Basic implementation of Copy-On-Write

src/GNUmakefile

@@ -55,6 +55,7 @@ OBJECTS += \
 	extension/extension.o	\
 	extension/kompat/kompat.o \
 	extension/fake_id0/fake_id0.o \
+	extension/cow/cow.o \
 	loader/loader-wrapped.o
 
 define define_from_arch.h

src/cli/proot.c

@@ -163,6 +163,12 @@ static int handle_option_k(Tracee *tracee, const Cli *cli UNUSED, const char *va
 	return 0;
 }
 
+static int handle_option_c(Tracee *tracee, const Cli *cli UNUSED, const char *value)
+{
+	(void) initialize_extension(tracee, cow_callback, value);
+	return 0;
+}
+
 static int handle_option_i(Tracee *tracee, const Cli *cli UNUSED, const char *value)
 {
 	(void) initialize_extension(tracee, fake_id0_callback, value);

src/cli/proot.h

@@ -57,6 +57,7 @@ static int handle_option_V(Tracee *tracee, const Cli *cli, const char *value);
 static int handle_option_h(Tracee *tracee, const Cli *cli, const char *value);
 static int handle_option_k(Tracee *tracee, const Cli *cli, const char *value);
 static int handle_option_0(Tracee *tracee, const Cli *cli, const char *value);
+static int handle_option_c(Tracee *tracee, const Cli *cli, const char *value);
 static int handle_option_i(Tracee *tracee, const Cli *cli, const char *value);
 static int handle_option_R(Tracee *tracee, const Cli *cli, const char *value);
 static int handle_option_S(Tracee *tracee, const Cli *cli, const char *value);
@@ -205,15 +206,28 @@ Copyright (C) 2014 STMicroelectronics, licensed under GPL v2 or later.",
 	},
 	{ .class = "Extension options",
 	  .arguments = {
-		{ .name = "-i", .separator = ' ', .value = "string" },
-		{ .name = "--change-id", .separator = '=', .value = "string" },
+		{ .name = "-0", .separator = '\0', .value = NULL },
+		{ .name = "--root-id", .separator = '\0', .value = NULL },
+		{ .name = NULL, .separator = '\0', .value = NULL } },
+	  .handler = handle_option_0,
+	  .description = "Make current user appear as \"root\" and fake its privileges.",
+	  .detail = "\tSome programs will refuse to work if they are not run with \"root\"\n\
+\tprivileges, even if there is no technical reason for that.  This\n\
+\tis typically the case with package managers.  This option allows\n\
+\tusers to bypass this kind of limitation by faking the user/group\n\
+\tidentity, and by faking the success of some operations like\n\
+\tchanging the ownership of files, changing the root directory to\n\
+\t/, ...  Note that this option is quite limited compared to\n\
+\tfakeroot.",
+	},
+	{ .class = "Extension options",
+	  .arguments = {
+		{ .name = "-c", .separator = '\0', .value = NULL },
+		{ .name = "--cow", .separator = '\0', .value = NULL },
 		{ .name = NULL, .separator = '\0', .value = NULL } },
-	  .handler = handle_option_i,
-	  .description = "Make current user and group appear as *string* \"uid:gid\".",
-	  .detail = "\tThis option makes the current user and group appear as uid and\n\
-\tgid.  Likewise, files actually owned by the current user and\n\
-\tgroup appear as if they were owned by uid and gid instead.\n\
-\tNote that the -0 option is the same as -i 0:0.",
+	  .handler = handle_option_c,
+	  .description = "Implement Copy-on-Write semantics",
+	  .detail = "\tThis option implments Copy-on-Write (COW) semantics",
 	},
 	{ .class = "Alias options",
 	  .arguments = {

