[PATCH] samples: user-trap: fix strict-aliasing warning

[PATCH] samples: user-trap: fix strict-aliasing warning

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

I started getting warnings for this one file, though I can't see what changed
since it was originally introduced in commit fec7b6690541 ("samples: add an
example of seccomp user trap").

samples/seccomp/user-trap.c: In function 'send_fd':
samples/seccomp/user-trap.c:50:11: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   50 |         *((int *)CMSG_DATA(cmsg)) = fd;
      |          ~^~~~~~~~~~~~~~~~~~~~~~~
samples/seccomp/user-trap.c: In function 'recv_fd':
samples/seccomp/user-trap.c:83:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   83 |         return *((int *)CMSG_DATA(cmsg));
      |                 ~^~~~~~~~~~~~~~~~~~~~~~~

Using a temporary pointer variable avoids the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 samples/seccomp/user-trap.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
index 20291ec6489f..a23fec357b5d 100644
--- a/samples/seccomp/user-trap.c
+++ b/samples/seccomp/user-trap.c
@@ -33,6 +33,7 @@ static int send_fd(int sock, int fd)
 {
 	struct msghdr msg = {};
 	struct cmsghdr *cmsg;
+	int *fd_ptr;
 	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
 	struct iovec io = {
 		.iov_base = &c,
@@ -47,7 +48,8 @@ static int send_fd(int sock, int fd)
 	cmsg->cmsg_level = SOL_SOCKET;
 	cmsg->cmsg_type = SCM_RIGHTS;
 	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
-	*((int *)CMSG_DATA(cmsg)) = fd;
+	fd_ptr = (int *)CMSG_DATA(cmsg);
+	*fd_ptr = fd;
 	msg.msg_controllen = cmsg->cmsg_len;
 
 	if (sendmsg(sock, &msg, 0) < 0) {
@@ -62,6 +64,7 @@ static int recv_fd(int sock)
 {
 	struct msghdr msg = {};
 	struct cmsghdr *cmsg;
+	int *fd_ptr;
 	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
 	struct iovec io = {
 		.iov_base = &c,
@@ -79,8 +82,9 @@ static int recv_fd(int sock)
 	}
 
 	cmsg = CMSG_FIRSTHDR(&msg);
+	fd_ptr = (int *)CMSG_DATA(cmsg);
 
-	return *((int *)CMSG_DATA(cmsg));
+	return *fd_ptr;
 }
 
 static int user_trap_syscall(int nr, unsigned int flags)
-- 
2.39.2

Re: [PATCH] samples: user-trap: fix strict-aliasing warning

[Kees Cook]

On Mon, 12 Feb 2024 12:17:31 +0100, Arnd Bergmann wrote:
> I started getting warnings for this one file, though I can't see what changed
> since it was originally introduced in commit fec7b6690541 ("samples: add an
> example of seccomp user trap").
> 
> samples/seccomp/user-trap.c: In function 'send_fd':
> samples/seccomp/user-trap.c:50:11: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
>    50 |         *((int *)CMSG_DATA(cmsg)) = fd;
>       |          ~^~~~~~~~~~~~~~~~~~~~~~~
> samples/seccomp/user-trap.c: In function 'recv_fd':
> samples/seccomp/user-trap.c:83:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
>    83 |         return *((int *)CMSG_DATA(cmsg));
>       |                 ~^~~~~~~~~~~~~~~~~~~~~~~
> 
> [...]

Applied to for-next/seccomp, thanks!

[1/1] samples: user-trap: fix strict-aliasing warning
      https://git.kernel.org/kees/c/9ad28ca5238d

Take care,

-- 
Kees Cook

Re: [PATCH] samples: user-trap: fix strict-aliasing warning

[Tycho Andersen]

On Mon, Feb 12, 2024 at 10:42:27AM -0800, Kees Cook wrote:
> On Mon, 12 Feb 2024 12:17:31 +0100, Arnd Bergmann wrote:
> > I started getting warnings for this one file, though I can't see what changed
> > since it was originally introduced in commit fec7b6690541 ("samples: add an
> > example of seccomp user trap").
> > 
> > samples/seccomp/user-trap.c: In function 'send_fd':
> > samples/seccomp/user-trap.c:50:11: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> >    50 |         *((int *)CMSG_DATA(cmsg)) = fd;
> >       |          ~^~~~~~~~~~~~~~~~~~~~~~~
> > samples/seccomp/user-trap.c: In function 'recv_fd':
> > samples/seccomp/user-trap.c:83:18: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
> >    83 |         return *((int *)CMSG_DATA(cmsg));
> >       |                 ~^~~~~~~~~~~~~~~~~~~~~~~
> > 
> > [...]
> 
> Applied to for-next/seccomp, thanks!
> 
> [1/1] samples: user-trap: fix strict-aliasing warning
>       https://git.kernel.org/kees/c/9ad28ca5238d
> 
> Take care,

If you happen to update the trailers, looks good to me too:

Acked-by: Tycho Andersen <tandersen@netflix.com>

I also don't understand what changed, or why this really fixes it.
We're still "violating" strict aliasing as far as I can tell, since we
just introduce `int *fd_ptr` insted of memcpy()-ing out the fd into
an int?

But whatever shuts the compiler up works for me.

Tycho