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    SubjectRe: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3
    On 03/08, Cyrill Gorcunov wrote:
    >
    > Hi Oleg, could you please take a look once you get a minute (no urgency).

    Add Matt. I won't touch the text below to keep the patch intact.

    With this change

    down_write(&mm->mmap_sem);
    if (mm->num_exe_file_vmas) {
    fput(mm->exe_file);
    mm->exe_file = exe_file;
    exe_file = NULL;
    } else
    set_mm_exe_file(mm, exe_file);
    up_write(&mm->mmap_sem);

    I simply do not understand what mm->num_exe_file_vmas means after
    PR_SET_MM_EXE_FILE.

    I think that you should do

    down_write(&mm->mmap_sem);
    if (mm->num_exe_file_vmas) {
    fput(mm->exe_file);
    mm->exe_file = exe_file;
    exe_file = NULL;
    }
    up_write(&mm->mmap_sem);

    to keep the current "mm->exe_file goes away after the final
    unmap(MAP_EXECUTABLE)" logic.

    OK, may be this doesn't work in c/r case because you are actually
    going to remove the old mappings? But in this case the new exe_file
    will go away anyway, afaics PR_SET_MM_EXE_FILE is called when you
    still have the old mappings.

    And I don't think the unconditional

    down_write(&mm->mmap_sem);
    set_mm_exe_file(mm, exe_file);
    up_write(&mm->mmap_sem);

    is 100% right, this clears ->num_exe_file_vmas. This means that
    (if you still have the old mapping) the new exe_file can go away
    after added_exe_file_vma() + removed_exe_file_vma(). Normally this
    should happen, but afaics this is possible. Note that even, say,
    mprotect() can trigger added_exe_file_vma().

    May be we can do something like

    down_write(&mm->mmap_sem);
    set_mm_exe_file(mm, exe_file);
    // we are cheating anyway, make sure it can never == 0
    // if we have the "old" VM_EXECUTABLE vmas.
    mm->num_exe_file_vmas = LONG_MAX;
    up_write(&mm->mmap_sem);

    I dunno. Matt, could you help?

    > From: Cyrill Gorcunov <gorcunov@openvz.org>
    > Subject: c/r: prctl: Add ability to set new mm_struct::exe_file v3
    >
    > When we do restore we would like to have a way to setup
    > a former mm_struct::exe_file so that /proc/pid/exe would
    > point to the original executable file a process had at
    > checkpoint time.
    >
    > For this the PR_SET_MM_EXE_FILE code is introduced.
    > This option takes a file descriptor which will be
    > set as new /proc/$pid/exe symlink.
    >
    > This feature is available iif CONFIG_CHECKPOINT_RESTORE is set.
    >
    > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    > CC: Oleg Nesterov <oleg@redhat.com>
    > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > CC: Pavel Emelyanov <xemul@parallels.com>
    > CC: Kees Cook <keescook@chromium.org>
    > CC: Tejun Heo <tj@kernel.org>
    > ---
    > include/linux/prctl.h | 1 +
    > kernel/sys.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
    > 2 files changed, 51 insertions(+)
    >
    > Index: linux-2.6.git/include/linux/prctl.h
    > ===================================================================
    > --- linux-2.6.git.orig/include/linux/prctl.h
    > +++ linux-2.6.git/include/linux/prctl.h
    > @@ -118,5 +118,6 @@
    > # define PR_SET_MM_ENV_START 10
    > # define PR_SET_MM_ENV_END 11
    > # define PR_SET_MM_AUXV 12
    > +# define PR_SET_MM_EXE_FILE 13
    >
    > #endif /* _LINUX_PRCTL_H */
    > Index: linux-2.6.git/kernel/sys.c
    > ===================================================================
    > --- linux-2.6.git.orig/kernel/sys.c
    > +++ linux-2.6.git/kernel/sys.c
    > @@ -36,6 +36,8 @@
    > #include <linux/personality.h>
    > #include <linux/ptrace.h>
    > #include <linux/fs_struct.h>
    > +#include <linux/file.h>
    > +#include <linux/mount.h>
    > #include <linux/gfp.h>
    > #include <linux/syscore_ops.h>
    > #include <linux/version.h>
    > @@ -1701,6 +1703,50 @@ static bool vma_flags_mismatch(struct vm
    > (vma->vm_flags & banned);
    > }
    >
    > +static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
    > +{
    > + struct file *exe_file;
    > + struct dentry *dentry;
    > + int err;
    > +
    > + exe_file = fget(fd);
    > + if (!exe_file)
    > + return -EBADF;
    > +
    > + dentry = exe_file->f_path.dentry;
    > +
    > + /*
    > + * Because the original mm->exe_file
    > + * points to executable file, make sure
    > + * this one is executable as well to not
    > + * break "big" picture and proc/pid/exe
    > + * symlink will be still pointing to
    > + * executable one.
    > + */
    > + err = -EACCES;
    > + if (!S_ISREG(dentry->d_inode->i_mode) ||
    > + exe_file->f_path.mnt->mnt_flags & MNT_NOEXEC)
    > + goto exit;
    > +
    > + err = inode_permission(dentry->d_inode, MAY_EXEC);
    > + if (err)
    > + goto exit;
    > +
    > + down_write(&mm->mmap_sem);
    > + if (mm->num_exe_file_vmas) {
    > + fput(mm->exe_file);
    > + mm->exe_file = exe_file;
    > + exe_file = NULL;
    > + } else
    > + set_mm_exe_file(mm, exe_file);
    > + up_write(&mm->mmap_sem);
    > +
    > +exit:
    > + if (exe_file)
    > + fput(exe_file);
    > + return err;
    > +}
    > +
    > static int prctl_set_mm(int opt, unsigned long addr,
    > unsigned long arg4, unsigned long arg5)
    > {
    > @@ -1715,6 +1761,9 @@ static int prctl_set_mm(int opt, unsigne
    > if (!capable(CAP_SYS_RESOURCE))
    > return -EPERM;
    >
    > + if (opt == PR_SET_MM_EXE_FILE)
    > + return prctl_set_mm_exe_file(mm, (unsigned int)addr);
    > +
    > if (addr >= TASK_SIZE)
    > return -EINVAL;
    >
    > @@ -1837,6 +1886,7 @@ static int prctl_set_mm(int opt, unsigne
    >
    > return 0;
    > }
    > +
    > default:
    > error = -EINVAL;
    > goto out;



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