFIX/bin/login` program which is used to create new Termux sessions checks for
+`$PREFIX/lib/libtermux-exec.so` and if so sets up `LD_PRELOAD` before launching the login shell.

run-tests.sh

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+#!/data/data/com.termux/files/usr/bin/bash
+
+set -u
+
+for f in tests/*.sh; do
+	printf "Running $f..."
+
+	EXPECTED_FILE=$f-expected
+	ACTUAL_FILE=$f-actual
+
+	rm -f $ACTUAL_FILE
+	$f $ACTUAL_FILE > $ACTUAL_FILE
+
+	if cmp --silent $ACTUAL_FILE $EXPECTED_FILE; then
+		printf " OK\n"
+	else
+		printf " FAILED - compare expected $EXPECTED_FILE with ${ACTUAL_FILE}\n"
+	fi
+done
+

termux-exec.c

@@ -0,0 +1,95 @@
+#include <dlfcn.h>
+#include <libgen.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+const char* termux_rewrite_executable(const char* filename, char* buffer, int buffer_len)
+{
+	strcpy(buffer, "/data/data/com.termux/files/usr/bin/");
+	char* bin_match = strstr(filename, "/bin/");
+	if (bin_match == filename || bin_match == (filename + 4)) {
+		// We have either found "/bin/" at the start of the string or at
+		// "/xxx/bin/". Take the path after that.
+		strncpy(buffer + 36, bin_match + 5, buffer_len - 37);
+		filename = buffer;
+	}
+	return filename;
+}
+
+// Examples:
+// [1] "#!/bin/sh" should exec "$PREFIX" without argument.
+// [2] "#! /bin/sh" should exec "$PREFIX" without argument.
+// [3] "#!/bin/sh a" should exec "$PREFIX" with single argument "a"
+// [4] "#! /bin/sh   a    " should exec "$PREFIX" with single argument "a"
+// [5] "#! /bin/sh   a  b " should exec "$PREFIX" with single argument "a  b"
+int execve(const char* filename, char* const* argv, char *const envp[])
+{
+	int fd = -1;
+
+	char filename_buffer[512];
+	filename = termux_rewrite_executable(filename, filename_buffer, sizeof(filename_buffer));
+
+	fd = open(filename, O_RDONLY);
+	if (fd == -1) goto final;
+
+	// execve(2): "A maximum line length of 127 characters is allowed
+	// for the first line in a #! executable shell script."
+	char shebang[128];
+	ssize_t read_bytes = read(fd, shebang, sizeof(shebang) - 1);
+	if (read_bytes < 5 || !(shebang[0] == '#' && shebang[1] == '!')) goto final;
+
+	shebang[read_bytes] = 0;
+	char* newline_location = strchr(shebang, '\n');
+	if (newline_location == NULL) goto final;
+
+	// Strip whitespace at end of shebang:
+	while (*(newline_location - 1) == ' ') newline_location--;
+
+	// Null-terminate the shebang line:
+	*newline_location = 0;
+
+	// Skip whitespace to find interpreter start:
+	char* interpreter = shebang + 2;
+	while (*interpreter == ' ') interpreter++;
+	if (interpreter == newline_location) goto final;
+
+	char* arg = NULL;
+	char* whitespace_pos = strchr(interpreter, ' ');
+	if (whitespace_pos != NULL) {
+		// Null-terminate the interpreter string.
+		*whitespace_pos = 0;
+
+		// Find start of argument:
+		arg = whitespace_pos + 1;;
+		while (*arg != 0 && *arg == ' ') arg++;
+		if (arg == newline_location) {
+			// Only whitespace after interpreter.
+			arg = NULL;
+		}
+	}
+
+	const char* new_argv[4];
+	new_argv[0] = basename(interpreter);
+
+	char interp_buf[512];
+	const char* new_interpreter = termux_rewrite_executable(interpreter, interp_buf, sizeof(interp_buf));
+
+	if (arg) {
+		new_argv[1] = arg;
+		new_argv[2] = filename;
+		new_argv[3] = NULL;
+	} else {
+		new_argv[1] = filename;
+		new_argv[2] = NULL;
+	}
+
+	filename = new_interpreter;
+	argv = (char**) new_argv;
+
+final:
+	if (fd != -1) close(fd);
+	int (*real_execve)(const char*, char *const[], char *const[]) = dlsym(RTLD_NEXT, "execve");
+	return real_execve(filename, argv, envp);
+}

tests/args-with-spaces.sh

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+#!/bin/echo     hello  world   bye 

tests/args-with-spaces.sh-expected

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+hello  world   bye tests/args-with-spaces.sh

tests/initial-whitespace.sh

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+# !/bin/sh
+
+echo hi2

tests/initial-whitespace.sh-expected

@@ -0,0 +1 @@
+hi2

tests/simple.sh

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+#!/bin/sh
+
+echo hi

tests/simple.sh-expected

@@ -0,0 +1 @@
+hi

tests/usr-bin-env.sh

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+#!/usr/bin/env sh
+
+echo hello-user-bin-env-sh

tests/usr-bin-env.sh-expected

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+hello-user-bin-env-sh