Re: [uml-devel] Host panic when UML reads host /dev/shm

[James W McMechan <mcmechanjw@juno.com>]

I have been tracing down the Oops on a vanilla 2.4.22 kernel
This appears to be a Oops with a semaphore held that will
lock out all other accesses to the tmpfs/shmfs filesystem.

I traced it to read_dir in hostfs_user.c by setting
breakpoints on all the hostfs_user.c functions.

Ok I now have a simple test program which Oops
the host kernel instantly (3 times through loop)

This is very strange my attached test program when run
as a unprivileged user Oops the kernel and locks /dev/shm
in general a unprivileged user should not be able to do this

It has taken a while in order to check that the kernel I was
running was without patches (i.e. no skas)
It still Oops on the third loop :(

I have also noted that the tmpfs/shmfs does not have a set
of files like other filesystems under fs/ it seems to be located
in mm/shmem.c instead... very strange

Oh well it is a problem with the vanilla 2.4.22 kernel so
more testing and off to the LKML...

Anybody have a better guess where to send this?

For your Enjoyment the test program

/* by James_McMechan at hotmail com */
/* test program to Oops shmfs usually mounted at /dev/shm */
/* yes it is dumb but unprivileged users should not be able */
/* to Oops the kernel regardless of how dumb the program */
#include <sys/types.h>                                                   
      
#include <dirent.h>
#include <stdio.h>

main()
{
        DIR *dir;
        struct dirent *ent;
        off_t pos = 0;

        do {
                dir = opendir("/dev/shm");
                seekdir(dir, pos);
                ent = readdir(dir);
                if (ent == 0) {
                        printf("end of directory\n");
                        perror("readdir ended with");
                } else {
                        printf("d_name is %s\n",ent->d_name);
                        pos = telldir(dir);
                        if (pos < 0)
                                perror("telldir failed with");
                }
                closedir(dir);
        } while (ent != 0);
}

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