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SubjectRe: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3
On 03/09, Cyrill Gorcunov wrote:
>
> On Fri, Mar 09, 2012 at 04:57:35PM +0400, Cyrill Gorcunov wrote:
> >
> > yeah, thanks, will update.
> >
>
> This one should fit all requirements I hope.

Oh, sorry Cyrill, I simply can't resist...

> +static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> +{
> + struct file *exe_file;
> + struct dentry *dentry;
> + int err;
> +
> + if (mm->num_exe_file_vmas)
> + return -EBUSY;
> +
> + 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 an overall picture.
> + */
> + 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;
> +
> + /*
> + * Setting new mm::exe_file is only allowed
> + * when no VM_EXECUTABLE vma's left. This is
> + * a special C/R case when a restored program
> + * need to change own /proc/$pid/exe symlink.
> + * After this call mm::num_exe_file_vmas become
> + * meaningless. If mm::num_exe_file_vmas will
> + * ever increase back from zero -- this code
> + * needs to be revised, thus WARN_ here, just
> + * to be sure.

To be shure in what??

> + */
> + down_write(&mm->mmap_sem);
> + WARN_ON_ONCE(mm->num_exe_file_vmas);

We already checked it is zero. Yes, it shouldn't grow. But why
do we need another check here?

If it can grow, it can grow after we drop mmap_sem as well and
this would be wrong. So may be we need another WARN_ON() at the
end?

I'd understand if you add something like

WARN_ON(!mm->num_exe_file_vmas && !current->in_exec);

into added_exe_file_vma().

Or

WARN_ON(mm->num_exe_file_vmas <= 0);

into removed_exe_file_vma().

But imho your WARN looks like "OK, I checked it lockless but I
am not sure this is correct".

Oleg.



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