* Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Fri, Dec 14, 2012 at 6:49 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, Dec 14, 2012 at 03:14:50AM -0800, Andy Lutomirski wrote:
> >
> >> > Wait a minute. get_user_pages() relies on ->mmap_sem being held. Unless
> >> > I'm seriously misreading your patch it removes that protection. And yes,
> >> > I'm aware of execve-related exception; it's in special circumstances -
> >> > bprm->mm is guaranteed to be not shared (and we need to rearchitect that
> >> > area anyway, but that's a separate story).
> >>
> >> Unless I completely screwed up the patch, ->mmap_sem is still held for
> >> read (it's downgraded from write). It's just not held for write
> >> anymore.
> >
> > Huh? I'm talking about the call of get_user_pages() in aio_setup_ring().
> > With your patch it's done completely outside of ->mmap_sem, isn't it?
>
> Oh, /that/ call to get_user_pages. That would qualify as screwing up...
>
> Since dropping and reacquiring mmap_sem there is probably a
> bad idea there, I'll rework this and post a v2.
It probably does not matter much, as aio_setup() is an utter
slowpath, but I suspect you could still use the downgrading
variant of do_mmap_pgoff_unlock() here too:
int downgraded = 0;
...
down_write(&ctx->mm->mmap_sem);
/*
* XXX: If MCL_FUTURE is set, this will hold mmap_sem for write for
* longer than necessary.
*/
info->mmap_base = do_mmap_pgoff_helper(NULL, 0, info->mmap_size,
PROT_READ|PROT_WRITE,
MAP_ANONYMOUS|MAP_PRIVATE, 0, &downgraded);
if (IS_ERR((void *)info->mmap_base)) {
up_read_write(&ctx->mm->mmap_sem, downgraded);
info->mmap_size = 0;
aio_free_ring(ctx);
return -EAGAIN;
}
dprintk("mmap address: 0x%08lx\n", info->mmap_base);
info->nr_pages = get_user_pages(current, ctx->mm,
info->mmap_base, nr_pages,
1, 0, info->ring_pages, NULL);
up_read_write(&ctx->mm->mmap_sem, downgraded);
Where up_read_write(lock, read) is a new primitive/wrapper that
does the up_read()/up_write() depending on the value of
'downgraded'.
Thanks,
Ingo
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