src/extension/cow/cow.c

@@ -0,0 +1,196 @@
+/*
+ * This file is part of PRoot.
+ *
+ * Copyright (C) 2016 Alejandro Liu
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301 USA.
+ */
+
+#include <stdint.h>        /* intptr_t, */
+#include <stdlib.h>        /* strtoul(3), */
+#include <linux/version.h> /* KERNEL_VERSION, */
+#include <assert.h>        /* assert(3), */
+#include <sys/utsname.h>   /* uname(2), utsname, */
+#include <string.h>        /* str*(3), memcpy(3), */
+#include <talloc.h>        /* talloc_*, */
+#include <fcntl.h>         /* AT_*,  */
+#include <sys/ptrace.h>    /* linux.git:c0a3a20b  */
+#include <errno.h>         /* errno,  */
+#include <linux/auxvec.h>  /* AT_,  */
+#include <linux/futex.h>   /* FUTEX_PRIVATE_FLAG */
+#include <sys/param.h>     /* MIN, */
+#include <sys/mman.h>	   /* mmap, munmap */
+#include <unistd.h>	   /* unlink, close, write, fchown */
+
+#include "extension/extension.h"
+#include "syscall/seccomp.h"
+#include "syscall/sysnum.h"
+#include "syscall/syscall.h"
+#include "syscall/chain.h"
+#include "tracee/tracee.h"
+#include "tracee/reg.h"
+#include "tracee/abi.h"
+#include "tracee/mem.h"
+#include "execve/auxv.h"
+#include "cli/note.h"
+#include "arch.h"
+
+typedef int Config;
+
+/* List of syscalls handled by this extensions.  */
+static FilteredSysnum filtered_sysnums[] = {
+  { PR_open,		FILTER_SYSEXIT },
+  { PR_openat,		FILTER_SYSEXIT },
+  FILTERED_SYSNUM_END,
+};
+
+
+static int check_flags(Tracee *tracee, Reg reg) {
+  int flags;
+  flags = peek_reg(tracee,CURRENT, reg);
+  switch (flags & O_ACCMODE) {
+  case O_WRONLY:
+  case O_RDWR:
+    return 1;
+  }
+  return 0;
+}
+
+static void check_inode_and_copy(Tracee *tracee,char const *name) {
+  struct stat stb;
+  int sfd, nfd;
+  if (stat(name,&stb) == -1) return;
+  if (stb.st_nlink < 2) return;
+  if (-1 == (sfd = open(name,O_RDONLY))) return;
+  if (unlink(name) == -1) {
+    close(sfd);
+    return;
+  }
+  if (-1 == (nfd = open(name,O_CREAT|O_EXCL|O_WRONLY,stb.st_mode))) {
+    note(tracee, ERROR, USER, "Error opening %s",name);
+    close(sfd);
+    return;
+  }
+  void *addr;
+  if ((addr=mmap(NULL,stb.st_size,PROT_READ,MAP_PRIVATE, sfd,0))==MAP_FAILED) {
+    note(tracee, ERROR, USER, "Error mmaping %s",name);
+    close(nfd);
+    close(sfd);
+    return;
+  }
+  if (write(nfd,addr,stb.st_size) != stb.st_size) {
+    note(tracee, ERROR, USER, "Error writing %s",name);
+  }
+  munmap(addr,stb.st_size);
+  close(sfd);
+  fchown(nfd,stb.st_uid,stb.st_gid);
+  close(nfd);
+}
+
+/**
+ * Adjust current @tracee's syscall parameters according to @config.
+ * This function always returns 0.
+ */
+static int handle_sysenter_end(Tracee *tracee, const Config *config UNUSED)
+{
+	word_t sysnum;
+	char old_path[PATH_MAX];
+	int status;
+
+	sysnum = get_sysnum(tracee, ORIGINAL);
+	switch (sysnum) {
+	case PR_open:
+	  if (!check_flags(tracee, SYSARG_2)) return 0;
+	  /* Extract the currrent path. */
+	  status = get_sysarg_path(tracee, old_path, SYSARG_1);
+	  if (status < 0) return status;
+	  //note(tracee, INFO, USER, "PR_open(%s)",old_path);
+	  check_inode_and_copy(tracee,old_path);
+	  return 0;
+	case PR_openat:
+	  if (!check_flags(tracee, SYSARG_3)) return 0;
+	  /* Extract the currrent path. */
+	  status = get_sysarg_path(tracee, old_path, SYSARG_2);
+	  if (status < 0) return status;
+	  //note(tracee, INFO, USER, "PR_openat(%s)",old_path);
+	  check_inode_and_copy(tracee,old_path);
+	  return 0;
+	default:
+		return 0;
+	}
+
+	/* Never reached  */
+	assert(0);
+	return 0;
+
+}
+
+/**
+ * Handler for this @extension.  It is triggered each time an @event
+ * occurred.  See ExtensionEvent for the meaning of @data1 and @data2.
+ */
+int cow_callback(Extension *extension, ExtensionEvent event, intptr_t data1, intptr_t data2 UNUSED)
+{
+	switch (event) {
+	case INITIALIZATION: {
+		Config *config;
+		extension->config = talloc(extension, Config);
+		if (extension->config == NULL)
+			return -1;
+
+		config = talloc_get_type_abort(extension->config, Config);
+		*config = 0;
+		extension->filtered_sysnums = filtered_sysnums;
+		return 0;
+	}
+
+	case INHERIT_PARENT: /* Inheritable for sub reconfiguration ...  */
+		return 1;
+
+	case INHERIT_CHILD: {
+		/* Copy the parent configuration to the child.  The
+		 * structure should not be shared as uid/gid changes
+		 * in one process should not affect other processes.
+		 * This assertion is not true for POSIX threads
+		 * sharing the same group, however Linux threads never
+		 * share uid/gid information.  As a consequence, the
+		 * GlibC emulates the POSIX behavior on Linux by
+		 * sending a signal to all group threads to cause them
+		 * to invoke the system call too.  Finally, PRoot
+		 * doesn't have to worry about clone flags.
+		 */
+
+		Extension *parent = (Extension *) data1;
+		extension->config = talloc_zero(extension, Config);
+		if (extension->config == NULL)
+			return -1;
+
+		memcpy(extension->config, parent->config, sizeof(Config));
+		return 0;
+	}
+
+	case SYSCALL_ENTER_END: {
+		Tracee *tracee = TRACEE(extension);
+		Config *config = talloc_get_type_abort(extension->config, Config);
+
+		return handle_sysenter_end(tracee, config);
+	}
+
+
+	default:
+		return 0;
+	}
+}

src/extension/extension.h

@@ -179,5 +179,5 @@ static inline int notify_extensions(Tracee *tracee, ExtensionEvent event,
 extern int kompat_callback(Extension *extension, ExtensionEvent event, intptr_t d1, intptr_t d2);
 extern int fake_id0_callback(Extension *extension, ExtensionEvent event, intptr_t d1, intptr_t d2);
 extern int care_callback(Extension *extension, ExtensionEvent event, intptr_t d1, intptr_t d2);
-
+extern int cow_callback(Extension *extension, ExtensionEvent event, intptr_t d1, intptr_t d2);
 #endif /* EXTENSION_H */