Re: [Qemu-devel] [PATCH] -chroot and -su options.

[Edgar E. Iglesias]

On Wed, Mar 05, 2008 at 12:51:36AM -0600, Rob Landley wrote:
> On Tuesday 04 March 2008 05:22:12 you wrote:
> > On Mon, Mar 03, 2008 at 06:28:22PM -0600, Rob Landley wrote:
> > > Quick and dirty patch to teach qemu application emulation how to chroot
> > > (and drop privs), so you don't have to pollute a target filesystem with
> > > host code, and/or figure out how to build qemu static in order to run a
> > > dynamic binary.
> >
> > Hi Rob,
> >
> > Right, doing the chroot from within qemu avoids the issue with polluting
> > the target/. Thanks for the example.
> >
> > The chroot approach still suffers from the need of initially having higher
> > privileges. Personally, I still prefer the sysroot option and avoid that
> > need but either way helps me.
> >
> > Best regards
> 
> Which sysroot option?  (I may have missed a patch, I'm a month behind on the 
> list.  This is just something I've meant to submit for... about a year, I 
> think.)
> 
> You can also teach a bunch of different qemu syscalls (open, unlink, mmap, 
> exec, fcntl, and 3 dozen others...) to append a prefix to its path, and 
> perhaps try to prevent them from playing games with symlinks or ".." to break 
> out of that subdir.  But that's a much, much, much more extensive/intrusive 
> patch.

Hi,

This is the updated example from my local git of how it could work, it only 
maps absolute paths.
I don't think taking care of relative paths involves much more code but so far 
this behaviour has been enough for me.
The sim simulators in GDB have a similar --sysroot option which I beleive 
behaves very similar (or equal).

Please note that I'm not trying to jail in a program for security purposes, 
just for test and debug purposes.

My original post can be found here:
http://lists.gnu.org/archive/html/qemu-devel/2008-03/msg00027.html

Best regards
-- 
Edgar E. Iglesias
Axis Communications AB

diff --git a/linux-user/main.c b/linux-user/main.c
index 0079c7a..8190dbf 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -1904,6 +1904,7 @@ void usage(void)
            "-h                print this help\n"
            "-g port           wait gdb connection to port\n"
            "-L path           set the elf interpreter prefix (default=%s)\n"
+           "-sysroot          Root for system calls with absolute file-names\n"
            "-s size           set the stack size in bytes (default=%ld)\n"
            "-cpu model        select CPU (-cpu ? for list)\n"
            "-drop-ld-preload  drop LD_PRELOAD for target process\n"
@@ -1943,6 +1944,7 @@ int main(int argc, char **argv)
     int gdbstub_port = 0;
     int drop_ld_preload = 0, environ_count = 0;
     char **target_environ, **wrk, **dst;
+    char *sysroot = NULL;
 
     if (argc <= 1)
         usage();
@@ -2014,6 +2016,8 @@ int main(int argc, char **argv)
             drop_ld_preload = 1;
         } else if (!strcmp(r, "strace")) {
             do_strace = 1;
+        } else if (!strcmp(r, "sysroot")) {
+            sysroot = argv[optind++];
         } else
         {
             usage();
@@ -2030,7 +2034,10 @@ int main(int argc, char **argv)
     memset(info, 0, sizeof(struct image_info));
 
     /* Scan interp_prefix dir for replacement files. */
-    init_paths(interp_prefix);
+    if (sysroot)
+        init_paths(sysroot, 1);
+    else
+        init_paths(interp_prefix, 0);
 
     if (cpu_model == NULL) {
 #if defined(TARGET_I386)
diff --git a/linux-user/path.c b/linux-user/path.c
index 7da0a8b..5b6abc9 100644
--- a/linux-user/path.c
+++ b/linux-user/path.c
@@ -25,6 +25,8 @@ struct pathelem
 };
 
 static struct pathelem *base;
+static int use_sysroot;
+static size_t sysroot_pathlen;
 
 /* First N chars of S1 match S2, and S2 is N chars long. */
 static int strneq(const char *s1, unsigned int n, const char *s2)
@@ -118,7 +120,7 @@ follow_path(const struct pathelem *cursor, const char *name)
     return NULL;
 }
 
-void init_paths(const char *prefix)
+void init_paths(const char *prefix, int sysroot)
 {
     char pref_buf[PATH_MAX];
 
@@ -135,15 +137,25 @@ void init_paths(const char *prefix)
         strcat(pref_buf, prefix);
         free(cwd);
     } else
-        strcpy(pref_buf,prefix + 1);
-
-    base = new_entry("", NULL, pref_buf);
-    base = add_dir_maybe(base);
-    if (base->num_entries == 0) {
-        free (base);
-        base = NULL;
+      strcpy(pref_buf, prefix + 1);
+
+    use_sysroot = sysroot;
+    if (sysroot) {
+        base = malloc(sizeof (*base));
+        sysroot_pathlen = strlen(pref_buf);
+        base->pathname = malloc(sysroot_pathlen + PATH_MAX + 1);
+        base->pathname[0] = '/';
+        memcpy(base->pathname + 1, pref_buf, sysroot_pathlen);
+        sysroot_pathlen++;
     } else {
-        set_parents(base, base);
+        base = new_entry("", NULL, pref_buf);
+        base = add_dir_maybe(base);
+        if (base->num_entries == 0) {
+            free (base);
+            base = NULL;
+        } else {
+            set_parents(base, base);
+        }
     }
 }
 
@@ -155,5 +167,12 @@ const char *path(const char *name)
     if (!base || name[0] != '/')
        return name;
 
-    return follow_path(base, name) ?: name;
+    if (use_sysroot) {
+        size_t name_len;
+        /* Prepend base->pathname to name.  */
+        name_len = strlen(name);
+        memcpy (base->pathname + sysroot_pathlen, name, name_len + 1);
+        return base->pathname;
+    } else
+        return follow_path(base, name) ?: name;
 }
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index b33ad89..9e1b4f4 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -166,7 +166,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
 void gemu_log(const char *fmt, ...) __attribute__((format(printf,1,2)));
 extern CPUState *global_env;
 void cpu_loop(CPUState *env);
-void init_paths(const char *prefix);
+void init_paths(const char *prefix, int sysroot);
 const char *path(const char *pathname);
 char *target_strerror(int err);