Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Sep 1, 2021 at 1:09 AM 'Michal Hocko' via kernel-team
<kernel-team@android.com> wrote:
>
> On Fri 27-08-21 12:18:57, Suren Baghdasaryan wrote:
> [...]
> > Userspace can set the name for a region of memory by calling
> > prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, start, len, (unsigned long)name);
> > Setting the name to NULL clears it.
>
> Maybe I am missing this part but I do not see this being handled
> anywhere.

It's handled in replace_vma_anon_name(). When name==NULL we call
free_vma_anon_name() which frees and resets anon_name pointer. Except
that, as you noticed, the check after strndup_user() will prevent NULL
to be passed here. I forgot to test this case after conversion to
strndup_user() and missed this important point. Thanks for pointing it
out. Will fix and retest.

>
> [...]
> > @@ -3283,5 +3283,16 @@ static inline int seal_check_future_write(int seals, struct vm_area_struct *vma)
> >       return 0;
> >  }
> >
> > +#ifdef CONFIG_ADVISE_SYSCALLS
> > +int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
> > +                       unsigned long len_in, const char *name);
> > +#else
> > +static inline int
> > +madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
> > +                   unsigned long len_in, const char *name) {
> > +     return 0;
> > +}
> > +#endif
>
> You want to make this depend on CONFIG_PROC_FS.

Ack.

>
> [...]
> > +#ifdef CONFIG_MMU
> > +
> > +#define ANON_VMA_NAME_MAX_LEN        64
> > +
> > +static int prctl_set_vma(unsigned long opt, unsigned long addr,
> > +                      unsigned long size, unsigned long arg)
> > +{
> > +     struct mm_struct *mm = current->mm;
> > +     char *name, *pch;
> > +     int error;
> > +
> > +     switch (opt) {
> > +     case PR_SET_VMA_ANON_NAME:
> > +             name = strndup_user((const char __user *)arg,
> > +                                 ANON_VMA_NAME_MAX_LEN);
> > +
> > +             if (IS_ERR(name))
> > +                     return PTR_ERR(name);
>
> unless I am missing something NULL name would lead to an error rather
> than a name clearing as advertised above.

Correct, I missed that. Will fix.

>
> > +
> > +             for (pch = name; *pch != '\0'; pch++) {
> > +                     if (!isprint(*pch)) {
> > +                             kfree(name);
> > +                             return -EINVAL;
> > +                     }
> > +             }
> > +
> > +             mmap_write_lock(mm);
> > +             error = madvise_set_anon_name(mm, addr, size, name);
> > +             mmap_write_unlock(mm);
> > +             kfree(name);
> > +             break;
> > +     default:
> > +             error = -EINVAL;
> > +     }
> > +
> > +     return error;
> --
> Michal Hocko
> SUSE Labs
>
> --
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