Rework immutability and overlays

overlayfs-tools/sh.c

@@ -0,0 +1,98 @@
+#define _GNU_SOURCE
+#include <stdarg.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <time.h>
+#include "sh.h"
+
+char * vars[NUM_VARS];
+const char * var_names[NUM_VARS] = {
+    "LOWERDIR",
+    "UPPERDIR",
+    "MOUNTDIR",
+    "LOWERNEW",
+    "UPPERNEW",
+};
+
+int quote(const char *filename, FILE *output);
+
+FILE* create_shell_script(char *tmp_path_buffer) {
+    int tmp_file = mkstemps(tmp_path_buffer, 3); // the 3 is for suffix length (".sh")
+    if (tmp_file < 0) { return NULL; }
+    fchmod(tmp_file, S_IRWXU); // chmod to 0700
+    FILE* f = fdopen(tmp_file, "w");
+    if (f == NULL) { return NULL; }
+    fprintf(f, "#!/usr/bin/env bash\n");
+    fprintf(f, "set -x\n");
+    time_t rawtime;
+    time (&rawtime);
+    fprintf(f, "# This shell script is generated by overlayfs-tools on %s\n", ctime (&rawtime));
+    for (int i=0; i < NUM_VARS; i++) {
+        if (vars[i]) {
+            fprintf(f, "%s=", var_names[i]);
+            if (quote(vars[i], f) < 0) { return NULL; }
+            if (fputc('\n', f) == EOF) { return NULL; }
+	}
+    }
+    // Non-empty *NEW vars make a backup copy and override *DIR vars
+    if (vars[LOWERNEW]) {
+        fprintf(f, "rm -rf \"$LOWERNEW\"\n");
+        fprintf(f, "cp -a \"$LOWERDIR\" \"$LOWERNEW\"\n");
+        fprintf(f, "LOWERDIR=");
+        if (quote(vars[LOWERNEW], f) < 0) { return NULL; }
+        if (fputc('\n', f) == EOF) { return NULL; }
+    }
+    if (vars[UPPERNEW]) {
+        fprintf(f, "rm -rf \"$UPPERNEW\"\n");
+        fprintf(f, "cp -a \"$UPPERDIR\" \"$UPPERNEW\"\n");
+        fprintf(f, "UPPERDIR=");
+        if (quote(vars[UPPERNEW], f) < 0) { return NULL; }
+        if (fputc('\n', f) == EOF) { return NULL; }
+    }
+    return f;
+}
+
+int quote(const char *filename, FILE *output) {
+    if (fputc('\'', output) == EOF) { return -1; }
+    for (const char *s = filename; *s != '\0'; s++) {
+        if (*s == '\'') {
+            if (fprintf(output, "'\"'\"'") < 0) { return -1; }
+        } else {
+            if (fputc(*s, output) == EOF) { return -1; }
+        }
+    }
+    if (fputc('\'', output) == EOF) { return -1; }
+    return 0;
+}
+
+int substitue(char what, const char *filename, FILE *output) {
+    int i;
+    for (i=0; i < NUM_VARS; i++)
+        if (vars[i] && var_names[i][0] == what)
+	    break;
+    if (i == NUM_VARS) { return -1; }
+    // filename prefix must match the var value
+    int prefix = strlen(vars[i]);
+    if (strncmp(filename, vars[i], prefix)) { return -1; }
+    filename += prefix;
+    fprintf(output, "\"$%s\"", var_names[i]);
+    return quote(filename, output);
+}
+
+int command(FILE *output, const char *command_format, ...) {
+    va_list arg;
+    va_start(arg, command_format);
+    for (size_t i = 0; command_format[i] != '\0'; i++) {
+        if (command_format[i] == '%') {
+            const char *s = va_arg(arg, char *);
+            if (substitue(command_format[++i], s, output) < 0) { return -1; }
+        } else {
+            if (fputc(command_format[i], output) == EOF) { return -1; }
+        }
+    }
+    va_end(arg);
+    if (fputc('\n', output) == EOF) { return -1; }
+    return 0;
+}