[PATCH] uml: make execvp safe for our usage

Reimplement execvp for our purposes - after we call fork() it is fundamentally
unsafe to use the kernel allocator - current is not valid there.  So we simply
pass to our modified execvp() a preallocated buffer.  This fixes a real bug
and works very well in testing (I've seen indirectly warning messages from the
forked thread - they went on the pipe connected to its stdout and where read
as a number by UML, when calling read_output().  I verified the obtained
number corresponded to "BUG:").

The added use of __cant_sleep() is not a new bug since __cant_sleep() is
already used in the same function - passing an atomicity parameter would be
better but it would require huge change, stating that this function must not
be called in atomic context and can sleep is a better idea (will make sure of
this gradually).

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 4 deletions

diff --git a/arch/um/os-Linux/helper.c b/arch/um/os-Linux/helper.c
index d13299cfa318..c7ad6306e22f 100644
--- a/arch/um/os-Linux/helper.c
+++ b/arch/um/os-Linux/helper.c
@@ -8,18 +8,21 @@
 #include <unistd.h>
 #include <errno.h>
 #include <sched.h>
+#include <limits.h>
 #include <sys/signal.h>
 #include <sys/wait.h>
 #include "user.h"
 #include "kern_util.h"
 #include "user_util.h"
 #include "os.h"
+#include "um_malloc.h"
 
 struct helper_data {
         void (*pre_exec)(void*);
         void *pre_data;
         char **argv;
         int fd;
+        char *buf;
 };
 
 /* Debugging aid, changed only from gdb */
@@ -41,9 +44,8 @@ static int helper_child(void *arg)
         }
         if (data->pre_exec != NULL)
                 (*data->pre_exec)(data->pre_data);
-        execvp(argv[0], argv);
-        errval = -errno;
-        printk("helper_child - execve of '%s' failed - errno = %d\n", argv[0], errno);
+        errval = execvp_noalloc(data->buf, argv[0], argv);
+        printk("helper_child - execvp of '%s' failed - errno = %d\n", argv[0], -errval);
         os_write_file(data->fd, &errval, sizeof(errval));
         kill(os_getpid(), SIGKILL);
         return 0;
@@ -84,11 +86,13 @@ int run_helper(void (*pre_exec)(void *), void *pre_data, char **argv,
         data.pre_data = pre_data;
         data.argv = argv;
         data.fd = fds[1];
+        data.buf = __cant_sleep() ? um_kmalloc_atomic(PATH_MAX) :
+                                        um_kmalloc(PATH_MAX);
         pid = clone(helper_child, (void *) sp, CLONE_VM | SIGCHLD, &data);
         if (pid < 0) {
                 ret = -errno;
                 printk("run_helper : clone failed, errno = %d\n", errno);
-                goto out_close;
+                goto out_free2;
         }
 
         close(fds[1]);
@@ -109,6 +113,8 @@ int run_helper(void (*pre_exec)(void *), void *pre_data, char **argv,
                 CATCH_EINTR(waitpid(pid, NULL, 0));
         }
 
+out_free2:
+        kfree(data.buf);
 out_close:
         if (fds[1] != -1)
                 close(fds[1]);