linux-kernel - Re: [PATCH 1/4] signalfd: add ability to return siginfo in a raw format

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Date:	Mon, 24 Dec 2012 17:53:06 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, criu@...nvz.org,
	linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: Re: [PATCH 1/4] signalfd: add ability to return siginfo in a raw
	format

On 12/24, Andrey Vagin wrote:
>
> signalfd should be called with the flag SFD_RAW for that.
>
> signalfd_siginfo is not full for siginfo with a negative si_code.
> copy_siginfo_to_user() is copied a full siginfo to user-space, if
> si_code is negative.  signalfd_copyinfo() doesn't do that and can't be
> expanded, because it has not compatiable format with siginfo_t.

Yes, but otoh perhaps we should change (fix) signalfd_siginfo/copyinfo,
its "default" case makes no sense if si_code < 0.

> Another problem is that a constant __SI_* is removed from si_code.

OK, so you add the additional put_user(kinfo->si_code). Again, in
this case we can extend signalfd_siginfo perhaps...

Anyway, the patch doesn't look right.

> +static int signalfd_copy_raw_info(struct signalfd_siginfo __user *siginfo,
> +					siginfo_t *kinfo)
> +{
> +	siginfo_t *uinfo = (siginfo_t *) siginfo;
> +	int err;
> +
> +	BUILD_BUG_ON(sizeof(siginfo_t) != sizeof(struct signalfd_siginfo));
> +
> +	err = __clear_user(uinfo, sizeof(*uinfo));
> +	err |= copy_siginfo_to_user(uinfo, kinfo);

This probably needs copy_siginfo_to_user32() if is_compat_task...

> +	err |= __put_user(kinfo->si_code, &uinfo->si_code);

__put_user() is not safe? This allows to write to the kernel memory.

> @@ -286,6 +308,11 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, user_mask,
>  				       O_RDWR | (flags & (O_CLOEXEC | O_NONBLOCK)));
>  		if (ufd < 0)
>  			kfree(ctx);
> +		else if (flags & SFD_RAW) {
> +			struct fd f = fdget(ufd);
> +			f.file->f_flags |= flags & SFD_RAW;

Well, but this is racy. How we can know that fdget(ufd) still
points to the same file created by anon_inode_getfd? Not to
mention f.file can be NULL.

Another CLONE_FILES thread can do close() right after fd_install().
And it can also do dup3().

Oleg.

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