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Date:   Fri, 7 Aug 2020 14:36:09 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Chris Palmer <palmer@google.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Christoph Hellwig <hch@lst.de>,
        Matt Denton <mpdenton@google.com>,
        Robert Sesek <rsesek@google.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Shuah Khan <shuah@kernel.org>, Tycho Andersen <tycho@tycho.ws>,
        Will Deacon <will@kernel.org>, Will Drewry <wad@chromium.org>,
        Yonghong Song <yhs@fb.com>
Subject: Re: wine fails to start with seccomp updates for v5.9-rc1
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On Fri, Aug 07, 2020 at 08:48:46AM -0700, Linus Torvalds wrote:
> On Fri, Aug 7, 2020 at 8:19 AM Alex Xu (Hello71) <alex_y_xu@yahoo.ca> wrote:
> >
> > On Linus' master, wine fails to start with the following error:
> >
> > wine client error:0: write: Bad file descriptor
> >
> > This issue is not present on 5.8. It appears to be caused by failure to
> > write to a pipe FD received via SCM_RIGHTS. Therefore, I tried reverting
> > 9ecc6ea491f0, which resolved the issue.
> 
> Would you mind trying to bisect exactly where it happens?
> 

This report [1] seemed related and pointed out at c0029de50982 ("net/scm:
Regularize compat handling of scm_detach_fds()"). The use of CMSG_USER_DATA
instead of CMSG_COMPAT_DATA seems fishy.

Alex, can you try applying the patch below?

Cascardo.

[1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-August/216156.html

> I don't think any of the commits in that pull are supposed to change
> semantics, and while reverting the whole merge shows that yes, that's
> what brought in the problems, it would be good to pinpoint just which
> change breaks so that we can fix just that thing.
> 
> Kees, ideas?
> 
>                  Linus

---
diff --git a/net/compat.c b/net/compat.c
index 703acb51c698..95ce707a30a3 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -294,7 +294,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
 		(struct compat_cmsghdr __user *)msg->msg_control;
 	unsigned int o_flags = (msg->msg_flags & MSG_CMSG_CLOEXEC) ? O_CLOEXEC : 0;
 	int fdmax = min_t(int, scm_max_fds_compat(msg), scm->fp->count);
-	int __user *cmsg_data = CMSG_USER_DATA(cm);
+	int __user *cmsg_data = CMSG_COMPAT_DATA(cm);
 	int err = 0, i;
 
 	for (i = 0; i < fdmax; i++) {